For those following this thread, you should probably ignore the curves Zac posted above until further notice. They were created using my data uploaded in his Lotus Suspension Analysis software, however I've since discovered several important errors that will change the nature of the curves. I've asked Zac to remove them until we can reconcile a few differences between the program's output and my data, so don't be surprised to see the original graphs disappear.
Once new ones are posted, one of us will let everyone know by posting a message stating as much. Thanks for your patience.
Before I went though the trouble of designing the double A-arm I wanted to try to solve the camber issue using the existing strut tower as this would significantly reduce the cost and build time. As it turns out I had no problem redesigning the lower control arm and engine cradle to accommodate my desired camber gains.
I was able to gain enough camber that even with body roll (between 1.7 and 3.5deg) I still had –1 degree to the road surface on the outside tire and +1 degree on the inside tire. Therefore for all those that wanted the double a-arm it seems that you don’t need it.
Keep in mind that although I checked for different body rolls there is separate geometry needed based on how much body roll you want (how stiff are your springs). I gained the camber by introducing a significant amount of anti roll. I didn’t use any anti-squat (as I don’t want any). However I will use anti-dive in the front.
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Update:
So I feel a little embarrassed about this, but I forgot to include for the tire deformation which adds an additional .6 degrees of body roll (275 tires). What I might do to compensate is reduce the anti-roll, as I'm back to the problem of not gaining enough negative camber. I could also increase static camber to -1 degree but I wanted to maintain it at -.2 to -.5. So back to the vector force diagrams for me.
If I can't solve this problem without making drastic (read expensive) changes I'll be on to the double control arm design...
Here are the suspension analysis results after the coordinates were confirmed by blooze. To begin I'll start with the layout views and then show each graph and any detail or information that may be pertinent in interpreting the graph.
Front View
Side View
Top View
Isometric View
Camber vs. Bump -This graph follows the standard convention for +'ve and -'ve camber -The number on either end of the axis scales correspond to the very end of the graph so in this case each graduation in the Y is 1 degree and each graduation in the X is 15mm
Castor vs. Bump -Rear castor is flat across the bottom at 0
Kingpin vs. Bump
Toe vs. Bump -Toe in is represented by -'ve values
Spring Motion Ratio vs. Bump -At ride height the front and rear wheels will move ~1.87 and 1.10 times more than the spring respectively.
Damper Motion Ratio vs. Bump
Roll Center Z vs. Bump -This is the height with a reference frame that is fixed to the body so ideally the movement in relation to the CG should be minimized for consistent predictable handling -The roll axis of a car should also correspond to it's weight distribution. ie. a rear heavy car should have a roll axis that is closer to the CG at the rear than the front
Camber vs. Roll -Only the Left side of the car is shown for clarity -+'ve roll would be caused by a right hand turn
Roll Center Z vs. Roll
Roll Center Y vs. Roll
Roll Center Position (XY) During Roll
[This message has been edited by Zac88GT (edited 12-03-2011).]
Here are the changes that Fieroguru proposed (moving the lateral and trailing links down by ~1.5" or 40mm). The new curve is shown in green and superimposed over the original in blue to better show the effects of the change.
Overall I would say the changes are an improvement. It certainly improves rear camber gain and roll center control but at the expense of toe out on bumps, although it's fairly minimal.
Originally posted by Zac88GT: -The roll axis of a car should also correspond to it's weight distribution. ie. a rear heavy car should have a roll axis that is closer to the CG at the rear than the front
The roll axis should be as close to parallel to the centroid axis as practical.
if i am reading this correctly, in the lower left quadrant, it is telling me that if the car rolls 6.0* (say to the right) the tire will be at -2.5* to the ground? better than -6* to the ground, and also better than the previous -3*, but not where it should be yet. and it should be around 0*?
things would be a lot easier to understand if our cars would just voluntarily lean into corners like McQueen does . . .
Originally posted by ricreatr: if i am reading this correctly, in the lower left quadrant, it is telling me that if the car rolls 6.0* (say to the right) the tire will be at -2.5* to the ground? better than -6* to the ground, and also better than the previous -3*, but not where it should be yet. and it should be around 0*?
At 6* roll the inside tires will be at -5.66* and -3.45* and the outside tires will be at 2.91* and 4.76* for front and rear respectively. Keep in mind this is with zero steering input and zero static camber so these values would change a little depending on the steering input angle due to castor. Everything is about compromises, if you increase the camber gain you'll be better off in the corners but straight line braking will suffer.
