If you compare the small performance differences that were measured by the different magazines in the day between the '88's and the earlier cars, I think any reasonable person would be hard pressed to justify the expense to convert an earlier car with '88 suspension. Especially since to achieve the same results you'd need to swap in both ends, not just the front. Besides, the rear end is where most of the improvements were made anyway, IMHO.
The early cars perform just fine for anything except at the extreme limits of performance, and certainly I believe an early car with new bushings and shocks would give a stock '88 with tired bushings a run for it's money. Again, IMHO you'd be far better off investing in renewing the suspension's key parts than in swapping in '88 sub frames.
yes because the strut towers moved inwards... With the '88 strut tower adapters you need to switch to 2.5" ID springs... You don't need an 88 cradle to benefit from that change though...
George Ryan, a Fiero autocrosser from back in the day, said that he thought the earlier car, with the 88 rear suspension, made the fastest car around a track. Better turn-in and cornering, etc. Of course, that's a very narrow set of parameters. There's a lot more to be considered.
Personally, I don't like the amount of kick-back in the earlier front end setups. I actually traded in an 85 GT that I had bought new, because I got tired of how it steered.
------------------ Raydar 88 Formula IMSA Fastback. 4.9, NVG T550 Praise the Lowered!
George Ryan, a Fiero autocrosser from back in the day, said that he thought the earlier car, with the 88 rear suspension, made the fastest car around a track. Better turn-in and cornering, etc. Of course, that's a very narrow set of parameters. There's a lot more to be considered.
Personally, I don't like the amount of kick-back in the earlier front end setups. I actually traded in an 85 GT that I had bought new, because I got tired of how it steered.
Yeah, I think it's even worse on my HELD Slalom front suspension... Has anyone ever discussed how to improve the pre-88 front end?
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)
[This message has been edited by Bloozberry (edited 06-02-2014).]
I had the good fortune to own both 1987 and 1988 GTs at the same time, so have a good basis for comparison. I am also a racer and very particular about handling.
The 1988 is significantly better in feel as well as comportment at the limit in a corner. I never ran a slalom course with both cars so can't give you objective numbers.
The king pin angle was created to reduce the scrub radius, but in so doing, it also created a secondary effect. The larger the king pin angle, the more quickly your wheels return to the straight ahead position on their own after a turn, similar to caster. The reason it does this is because turning the wheel in the steering axis (about the king pin inclination) actually lifts the nose of the car up slightly as well. The tendency is for the car's weight to return to the lowest position which in turn forces the wheel back to straight ahead.
The king pin angle was created to reduce the scrub radius, but in so doing, it also created a secondary effect. The larger the king pin angle, the more quickly your wheels return to the straight ahead position on their own after a turn, similar to caster. The reason it does this is because turning the wheel in the steering axis (about the king pin inclination) actually lifts the nose of the car up slightly as well. The tendency is for the car's weight to return to the lowest position which in turn forces the wheel back to straight ahead.
I have noticed this in our 88 vs the 86 we had, actually found it quite noticeable at times.
[This message has been edited by Mickey_Moose (edited 06-04-2014).]
I think tire size has a lot to do with how the handling "feels" too. I have an 85 2M4 with 195/70-14 tires and an 85 GT with 205/55-16 tires. The 2M4 feels nimbler and more controllable and is less affected by bumps.
Is it desirable to package everything well to reduce kingpin angle while maintaining the same scrub radius? Because the kingpin angle is going to create negative camber anytime the wheels aren't pointed straight, while caster is more friendly as it adds negative camber to outside wheel and positive camber to the inside wheel.
You can't really reduce the king pin angle because it's cast into the knuckle by the location of the upper and lower ball joint mounts.
