Originally posted by Will: [Tilting the front crossmember] dramatically improves the driving experience, but reduces caster and everything that goes along with that. I'm working on adjustable upper control arms to get my caster back.
Can you be more specific? What's the theoretical purpose and what aspect of the driving experience is improved?
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Originally posted by Silicoan86: I'm thinking the discrepancy between those spec sheets on the pre-88 cars is due to them being measured with different wheel options.
Possibly, but for something as simple to check as this it'd be better to find the specs on the steel wheel and come up with conclusive numbers rather than speculate.
I have the factory '84 SE wheels on that car; the factory '88 Formula wheels on that car and '88 Formula wheels on my '86 Coupe that came with steel wheels.
Are there any effects on suspension/handling geometry among the factory supplied wheels when swapped among any year car?
I finally found the specs on the 13" wheels... they're 13 x 5.5 ET42. Knowing this, then the scrub radius would be as follows on '84-'87 cars if the following wheels were installed on the front:
Read in two columns: Installed Wheel / Scrub Radius:
14 x 5.5 ET 42 steel / 47 mm 14 x 6 ET 35 Hi Tech Aluminum / 54 mm 15 x 6 ET 37 Lace Aluminum ('88 front) / 52 mm 15 x 7 ET 30 Lace Aluminum ('86-'87 front & rear, '88 rear) / 59 mm
This is assuming the overall tire diameters are roughly the same as the P185/80R/13's installed on the 13" rims. Rather eye-opening isn't it? It's no wonder the 40 mm (or 35 mm) scrub radius on the '88's is noticeable.
(Edit: corrected my math... it's been a long day shovelling snow.)
[This message has been edited by Bloozberry (edited 02-03-2015).]
Originally posted by Bloozberry: I finally found the specs on the 13" wheels... they're 13 x 5.5 ET42. Knowing this, then the scrub radius would be as follows on '84-'87 cars if the following wheels were installed on the front:
Read in two columns: Installed Wheel / Scrub Radius:
14 x 5.5 ET 42 steel / 47 mm 14 x 6 ET 35 Hi Tech Aluminum / 54 mm 15 x 6 ET 37 Lace Aluminum ('88 front) / 57 mm 15 x 7 ET 30 Lace Aluminum ('86-'87 front & rear, '88 rear) / 64 mm
This is assuming the overall tire diameters are roughly the same as the P185/80R/13's installed on the 13" rims. Rather eye-opening isn't it? It's no wonder the 40 mm (or 35 mm) scrub radius on the '88's is noticeable.
If we're assuming the overall tire diameter is the same, wouldn't it be:
14 x 5.5 ET 42 steel / 47 mm 14 x 6 ET 35 Hi Tech Aluminum / 54 mm 15 x 6 ET 37 Lace Aluminum ('88 front) / 57 mm 52 mm 15 x 7 ET 30 Lace Aluminum ('86-'87 front & rear, '88 rear) / 64 mm 59 mm
It's still a very significant difference to the '88. And as I mentioned, so many people run brake upgrades that add another 10mm to the equation, which is rarely compesated for in wheel choice.
[This message has been edited by Silicoan86 (edited 02-03-2015).]
Can 7 mm make that much difference? If changing to a wider tire and trying to compensate for it, is the wider tire changing something beside more friction and harder slow speed turns?
[This message has been edited by LornesGT (edited 02-03-2015).]
Originally posted by LornesGT: Can 7 mm make that much difference?
To answer that, I'll repeat the first sentence I posted in this thread: "Are you looking for subjective seat-of-the-pants opinions or objective numbers?" Some people add 3" spacers on the front of their cars, or 10" wide wheels with very low offsets to make their wide-bodied car "look" right. Most of these people won't complain about compromised handling (some won't even notice) because for them appearance is prime. So put on whatever wheels you want and if you enjoy the ride, forget about the nitty-gritty details that some of us enjoy discussing and trying to perfect. It's kind of like asking whether 610 horsepower is really any better than 590. You know the difference is there even if only a very few will ever be able to take advantage of it.
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Originally posted by LornesGT: If changing to a wider tire and trying to compensate for it, is the wider tire changing something beside more friction and harder slow speed turns?
That's a bit of a loaded question.
Easy answer: No, unless you count more drag. It's only when you change the location of the center of the tire contact patch by changing the rim offset that other things change.