If I were to lengthen my a arms, would the lower and upper both be extended by the same amount, or would it be a ratio of stock length?
Not a dumb question at all... in fact a really good one. The answer is, that it depends. You can do either but most aftermarket companies extend them by an equal amount since that keeps certain static parameters the same as stock, like the caster angle, camber, kingpin angle, scrub radius, etc. Equally extended control arms do however change a few dynamic properties though like the rate of camber change, for example. The longer the control arms, the less camber change you'll get for a given amount of wheel travel.
If you knew what you were doing, you could design control arms that were overall longer, but by a different amount to help compensate for the slower rate of camber change. As with any change though, it usually results in undesired effects on some other aspect of the geometry though, so I'd stick with equally lengthened arms.
Great! Thank you. It will only be 2" per side on the front and back. I was going to cut the arms, connect via rod stock then use angle iron on top and bottom to add more support. I will make sure it isn't too close to the wheel. Sound good?
I guess I should have asked you ,what you would do?
I was going to cut the arms, connect via rod stock then use angle iron on top and bottom to add more support. Sound good?
This isn't a good idea IMHO. I know from you're other thread that you're trying to get the best design features for the least cost, but cutting and lengthening the stock control arms is a dangerous idea. The stamped steel arms were designed as a continuous piece using finite element analysis and made from high strength steel to ensure sufficient structural integrity in bending, torsion, and compression. They would be significantly weakened by cutting them, and even more so by applying welding heat. Remember these are what keep the wheel pointed in the right direction and attached to the rest of the car under some fairly strenuous conditions.
quote
Originally posted by mram10:
I guess I should have asked you ,what you would do?
I have different constraints than you do since my build project isn't on a tight budget. I bought 3" extended tubular control arms to avoid the problems associated with wheel spacers, but they are quite expensive. Given your stated goals to stay within a certain cost, I would use wheel spacers/adapters in the rear since they have they least impact on suspension geometry back there.
For the front, it's a toss up. The right way to do it is with 2" longer control arms that have been designed by a reputable firm and welded by a certified welder. That way you keep the safety margins on a critical component of the car. If you really can't afford the longer arms, then wheel spacers/adapters are a far, far better way to get the right track width on your car than modifying the OEM arms. There will be a few handling quirks with wheel spacers on the front of the car, but nothnig that others haven't gotten used to in the past by going the same route.
The stamped steel arms were designed as a continuous piece using finite element analysis and made from high strength steel to ensure sufficient structural integrity in bending, torsion, and compression.
Uhh... remember that these parts were designed in the late '70's/early '80s. (First appeared on Citation/Celebrity). I don't think there was a lot of FEA involved...
Will, according to the Sept '83 issue of Road & Track Magazine in the Technical Analysis review: "Finite element analysis and high strength steels were used extensively...".
Now, with respect to the rear control arms, you may be right about them not being designed with FEA since they are virtually indistinguishable from X-car parts. Regardless, the discussion about cutting control arms was specific to the front suspension. If you read an earlier post by mram10 he asks: "If I were to lengthen my a arms, would the lower and upper both be extended by the same amount?" Clearly this is about the front.
As for the front, you might argue that the upper and lower control arms were designed long before the Fiero for the T-car (Chevette/Acadian) but the Fiero arms were designed specifically for the P-car. Again from R&T: "On the T-car, the shock mounts to the upper arm and stands very high in the wheel well. But to lower the Fiero's hoodline, the shock now mounts to an otherwise standard lower arm." Neither of the front control arms are carry-overs from earlier cars, so they may very well have used FEA.
Originally posted by Will: Uhh... remember that these parts were designed in the late '70's/early '80s. (First appeared on Citation/Celebrity). I don't think there was a lot of FEA involved...