I'm designing a new suspension which may be including custom knuckles. I was wondering if it is beneficial if starting from scratch. I'm still trying to decide if it would be worth it (custom knuckles), because I could use any bearings I want, any brakes and rotors I want, and do whatever I want with the geometry
I just found this post and I am attempting to align my 1988 kit car. I have an old vintage snapon aligning tables with caster/camber gauges and have aligned 86s before but I see the castor on the 88 is adjustable by moving the upper control arms in and out on either end. What I dont understand is the kingpin angle. If I move the upper arm all the way in it would increase and all the way out would decrease. Then the caster is adjusted by moving the front or rear in or out separately. So if I start all the way in I could increase the kingpin angle. Is this a good thing with wider kit car wheels or is there a magic number I should look for. And... how do I check the kingpin angle with the camber gauge? Nobody seems to mess with the kingpin angle but it seems important to me. thanks Wheelie
------------------ WHEELER'S PERFORMANCE MOTORCYCLE SHOP NEAR DEALS GAP check out TAILOFTHEDRAGON.COM 318 curves in 11 miles #1 RED 85 GT ... GONE still in the area and for sale, needs work # 2 white 86 gt fastback project. UPDATE 3/03 IT LIVES!! UPDATE 12/03 3.4 dohc is here,this winter's project 12/04 still this winter's project 12/05 still this winters project 01/06 96 3.4 dohc engine is in! full length headers built,al flywheel, 5 speed,konis,coilovers, n* throttle body this thing kicks *SS whoo hoo!! OBD2 ,HP tuner. magnaflow 2.5, ,400lb springs,88 cradle,245/17 rear, I AM NOT ADDICTED.... BUT Now # 2 formula project. runs but needs work(so what's new?) # 3 86 gt BLACK it's an auto I want a 5 speed used engine is in and runs, 2012. Been driving for 2 years, dual throttle bodies, roller rockers, 2 1/2 magnaflow... still auto. AND NOW for 2012/2013 a kitcar!! I found an 88 SCORPION.and it's red! I found a GREAT engine for this, 96 3800 sc NEW on the crate never been run, it now has a 4 speed, al flywheel,3.4 pully powerlog etc etc
You can't change the kingpin angle by moving the upper control arm further in or out... that only changes the wheel camber. The reason you can't change the kingpin angle that way is because the kingpin angle is measured between a line drawn through the upper and lower ball joint centerlines and a line that's parallel to the wheel flange. Moving the upper control arm tilts both an equal amount so the angle between then doesn't change. As mentioned before, the kingpin angle is cast into the knuckle so you can't do anything to change it.
Set your wheel camber in accordance with the OEM's recommendations for the best compromise between cornering and straight line traction.
Is it desirable to package everything well to reduce kingpin angle while maintaining the same scrub radius? Because the kingpin angle is going to create negative camber anytime the wheels aren't pointed straight, while caster is more friendly as it adds negative camber to outside wheel and positive camber to the inside wheel.
You can't really reduce the king pin angle because it's cast into the knuckle by the location of the upper and lower ball joint mounts.
The behavior of the kingpin angle mentioned above--the weight jacking and self-centering effect--is dependent on the kingpin angle relative to VERTICAL. The kingpin angle does change with camber when the camber is adjusted by pulling the upper ball joint inward. IE, if camber is zero and kingpin is 6 degrees, then when static camber is -1.0, then kingpin will be 7 degrees.
And... how do I check the kingpin angle with the camber gauge? Nobody seems to mess with the kingpin angle but it seems important to me. thanks Wheelie
Kingpin angle is fixed relative to static camber. Per the above drawing, if static camber is zero, kingpin will be 6 degrees. If static camber is -1.0, kingpin will be 7 degrees.
EDIT: Does anyone know what the kingpin angle for the '84-'87 suspension is? I don't think I've ever seen it in print.
[This message has been edited by Will (edited 01-31-2015).]
Just if the pre 88s could be upgraded at a reasonable price instead of what all is involved in swapping the 88 in.