Complicated answer: No, but while you're not changing anything other than more friction, adding more friction can change things such as suspension dynamics (ex: location of the traction center), stress on mechanical components (ex: greater potential braking, acceleration, and lateral forces on fasteners and bearings), and operating characteristics (ex: likelihood of hydroplaning). How's that for taking a simple question and making it complicated?
I have an article in one of my car mags where they took a subaru BRZ to a track to play around with tires .The subie stock comes with low rolling resistance tires that are not great on the skidpad or the track .So they put on high performance summer tires in the same size as stock and the improvement was huge , close to 5 seconds a lap faster than OEM tires and big improvement on the skid pad .So thinking that more should be better , they went to a wider tire , same brand .Skid pad went up to close to 1G but the lap times went down to only a little better than stock .The straight away speeds were down considerably because the stock engine could not over come the increased friction .Think of that before you put huge tires on a low powered car .(edited to change larger tire to wider tire , sorry for the confusion )
[This message has been edited by wftb (edited 02-04-2015).]
I have an article in one of my car mags where they took a subaru BRZ to a track to play around with tires .The subie stock comes with low rolling resistance tires that are not great on the skidpad or the track .So they put on high performance summer tires in the same size as stock and the improvement was huge , close to 5 seconds a lap faster than OEM tires and big improvement on the skid pad .So thinking that more should be better , they went to a larger tire , same brand .Skid pad went up to close to 1G but the lap times went down to only a little better than stock .The straight away speeds were down considerably because the stock engine could not over come the increased friction .Think of that before you put huge tires on a low powered car .
In this case it was probably the extra weight of the wheel and tire combination that was causing the reduction in performance, not the wider tire causing extra friction.
Larger wheels and wider tires generally weigh more, and rotating mass has a big effect on performance, both acceleration and cornering.
I wonder how much difference using the 88-lace style wheel would be on a pre-88 suspension, if it would be noticeable.
The kick-back in the steering is due partly to the scrub radius and partly due to the amount of caster. Since the caster settings between the '88 and earlier cars is the same (+5 deg), then the difference in kickback must be due to the difference in scrub radius.
Scrub radius is the distance between the centerline of the tire contact patch and an imaginary line extended to the ground drawn between the upper and lower ball joints (king pin angle), as viewed from the rear of the car. The larger the distance between those two points on the ground, the greater the kick back. Here's a drawing of an '88 to illustrate what scrub radius is:
On the '84-'87's the scrub radius is 47 mm with the P185/80R/13 tires and is probably several mm's more with the later 14" Hi-Tech and 15" lace wheels. On the '88's, the scrub radius with the stock 15" lace wheels is only 40 mm's as shown above. That's a 15% reduction in scrub radius so it surely would be felt. It's also the most likely reason why GM left the steering damper off of the '88's.
The reason a larger scrub radius gives more kickback is because the tire pivots on the ground not about the center of the tire contact patch, but rather around the point on the ground drawn between the ball joints. When you hit a bump though, the force acts through the center of the tire contact patch, so the further the center of the patch is from the actual pivot point, the longer the lever arm trying to wrench the wheel around the pivot point.
To improve the kick back in earlier cars, the trick is to find wheels with a greater offset than stock. That's part of the way GM reduced the kickback on the '88's. Higher offset wheels move the center of the tire contact patch further inboard, closer to the ball joint line reducing the scrub radius. But everything is a compromise. Higher offset wheels will tuck under the fender further and most people want a wider looking stance, not a narrower one. By playing with wider rims and offsets though, you can achieve a reduced scrub radius and still maintain the sidewall at the outside edge.
(Edit for clarity)
So Rodney's Lowering ball joints raise the upper control arm angle decreasing the scrub radius correct?
So Rodney's Lowering ball joints raise the upper control arm angle decreasing the scrub radius correct?
No.
AIUI, the actual ball, and therefore pivot, moves up relative to the control arm. As we discussed above, kingpin angle is fixed relative to camber. the locations of the pivots don't change relative to the knuckle. Have the car realigned to the same static camber as before the lowering ball joint installation and you won't change the scrub radius any.
[This message has been edited by Will (edited 02-12-2015).]
This was a highly educational thread. I'm going to have to re-read this one though. For some reason I have always had a hard time with wrapping my head around suspension geometry... Thanks for everyone's contributions and a special thanks to Will and Blooz!
Brake dive can be basically eliminated, but at the expense of reducing caster to almost nothing.
I'm going through my front suspension on my '87 GT to optimize the geometry for racing. My mod for anti-dive is a lot simpler, just involves slotting the lower control arm pivot points to move the front of the arm down and the rear of the arm up. No change in caster and the anti-dive went up to about 30%.