Well didn't they do FEA with a slide ruler and abacus back then? *I kid, I kid*
i am installing an HT motorsports bumpsteer kit on my 86 gt .i noticed that as i jack it up and down that it has some negative camber gain under compression .kind of surprised me ,since the 84 -87 rear suspension is supposed to have terrible geometry and all .anyways , if this kit gets rid of my bumpsteer and works as well as my rear suspension did before , then i am done modifying the rear suspension .this picture shows tire to roadangle going through a corner at speed (with old suspension ) : i could have gone a lot faster but the can am spyder in front of me kept slowing down , he was having trouble staying in the lane .i now own a spyder RS and i dont think he had many hours on it .takes a lot of practise to go through a corner at speed .this thread has kind of become an 88 suspension discussion so i thought i would inject a bit of 84 t0 87 in to it .after all , close to 92% of all fieros are in that category .
i made an attempt at the double wishbone but gave up when i could not get the arms to line up the way i wanted .on the passenger side it would be easy to do .but on the driver side , a part of the f23 transmission i use hangs right over the cradle and is in the way of an easy way to do it .i have not totally given up on the idea , and now that i have the HT bumpsteer correction lower arms it means the uppers can be less complicated .but still have not figured out a simple way to do it .
Will, according to the Sept '83 issue of Road & Track Magazine in the Technical Analysis review: "Finite element analysis and high strength steels were used extensively...".
Now, with respect to the rear control arms, you may be right about them not being designed with FEA since they are virtually indistinguishable from X-car parts. Regardless, the discussion about cutting control arms was specific to the front suspension. If you read an earlier post by mram10 he asks: "If I were to lengthen my a arms, would the lower and upper both be extended by the same amount?" Clearly this is about the front.
As for the front, you might argue that the upper and lower control arms were designed long before the Fiero for the T-car (Chevette/Acadian) but the Fiero arms were designed specifically for the P-car. Again from R&T: "On the T-car, the shock mounts to the upper arm and stands very high in the wheel well. But to lower the Fiero's hoodline, the shock now mounts to an otherwise standard lower arm." Neither of the front control arms are carry-overs from earlier cars, so they may very well have used FEA.
I guess they could have done primitive FEA on a Cray that has fewer flops than a modern cell phone.
Lol... welding a u-bracket on the LCA to mount the shock is hardly a "redesign". I'm not sure there was *ANY* CAD involved in designing the front suspension... the two LCA pivots are not coaxial, for example.
[This message has been edited by Will (edited 01-05-2012).]
I guess they could have done primitive FEA on a Cray that has fewer flops than a modern cell phone.
Lol... welding a u-bracket on the LCA to mount the shock is hardly a "redesign". I'm not sure there was *ANY* CAD involved in designing the front suspension... the two LCA pivots are not coaxial, for example.
why does having the pivots non coaxial mean anything ? they are on the same plane and they go up and down without moving the ball joint out of its intended path .look at the mustang 2 front suspension , it uses a similar idea :a main arm coming straight out at the wheel centre and then an arm that braces to the back to prevent distortion .it works .
If they're non-coaxial, the shorter leg will prescribe a tigher arc than the longer leg as it tries to pivot, but the trouble is that they are both part of the same rigid control arm. That means the only way the lower control arm can rotate is through the compliance of the rubber bushings.
so there is too much binding ? there are a lot of similar designs in use here , i think mostly for packaging concerns but they still seem too work well .the perfect setup seldom fits in the allotted space .i have to go out to the garage , take a coilover off and run the arm up and down and see what happens .see you later .
going up and down and i apoligize for poor pics but not great light at night .but there is no camber gain or loss from bottom to top of travel of my 86 gt front suspension .not good but not terrible compared to most older north american built cars .it is hard to tell how much binding is going on since i have tubular arms with poly .but here is a challenge for those of you that have the software ....tell me what the affect is of the non coaxial inner mounts .the last pic is just showing what my suspension looks like .
why does having the pivots non coaxial mean anything ? they are on the same plane and they go up and down without moving the ball joint out of its intended path .look at the mustang 2 front suspension , it uses a similar idea :a main arm coming straight out at the wheel centre and then an arm that braces to the back to prevent distortion .it works .
If you have rubber bushings or spherical bearings, it's not a big deal.
If you have urethane or UHMW bushings, then the suspension will start to bind as it gets away from the neutral position.
there is some binding in the motion of the lower control arm .it is not extreme .i noticed that when i first put on the tubular arms and i chalked it up to the way the poly is jammed against the mounts .i have some delrin bushings i was going to put in sometime , i am thinking they might not be such a good idea because they are so rigid .