For feel, the '88 is far superior. The lower scrub radius only tells part of the story. I've read that the kingpin angle changed as well. The hub offset from the steering axis is certainly much larger on the early suspension than on the '88.
Originally posted by Will: Does anyone know what the kingpin angle for the '84-'87 suspension is?
7.5 degrees.
Edit to add:
quote
Originally posted by Will: The kingpin angle does change with camber when the camber is adjusted by pulling the upper ball joint inward. IE, if camber is zero and kingpin is 6 degrees, then when static camber is -1.0, then kingpin will be 7 degrees.
For all intents and purposes kingpin inclination is specified relative to camber. For instance, the reference material I have for both '84-'87 and '88 specify 7.5 and 6.0 degrees kingpin inclination respectively. When measured on the actual knuckles, those are the angles found between the wheel flange and ball joint axes. If the kingpin inclination were specified the way you suggest, then the reference material should have shown inclination angles of 8.5 and 7.0 degrees respectively since stock camber is -1.0 degree. Regardless, none of this changes the discussion relative to WHEELIE's desire to reduce scrub radius by altering the camber.
[This message has been edited by Bloozberry (edited 02-01-2015).]
Originally posted by Gall757: Sometimes it helps to know where these terms came from. The Kingpin used to be an actual pin!.....not just an angle...
Many Trucks still use Kingpins, including many Ford light to medium trucks w/o 4WD.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
So would changing caster or tilting the front crossmember to afftect mechanical trail to any signifigant degree if at all? I am curious because I have wondered if the front end dive could be reduced by tilting the front crossmember.
Also, would getting a higher offset wheel change the scrub radius enough to make a difference in feel and kickback? If so, how much would be good? The 14s and 15s are 30mm right? A Subaru legacy (a cheap source of 55mm 16 x 6.5in rims) rim i think have been used by some members. Aside from increasing inner bearing loads, is there a downside to increasing negative offset? I was actually wondering about getting 1" (or longer if i do widen the fenders) longer aftermarket a-arms and using a 55mm offset rim. I would then run 205 or 215s in front. I currently run 195/60/15 on the front and have acceptable kickback with 30psi
Still in use on Medium Duty trucks. Isuzu uses this on their cab overs. Works great. But some customers were either lazy or stupid and didnt follow maintence recommendation and had to have them replaced around 100,000 miles. LOL
quote
Originally posted by Gall757:
Sometimes it helps to know where these terms came from. The Kingpin used to be an actual pin!.....not just an angle...
[This message has been edited by solotwo (edited 02-01-2015).]
Originally posted by Knight: So would changing caster or tilting the front crossmember to afftect mechanical trail to any signifigant degree if at all?
I'm not sure what you're asking... you're missing a word or two in your sentence. If I remember correctly there was a thread that discussed the merits of tilting the entire front cross member ( think Will was involved), but I'm not sure what they were trying to improve by doing so, nor whether it had any appreciable effect. You might try searching for it.
quote
Originally posted by Knight: Would getting a higher offset wheel change the scrub radius enough to make a difference in feel and kickback?
Definitely.
quote
Originally posted by Knight: If so, how much would be good?
Well, if you look at the '88's for comparison, GM reduced the scrub radius by just 7 mm and did away with the steering damper. You could aim for that level of reduction or even better. I'm not aware of any inherent benefit to having any scrub radius at all (except for the centering effect) though the easiest way to find out is to state definitively that there are no advantages... someone will be sure to set the record straight! If you're dealing with an '84-'87 then as mentioned the scrub radius with stock 13" rims is 47mm. I don't recall what the width or offset was of those steel wheels, but it would be easy to calculate how much more or less scrub radius any other wheel would create if someone posted the specs on the 13" rims. In general though, the higher the positive offset, the closer you'll get to zero scrub radius since that will move the center of the tire patch closer to the actual pivot point of the tire on the ground. Of course it's going to be tough finding a wheel that meets this criteria and sits in the wheel well correctly.
quote
Originally posted by Knight: Aside from increasing inner bearing loads, is there a downside to increasing negative offset?