Big improvement under braking; before the tires would tuck up into the wheel wells enough to rub through the plastic fender liners. The camber change in the last bit of compression was excessive enough to cause the fronts to wash out. Now with the anti-dive the nose only drops about 1 1/2" despite the sticky tires and hard braking. Tire wear is much better as well.
I've moved the upper ball joint to get the static camber that I want. I'm going to move it back to get more caster, then weld in a plate to reinforce the control arm. Final step will be to move the rack to minimize the bumpsteer that will result from the geometry changes.
[This message has been edited by mender (edited 11-24-2017).]
Did you do the spherical bearing mod to the front wishbones as well?
Actually, from the factory, the pivots didn't line up...
Haven't yet but will be doing that. Mine presently have poly and I can feel the resistance because of the misaligned axes.
I just got the car on the hoist and leveled with the springs out and sway bars disconnected to do the suspension measurements. Did the rear camber curves and bumpsteer and will be doing the front next.
I wanted to chime in here with my 1 cent opinion (That's probably all it's worth!)
The difference between the REAR 84-87 vs 88 is huge- big benefit to change over...The front is just not that big...Pontiac could have saved a lot of money by making a stand-alone separate hub with less offset (Bring the mounting flange in say a full inch, which, with the added bolt-on disc thickness, would have improved scrub radius by almost 3/4")......The other thing would be to simply mount the inner A-arm mounts in different locations to improve the geometry.....Somewhat like what Mender is doing.
I grew up with Ford/Mustang cars, so I knew about the "Shelby mod" to the front of first-gen mustangs; Pull the upper a-arm out and re-drill the mounting holes one inch lower, so that you get better camber gain during suspension deflection....Also, there is benefit from a lower roll center......On the Mustangs it's simply drilling holes, but on the Fiero, the mounting tube sits on top of the subframe/Spring-perch....A future project of mine is to drill thru the subframe and weld a new mounting tube below the spring perch (Basically resting on it from underneath, lowering the mounting point one inch....I have a spare subframe, so I just have to find the time.....Once I finish it, I will let everyone know how it turned out....(I want to align everything properly, so I am thinking of using angle-iron bars, with alignment tubes welded on, then mount one end in the original position on one side, and then use the far end to align the new hole/tube on the other side, then reverse the process to finish the first side)(Obviously, this fixture will allow the job to be repeated...)
Originally posted by mender: I'm going through my front suspension on my '87 GT to optimize the geometry for racing. My mod for anti-dive is a lot simpler, just involves slotting the lower control arm pivot points to move the front of the arm down and the rear of the arm up. No change in caster and the anti-dive went up to about 30%.
Any more feedback about this five years later?
Did you leave the steering rack at the in the stock position on the crossmember, or did you move it down a bit to follow the front LCA pivot?
I'm going through my front suspension on my '87 GT to optimize the geometry for racing. My mod for anti-dive is a lot simpler, just involves slotting the lower control arm pivot points to move the front of the arm down and the rear of the arm up. No change in caster and the anti-dive went up to about 30%.
Big improvement under braking; before the tires would tuck up into the wheel wells enough to rub through the plastic fender liners. The camber change in the last bit of compression was excessive enough to cause the fronts to wash out. Now with the anti-dive the nose only drops about 1 1/2" despite the sticky tires and hard braking. Tire wear is much better as well.
I've moved the upper ball joint to get the static camber that I want. I'm going to move it back to get more caster, then weld in a plate to reinforce the control arm. Final step will be to move the rack to minimize the bumpsteer that will result from the geometry changes.
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Originally posted by pmbrunelle:
Any more feedback about this five years later?
Did you leave the steering rack at the in the stock position on the crossmember, or did you move it down a bit to follow the front LCA pivot?
Building wedge-shaped spacers that rotate the entire front suspension around the lower control arm rear pivot was the proof of concept that showed that brake dive could be eliminated. That first method had the aforementioned drawbacks.
Like Mender, I've been thinking that just raising the rear pivot without spacing the crossmember may get most of the way there without the additional problems.
Lowering the LCA forward pivot without also moving the rack would definitely cause bump steer. Moving the LCA rear pivot up may also cause bump steer, as it rotates the plane of the LCA so that the inner tie rod end moves "up" (further from the plane), even though the inner tie rod end itself does not move.