So I am new to all of this but I am in school to be a Mechanical engineer and in my kinematics class right now we are doing lots of problems that are related to suspension geometries so only recently I have gained some real knowledge on the subject of suspension design and I have been coming up with some ideas. I have been doing drawings of proposed geometries of a strut type suspension, and it seems to me that an ideal set up would utilize a steeply angled strut with long lower links at a slight downward angle when the car is siting at ride height. So would lengthening the lower links and raising the attachement points to the cradle not only increase the track width and lower the lateral load, but also increase the camber curve because the longer links will displace more, the angle of the links will create a decent amount of negative camber, andd pushing the knuckle out farther will also steepen the angle of the strut? I have an 85 so for me its all A arms and such. Is there a way to move the mounting point of the strut to the tower in the pre 88s to move them farther in and put the struts at a steeper angle?
So I am new to all of this but I am in school to be a Mechanical engineer and in my kinematics class right now we are doing lots of problems that are related to suspension geometries so only recently I have gained some real knowledge on the subject of suspension design and I have been coming up with some ideas. I have been doing drawings of proposed geometries of a strut type suspension, and it seems to me that an ideal set up would utilize a steeply angled strut with long lower links at a slight downward angle when the car is siting at ride height. So would lengthening the lower links and raising the attachement points to the cradle not only increase the track width and lower the lateral load, but also increase the camber curve because the longer links will displace more, the angle of the links will create a decent amount of negative camber, andd pushing the knuckle out farther will also steepen the angle of the strut? I have an 85 so for me its all A arms and such. Is there a way to move the mounting point of the strut to the tower in the pre 88s to move them farther in and put the struts at a steeper angle?
The problem with your proposed solution is how it will affect roll center placement (factors of the instant centers and force vectors), migration and control. The difference in the 88 to the citation derived suspensions is incorporating modest changes like what your suggesting. But doing so to an extreme wont cure all the handling problems it can make them worse. There is more to the solution of stable and predictable handling than camber gain, Ideally it is maintaining the tire perpendicular to the road during roll. Roll stiffness is important too, which is what makes roll center placement important. Anti dive and squat are used to maintain perpendicularity of the tire during accel/deccel forces causing suspension movement and therefor camber change. Another important aspect of suspension design is to incorporate an amount of rear toe-in and front tow out during roll on the outside wheel. This will fight oversteer. Does your university offer any vehicle dynamics class? I really suggest you take one if you have the opportunity. Mine wasn't very in depth, but a decent start. Try to get a hold of Race Car Vehicle Dynamics by Milliken, everything your wondering, and interested in knowing can be explained to some extent within that book, and you will have the understand to take advantage of it. Try to get involved in SAE Formula and Formula Hybrid, and you'll learn tons though application.
Changing the pre 88 to extremes would be hard, there is only so much room on the knuckle to regain static camber to a reasonable amount if the strut top is moved inboard (unless your looking to be some baller hellaflush cruiser), and there are few knuckles with the same or more inclination to the strut mounting than what the Fiero has. raising the inner control arm pivot could be done with some work on the cradle by welding some rectangular stock to place the bushings (1+" of gain?), but the toe-link needs major rework in length and position to control excessive toe-change (i guess it does already though).
Edit to add: If you were to move the strut top inboard, the control arm should become much longer with a slightly higher inner pivot, and roll center will not be as terrible, but still not great ( a major downfall of strut based suspensions) The fiero rear doesn't have the room for long control arms.
[This message has been edited by FieroWannaBe (edited 01-26-2012).]
I go to WPI and I am part of fsae but unfortunately when it comes to design it is mostly done by a small team of seniors who work on the car for their MQP and the ones of us in the club who are not on the MQP team just help build the car drive it and test it. I do have some courses like that but kinematics is a prerequisit so I will look into taking one next year. And I have race car vehicle dynamics its a huge book I downloaded the pdf and printed it and hole punched it and put it in a binder. Im slowly working my way through it. I thought angling the A arms and steepening the pitch of the lower links would move the roll center up closer to the cog? I was under the impression that you want roll center close to cog?
Originally posted by zkhennings: I was under the impression that you want roll center close to cog?
What's more important is to have the roll center located at the ideal vertical distance from the CofG (bear in mind that the location of the roll center is always changing in most cases). It's about compromises. If you agree that you want a suspension that absorbs road shocks, then you must accept body roll as a side effect. To counter body roll, you need a suspension that generates the optimum amount of camber to offset the body roll. One of the ways this is achieved is by tuning, not eliminating the vertical distance between the CofG and the roll center. Reducing the vertical distance between the roll center and the CofG reduces the amount of roll for any given speed around a corner, but isn't necessarily a good thing depending on the design of the suspension. What you want is the right amount of body roll to cause the suspension to induce the right amount of camber. In any case, raising the roll center is not usually the best way to tune this characteristic... the best results come from lowering the CofG instead.