I can't think of any, and within the possible range of wheel widths and offsets that you're talking about, the change in bearing loads isn't going to be a factor either.
Also, keep in mind that anyone who has performed a brake swap on the front of their pre-88 that involves adding a rotor over a modified front hub has increased their scrub radius by the thickness of the rotor hat (roughly 9 to 10mm for grand am rotors, others are likely similar). I rarely see anyone compensate for this thickness by using higher offset wheels.
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?
Originally posted by David Hambleton: Are there any effects on suspension/handling geometry among the factory supplied wheels when swapped among any year car?
If it could be shown that the center of the tire patch were moved relative to where the kingpin angle intersects the ground, then yes... there would be some effect on suspension/handling between factory wheels. I still don't know what the rim width and offset was on the 13" steel wheels... that would be crucial to make any comparisons since the scrub radius of 47 mm was specifically for that configuration. If any of your wheel combos uses the '88 front wheels, then you can be sure there's an impact vs using any other wheel since they would definitely move the contact patch further inboard than say, the same 15" lace wheels on an '84-'87.
So would changing caster or tilting the front crossmember to afftect mechanical trail to any signifigant degree if at all?
Yes, it will.
quote
Originally posted by Knight: I am curious because I have wondered if the front end dive could be reduced by tilting the front crossmember.
Yes, it can be. I've done it. It 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.
quote
Originally posted by Knight:
Also, would getting a higher offset wheel change the scrub radius enough to make a difference in feel and kickback? If so, how much would be good? The 14s and 15s are 30mm right? A Subaru legacy (a cheap source of 55mm 16 x 6.5in rims) rim i think have been used by some members. Aside from increasing inner bearing loads, is there a downside to increasing negative offset? I was actually wondering about getting 1" (or longer if i do widen the fenders) longer aftermarket a-arms and using a 55mm offset rim. I would then run 205 or 215s in front. I currently run 195/60/15 on the front and have acceptable kickback with 30psi
Yes, the higher offset reduces effort and kickback. I'm running 16x6-$) wheels from a Chrysler LeBaron on the front with 205/55 (I think) tires. Effort and kickback were notably reduced vs. the stock fitment.
Originally posted by Bloozberry: Well, if you look at the '88's for comparison, GM reduced the scrub radius by just 7 mm and did away with the steering damper. You could aim for that level of reduction or even better. I'm not aware of any inherent benefit to having any scrub radius at all (except for the centering effect) though the easiest way to find out is to state definitively that there are no advantages... someone will be sure to set the record straight! If you're dealing with an '84-'87 then as mentioned the scrub radius with stock 13" rims is 47mm.
Per the spec sheet above, the scrub radius changed from 49 -> 35, which is a 14mm change AND, as the spec sheet notes, 30%.
For a car that's supposed to handle rather than put you to sleep, I haven't heard of a disadvantage to having zero steering offset.
The right amount of mechanical trail is what generates steering feel and contact patch feedback at the limit of tire adhesion.
Originally posted by Will: Per the spec sheet above, the scrub radius changed from 49 -> 35, which is a 14mm change AND, as the spec sheet notes, 30%.
Yeah... I question the accuracy of the spec sheets regarding scrub radii. One lists the early cars with 47 mm scrub and the other 49 mm. Then, the '88 is specified as having a 35 mm scrub radius but I've actually measured it at 40 mm (see my diagram above).
Originally posted by Bloozberry: Yeah... I question the accuracy of the spec sheets regarding scrub radii. One lists the early cars with 47 mm scrub and the other 49 mm. Then, the '88 is specified as having a 35 mm scrub radius but I've actually measured it at 40 mm (see my diagram above).
I'm thinking the discrepancy between those spec sheets on the pre-88 cars is due to them being measured with different wheel options, especially since the second sheet calls out the 13" tire option.