However, the LCA forward pivot is pretty close to the wheel centerline, so the projection of steering components into the vertical transverse plane at the front wheel centerline wouldn't move around much.
[This message has been edited by Will (edited 03-30-2022).]
Originally posted by Will: Building wedge-shaped spacers that rotate the entire front suspension around the lower control arm rear pivot was the proof of concept that showed that brake dive could be eliminated. That first method had the aforementioned drawbacks.
Like Mender, I've been thinking that just raising the rear pivot without spacing the crossmember may get most of the way there without the additional problems.
Lowering the LCA forward pivot without also moving the rack would definitely cause bump steer. Moving the LCA rear pivot up may also cause bump steer, as it rotates the plane of the LCA so that the inner tie rod end moves "up" (further from the plane), even though the inner tie rod end itself does not move.
However, the LCA forward pivot is pretty close to the wheel centerline, so the projection of steering components into the vertical transverse plane at the front wheel centerline wouldn't move around much.
I'm leaning towards doing the mender mod now.
Stock, the Fiero/Chevette knuckle seems not too bad, in the sense that the tie-rod-ball is pretty down low, perhaps almost in the LCA plane (plane defined by pivots + ball joint).
So I might move the rack downwards by the distance equal to the change in LCA plane height, evaluated at the position of the tie-rod ball. Or something like that... I am imagining new steel rack-to-crossmember brackets, welded on.
I suppose that I also want the steering column to penetrate the "firewall" at the same position as stock, so that the weather sealing boot works correctly.
To keep the same firewall penetration point with a lowered steering rack, the steering rack shall be slightly tilted forwards in the pitch axis.
This will affect the lower U-joint angle, but I think it should be workable, in terms of the U-joint ratio variation.
I still have your spherical bearing kit for the LCA sitting in a box, but it only makes sense to tack (and fully weld) the parts in place once I have determined the new LCA pivot positions.
I too am going to try and achieve this by lowering the front mounting holes at a ratio to what I raise the rear mounting holes that hopefully keeps the axis of rotation of the LCA the same distance from the inner tie rod. I'll have to do some maths. I am also planning to weld a plate onto the front crossmember to make mounting locations for the upper control arm that are lower down and probably towards the rear of the car to add a little caster. Might add mounting provisions for a coilover into this plate as well. I plan to make front lower and upper control arms with heim joints, and I could make them slightly longer so I could run a wheel with a little more offset and get a smaller scrub radius without having the front end look really narrow. I could be wrong, but it doesn't seem beneficial to have a higher kingpin angle if you can run a wheel with less offset.
I will still probably have to play with steering rack position, but I would like to measure everything and put it all in CAD and see. Have to make some kind of jig to give me a good datum to measure to.
I currently run 88GT wheels on my 85, and the reduced scrub is noticeable, I do have grand am rotors though so I am not sure if I am back where I started even with the additional offset of the wheel.
my current setup: Koni reds, stock 1985 SE springs (F41 package), Will's Anti dive setup, Addco heavy duty front swaybar, Spherical bearing LCA's, 2" belltech drop spindles, stock UCA with poly bushings, in the rear, I have a 1988 cradle, KYB struts, coilovers, and rod end lateral links.
having installed and driven Will's anti-dive setup, I will make the following installation comments:
rotating the crossmember forward 6 degrees is a little more involved than that sounds, I did this mod, and installed spherical bearings in the LCA's, at the same time, the spherical bearings have no give, when I put the control arms back on, they didn't quite line up. as rotating the crossmember moved the control arm mounts slightly closer together. I had to slot the bolts in the crossmember, and slide it forward a bit to make everything fit.
Not sure if it's been mentioned here yet or not, but the steering shaft just barely still fits
This mod lifts the front of the car, I installed drop spindles at the same time to minimize the lift. drop spindles and factory spindles are not all equal, stock brakes may not be a problem, custom brake setups like the C5 fronts I have on my car, may no longer bolt on in the same manner, and may require shimming to align the caliper to the rotor. eventually, I will make my own spindles to eliminate the shims I needed to install to gain alignment.
the front braces on the crossmember are not long enough to meet the crossmember, they need to be lengthened to fit.