Originally posted by Bloozberry: What's more important is to have the roll center located at the ideal vertical distance from the CofG .
I agree with Bloozberry here. A high roll center creates jacking forces on the body, it does reduce roll moment though. Roll center placement is not an exact right or wrong, designers have their own opinions and reasons on where they like it, some prefer them to be low, others around CoG, it also can go off of driver preference. But a moving roll center is never good, it leads to unpredictable handling, so its best to try and keep it around the same area during roll. Speed in corners comes more from driver confidence and car control than all out grip. Fieros arent Formula racers, so all out grip is NOT as important as driver confidence (IMO), predictability, and durability. I would rather race an always understeering car than a nuetral car that snaps into oversteer and inoportune moments. I can fight understeer, and adjust my driving style to suit, I will underdrive a car that I feel will not remain stable at its limits.
[This message has been edited by FieroWannaBe (edited 01-27-2012).]
Originally posted by Bloozberry: Reducing the vertical distance between the roll center and the CofG reduces the amount of roll for any given speed around a corner, but isn't necessarily a good thing depending on the design of the suspension. What you want is the right amount of body roll to cause the suspension to induce the right amount of camber. In any case, raising the roll center is not usually the best way to tune this characteristic... the best results come from lowering the CofG instead.
A reduced roll center to CofG height can also combat dynamic body roll, making it harder for the CofG to roll to the outside relative to the tires, which for extremely soft suspensions mean less load transfer (say, in a very wide and heavy big block trans am). But as the roll center is raised the force vector from the tire patch to the roll center will try to raise the body, and you will lose load transfer on the outside tire (if I remeber correctly).
Ok, so should I bother trying to move the strut inwards at all? (pre 88) to at least the point of the 88? Has anyone done this? I know when people swap an 88 cradle they have to relocate the rear strut attachment point, I figured Id do something similar. Also, do you want positive camber gain to be equal to negative camber gain? (when suspension is dropping of course) or do you want the inside wheel to end up flat and so positive camber a little more to compensate for the static camber? I know that the outside wheel should not be totally flat against the pavement, the whole "a tire on edge wants to turn" like on a motorcycle, but should the inside be flat?
Ok, so should I bother trying to move the strut inwards at all? (pre 88) to at least the point of the 88? Has anyone done this? I know when people swap an 88 cradle they have to relocate the rear strut attachment point, I figured Id do something similar. Also, do you want positive camber gain to be equal to negative camber gain? (when suspension is dropping of course) or do you want the inside wheel to end up flat and so positive camber a little more to compensate for the static camber? I know that the outside wheel should not be totally flat against the pavement, the whole "a tire on edge wants to turn" like on a motorcycle, but should the inside be flat?
Idealy the rear tires should be flat as much as possible, anything less than flat and you will lose tractive capability. Camber thrust is more usefull on the front axle to aid slip angle turning force. Strut inclination like the 88 wouldnt be terrible, it can help the camber curve, but how will you regain static camber? the knuckle is now pivoted inboard. linear camber gain is hard to accomplish while maintaining a decent roll center, but idealy yes, camber lost and camber gained in bump and jounce should mimic body roll angle. So if the body rolls 5 degrees, the outside tire would loss 5 degrees of camber, and the inside gain 5. Before anything is done, take a look at where the roll center for such a setup would be compared to stock, take some simple measurements when the car is on the ground of where all the pivots are. The best improvement you can make to the pre 88 suspension is to limit movement. it's BIGGEST flaw is a solidly mounted toe link and compliantly mounted control arms and struts. The unloading of the rear suspension will cause rear toe-in , and mid turn, while lifting the throttle, the car will move from an understeer situation into oversteer. The lack of adequate anti-dive/anti-squat makes this very hard to control. I would concentrate some kinematic analysis on how the toe link affects toe change throughout the suspension movement, including some level of bushing deflection.
Ok, I was actually doing that (seeing where I should put the tierod/how long it should be for useable suspension travel) But I have poly bushings and Kyb struts and an anti roll bar in the back along with solid mounted cradle bushings. So I figured the next step was to improve camber curve and then put the toe link in the right place for the amount my suspension moves. Is there a way that I can move the strut in more? And can antidive/squat be clarified for me? This is all really helpful though thanks