Shocks no longer meet the control arm at the same angle, I found it easier to install the shock on the car, then bolt the shock to the control arm, then the control arm on the car, your results may vary.
don't break any of the bolts holding the crossmember to the frame... it's a PITA to fix them. Now, Driving comments:
the car stops dead flat, it's pretty nice, and so far, very predictable, that said, I have not attempted to hammer the brakes hard enough to induce lockup, but I have braked fairly aggressively several times, and each time was very predictable and flat.
the biggest con, right now as mentioned above, doing nothing else, rotating the crossmember eliminates all of your caster. the car now has a tendency to wander, and follow ruts in the road a bit. custom upper control arms should solve that problem, but are another feature that needs to be addressed, similar to the drop spindles minimizing lift.
without custom upper control arms restoring caster, and 2" drop spindles. the tires are also SUPER close to the wheel well liners and do occasionally contact them.
Overall, I like it, but I definitely need to gain more caster, my 3/4 ton truck feels more stable, if I end up with another 84-87 car, I'll do it again, but I may go about it in other ways, like raising the rear LCA point.
------------------ "I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
Has the anti-dive had an effect on the nose of the car raising under acceleration? I imagine that it has stayed relatively the same?
My thoughts being that while the front wheels during braking are putting a lot of force on the control arms which are being "pulled" to the rear of the car (imagining forces at the ball joints), and with stock pro-dive geometry as the control arms travel rearwards, they also travel up due to the angle of the control arm.
With my WRX, people install an "anti lift kit" which is basically a pro-dive kit by dropping the rear pivot of the front control arm. This makes the car pitch more on braking, but during accelerating the forces of the front wheels pulling the control arm forwards forces the wheel into the ground giving more traction on corner exit.
However during accelerating with a RWD car the wheels are not applying much force at all to the control arms. The Fiero front wheels if anything are applying an extremely mild "braking" force to the front control arms which would actually make it lift less during acceleration. But I imagine it is relatively negligible and the anti-dive change has not really affected the lift of the front end, but would not do anything to improve it.
Is the only way to really control front end lift under acceleration with stiffer shorter springs that do not compress as much as therefore do not extend as far under accelerating?
Anti-dive describes how the suspension reacts during braking, when the road pushes rearwards on the contact patch.
During acceleration, unless your foot is on the brake pedal, the front wheels on a Fiero are just rolling along for the ride. Anti-dive doesn't play a part here.
Yeah, you need stiffer front springs to reduce the front-end lift during acceleration. That's just the result of weight transfer.
Part of the motorboat feeling though comes from the pro-squat in the rear of stock 84-87 Fieros, so you may want to work on an anti-squat mod in the rear. It's on my agenda, but not for a while...
Has the anti-dive had an effect on the nose of the car raising under acceleration? I imagine that it has stayed relatively the same?
My thoughts being that while the front wheels during braking are putting a lot of force on the control arms which are being "pulled" to the rear of the car (imagining forces at the ball joints), and with stock pro-dive geometry as the control arms travel rearwards, they also travel up due to the angle of the control arm.
With my WRX, people install an "anti lift kit" which is basically a pro-dive kit by dropping the rear pivot of the front control arm. This makes the car pitch more on braking, but during accelerating the forces of the front wheels pulling the control arm forwards forces the wheel into the ground giving more traction on corner exit.
However during accelerating with a RWD car the wheels are not applying much force at all to the control arms. The Fiero front wheels if anything are applying an extremely mild "braking" force to the front control arms which would actually make it lift less during acceleration. But I imagine it is relatively negligible and the anti-dive change has not really affected the lift of the front end, but would not do anything to improve it.
Is the only way to really control front end lift under acceleration with stiffer shorter springs that do not compress as much as therefore do not extend as far under accelerating?
it stays flat as Patrick said, the fronts are just rolling at that point unless you're inputting force some other way, braking, driving the front wheels, ect. the rear squatting could give the impression of front end lift though.
in this clip, I hit it from about 15-80, you can see there's pretty much no rise from the nose of the car.
------------------ "I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
Do you know what rate front spring you are running? Because base model springs with 1 coil removed are still too soft.
And yea I did say the rolling resistance is basically negligible, I wonder how much force is applied at the contact patch during hard acceleration when you take into account the change in inertia of the (front) wheel. Still probably negligible in regards to suspension design but interesting to think about the full fbd.
Do you know what rate front spring you are running? Because base model springs with 1 coil removed are still too soft.
And yea I did say the rolling resistance is basically negligible, I wonder how much force is applied at the contact patch during hard acceleration when you take into account the change in inertia of the (front) wheel. Still probably negligible in regards to suspension design but interesting to think about the full fbd.
all I know about my front springs, is that they're the springs that were installed in the car when I got it, and presumably stock, 1985 spec springs with no coils removed, Right now, I'm not sold that I would want to go a whole lot stiffer than what I have with the anti dive kit installed.
------------------ "I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
Has the anti-dive had an effect on the nose of the car raising under acceleration? I imagine that it has stayed relatively the same?
Is the only way to really control front end lift under acceleration with stiffer shorter springs that do not compress as much as therefore do not extend as far under accelerating?
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Originally posted by pmbrunelle: Part of the motorboat feeling though comes from the pro-squat in the rear of stock 84-87 Fieros, so you may want to work on an anti-squat mod in the rear. It's on my agenda, but not for a while...
Correct, most of the nose lift under acceleration comes from the rear. The rear end is the front end of a X-body or A-body FWD car. The forward pivot of the control arm is lower than the rear pivot in this design. At the front of a car, this results in anti-dive. At the rear, it results in pro-squat.
The control arm pivots need to be moved to level or the cradle rotated until the control arm pivots are level in order to reduce/eliminate rear pro-squat.
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Originally posted by ericjon262: it stays flat as Patrick said, the fronts are just rolling at that point unless you're inputting force some other way, braking, driving the front wheels, ect. the rear squatting could give the impression of front end lift though.
in this clip, I hit it from about 15-80, you can see there's pretty much no rise from the nose of the car.
The '88 cradle doesn't have nearly as much bad behavior as the early car's cradle. Also, you can raise the forward pivot of the trailing arm to give you more anti-squat if you want it.
Cool, so the rear also contributes to the front dive as well then in the pre-88, under braking the rear must lift and transfer some weight to the front suspension. I was not aware that the rear end was pro-squat, the rear control arm looks like the pivots are relatively level but I have never measured it. It is very apparent up at the front control arm that the front pivot is higher than the rear.
I have to buy some DOM tubing and rod ends and start fabricating. I need spherical joints to mess around with misalignment. I was thinking 1.25" x 0.125" DOM tubing for lower control arms and 1" x 0.125" DOM for the uppers based off reading on motorsports forums. I can do the FEA for forces during cornering accelerating and braking, but I am not sure how to set it up for hitting pot holes and that sort of thing.
Originally posted by zkhennings: Cool, so the rear also contributes to the front dive as well then in the pre-88, under braking the rear must lift and transfer some weight to the front suspension.
Lifting the rear, or changes in the pitch angle of the car, do not materially change the steady-state weight transfer to the front tires.
The weight transfer will happen, and just depends on wheelbase and CG height, and acceleration.
You cannot easily change the weight transfer (not without shifting mass around the car), but you can change how the suspension extends or compresses at each end.
Originally posted by zkhennings: I need spherical joints to mess around with misalignment.
You know about Will's spherical bearing kit for the front lower A-arms?
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Originally posted by zkhennings: I can do the FEA for forces during cornering accelerating and braking, but I am not sure how to set it up for hitting pot holes and that sort of thing.
I don't know how things are at other shops, but in my version of the truth, the OEM will instrument a test vehicle, and then datalog it while doing different types of driving. You could have datalogs for highway, city, etc, and each datalog would represent a certain number of driving hours in the vehicle's life.
We then study the datalogs, and then make a simplified/condensed test which attempts to simulate the datalog. Before prototypes and testing, while still in the paper/theory stage, the simplified test could translate into a series of stresses for different numbers of cycles, which could be compared to the SN curve of the material and Miner's rule.
So for example, you could say in the life of the vehicle, there will be A number of bumps at X intensity, B number of bumps at Y intensity, and C number of bumps at Z intensity.
Almost always the OEMs specify a life cycle for their own vehicle; it is not universal for all vehicles.
Probably for a low-budget DIY project, you'll use empirical vertical/longitudinal/lateral fudge factors. Look around, people use different fudge factors.
[This message has been edited by pmbrunelle (edited 04-06-2022).]
Hmmm I understand that there is weight transfer even if the suspension had 0 movement from the moments created at the contact patches, but for example when you set up cross weights on a racecar by adjusting ride height at each corner it only takes a minor change in ride height in the rear to put more weight on the front wheels as the CG is physically moving forwards unless CG is at the height of the wheel center (unlikely).
I am a$$uming that since the rear will physically raise more during braking that that would additionally load the front wheels creating even further dive. And same thing with squatting further under acceleration, the front will become even lighter than it normally would be as the CG moves further to the rear of the car further exaggerating front end lift. The higher the overall CG for the car the more pronounced this would be.
Is this not the same pro-squat phenomenon used in drag cars to get more weight over the rear wheels during a launch?
In Fiero terms, for the pre-88 Fiero, if you left the front suspension alone but put an 88 rear cradle in it, do you think you would measure the same vertical displacement in the front end under acceleration and braking as with the stock rear? I am suggesting you will measure smaller displacements purely from a physics point of view, but I am not an automotive engineer like yourself so I may be missing some variables. I also have not done the math and could imagine it being imperceptible to the driver.
And edit to say yes about Will's spherical bearings but I would like to make my own control arms to gain further adjustability/run a coilover up front. If I had an 88 where I wouldn't be messing with suspension geometry much I think it is an ideal solution. I guess there is always the possibility of modifying the stock lower control arm to accept a coilover, and I do like that the spherical bearings are protected with seals.
[This message has been edited by zkhennings (edited 04-06-2022).]
Originally posted by zkhennings: Hmmm I understand that there is weight transfer even if the suspension had 0 movement from the moments created at the contact patches, but for example when you set up cross weights on a racecar by adjusting ride height at each corner it only takes a minor change in ride height in the rear to put more weight on the front wheels as the CG is physically moving forwards unless CG is at the height of the wheel center (unlikely).
I am a$$uming that since the rear will physically raise more during braking that that would additionally load the front wheels creating even further dive. And same thing with squatting further under acceleration, the front will become even lighter than it normally would be as the CG moves further to the rear of the car further exaggerating front end lift. The higher the overall CG for the car the more pronounced this would be.
Yes, technically the CG moves around and weight transfer is affected. But for the kinds of analyses you might do on your Fiero, is the effect really significant enough to matter? You can estimate how much the CG moves, and see how much that affects your calculations, then judge if you want to consider CG movement or not.
It is like with finite element analysis; you can study with "small displacements", or with "large displacements" in which the system of equations is constantly being modified to reflect the deformation of the structure.
Besides modeling errors/inaccuracies, a most common source of calculation error comes from the human making calculation mistakes. Keeping things simple (but not too simple) reduces the chance of the human screwing things up.
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Originally posted by zkhennings: Is this not the same pro-squat phenomenon used in drag cars to get more weight over the rear wheels during a launch?
I don't hang around dragstrips enough to see what drag cars do.
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Originally posted by zkhennings: In Fiero terms, for the pre-88 Fiero, if you left the front suspension alone but put an 88 rear cradle in it, do you think you would measure the same vertical displacement in the front end under acceleration and braking as with the stock rear?
Assuming the same wheel rates, yes. Where "the same" displacement means close enough that you can't practically tell the difference.
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Originally posted by zkhennings: I am suggesting you will measure smaller displacements purely from a physics point of view
Right, of course.
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Originally posted by zkhennings: but I am not an automotive engineer like yourself so I may be missing some variables.
"The car" is a broad topic... even though I work in the vehicle world (never touched a passenger car though), I'm still missing a bunch of know-how.
Hmmm I understand that there is weight transfer even if the suspension had 0 movement from the moments created at the contact patches, but for example when you set up cross weights on a racecar by adjusting ride height at each corner it only takes a minor change in ride height in the rear to put more weight on the front wheels as the CG is physically moving forwards unless CG is at the height of the wheel center (unlikely).
Nope, not correct. Cross weight changes because the springs are being compressed differentially. If you add weight to the left rear, the right front gains weight as well, but the left front and right rear lose weight. The front/rear totals and left/right totals remain the same. Horizontal movement of the CG is the sine of the roll or pitch angle times the height of the CG. If the car rolls or pitches 3 degrees, and the Fiero CG is 19" high, then the horizontal CG movement would be about 1 inch. This does not materially affect weight distribution, especially not during corner-weighting.
A go-kart with the same wheelbase and CG location as the Fiero would have the same degree of weight transfer under acceleration/braking as a Fiero, if the minimal movement of the CG is neglected in the math.
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Originally posted by zkhennings: I am a$$uming that since the rear will physically raise more during braking that that would additionally load the front wheels creating even further dive. And same thing with squatting further under acceleration, the front will become even lighter than it normally would be as the CG moves further to the rear of the car further exaggerating front end lift. The higher the overall CG for the car the more pronounced this would be.
The horizontal CG movement is not a significant factor in weight transfer. Pro-dive and pro-squat do not materially affect weight transfer... just suspension and body motion.
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Originally posted by zkhennings: Is this not the same pro-squat phenomenon used in drag cars to get more weight over the rear wheels during a launch?
Drag race cars use ANTI-squat. A well tuned drag race suspension will leave the line with the fronts just barely off the ground, but won't go higher than that. The geometry of the suspension changes the direction in which the tractive force of the tires is applied to the body.
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Originally posted by zkhennings: In Fiero terms, for the pre-88 Fiero, if you left the front suspension alone but put an 88 rear cradle in it, do you think you would measure the same vertical displacement in the front end under acceleration and braking as with the stock rear? I am suggesting you will measure smaller displacements purely from a physics point of view, but I am not an automotive engineer like yourself so I may be missing some variables. I also have not done the math and could imagine it being imperceptible to the driver.
An early car with an '88 cradle will show similar front end lift to an '88 under acceleration. The early car might actually have a little less, since the early front springs are slightly stiffer than the '88 front springs. Under braking, the same car may show the same front end drop in terms of distance, but less rear end rise due to the lesser anti-squat geometry in the rear. This would translate to a lesser pitch change from braking.
Squat and dive geometry uses tractive forces on the tire to apply extension/compression loads to the suspension... Since these loads are in addition to the spring, but can be positive or negative, then they will change the pitch attitude of the body relative to the steady-state (constant acceleration) cases without the squat or dive geometric effects.
[This message has been edited by Will (edited 04-07-2022).]
Alright I trust you guys but I could swear that I have seen drag cars with lower front pivots in the rear suspension linkage, and I could have sworn my friend who races spec Miata raises the rear for extra twisty tracks to help the rear end be looser, and he lowers it for racing in the rain to get more rear end traction. He did confirm that it affects the handling of the car drastically but also confirmed the front to rear weight distribution only minorly changes. Very anecdotal information, I know.
I guess I am still unsure how the front end on a pre-88 with a 88 rear end will have less front end lift under acceleration than a stock pre-88 from this conversation though, I can see it having less overall angular pitch because the rear won't drop, but why would the front raise any less if the weight transfer is unaffected by the lack of rear squat? And I am defining "front" as the point directly above the front suspension, if you are referring to the extreme forward point at the end of the front bumper will raise less, that makes sense due to the overall angular pitch of the car being reduced.
But if the actual front spring extends less under accel with an 88 rear end then I need answers lol.
This has been an enlightening conversation! I want to do out some of the math to see how minor the the weight transfer actually is. Would be interesting to work backwards and adjust suspension and measure the pitch of the car static while wheels are on scales, see where CG ends up via scale weights, and using the angular change of the car apply some trig to figure out the CG height.
Alright I trust you guys but I could swear that I have seen drag cars with lower front pivots in the rear suspension linkage
Pic? Also, was it a successful drag racer?
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Originally posted by zkhennings: , and I could have sworn my friend who races spec Miata raises the rear for extra twisty tracks to help the rear end be looser, and he lowers it for racing in the rain to get more rear end traction. He did confirm that it affects the handling of the car drastically but also confirmed the front to rear weight distribution only minorly changes. Very anecdotal information, I know.
While that does move the CG vertically a smidge, that has a much larger effect on the height of the rear roll center. Raising the rear roll center is one effort that has been demonstrated to significantly improve a Fiero's handling, moving the car from limit oversteer toward neutral, because it carries its weight higher in the rear. FieroGuru has a bracket kit that does this for the '88 cars. My fabbed knuckle design for '84-'87 cars will lower the outer ball joint for a similar effect. There are also "roll center kits" for front wheel drive cars, which mostly consist of rather scary looking ball joint extensions.
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Originally posted by zkhennings: I guess I am still unsure how the front end on a pre-88 with a 88 rear end will have less front end lift under acceleration than a stock pre-88 from this conversation though,
IIRC, the '84-'87 cars have slightly stiffer springs, which would result in less suspension deflection for the same load changes.
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Originally posted by zkhennings: This has been an enlightening conversation! I want to do out some of the math to see how minor the the weight transfer actually is. Would be interesting to work backwards and adjust suspension and measure the pitch of the car static while wheels are on scales, see where CG ends up via scale weights, and using the angular change of the car apply some trig to figure out the CG height.
The magazine articles I've read from back in the day quote the CG height at 19". 1" lowering makes that 18" of course, but engine swaps might change it that much as well. Because of the narrow bank angle, the V6/60 carries its cylinder heads high up, which may give it a higher CG than one might expect.
I'll edit my original post so that no one accidentally uses the wrong numbers.
