84-87 vs 88 front suspension (Page 4/10)
Knight FEB 01, 01:40 PM
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
solotwo FEB 01, 07:33 PM
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).]

Bloozberry FEB 01, 08:48 PM

quote
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.
Silicoan86 FEB 02, 12:05 PM
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.
David Hambleton FEB 02, 10:09 PM
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?
Bloozberry FEB 02, 10:30 PM

quote
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.
Will FEB 02, 10:45 PM

quote
Originally posted by Knight:

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.
Will FEB 02, 10:53 PM

quote
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.
Bloozberry FEB 03, 08:30 AM

quote
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).
Silicoan86 FEB 03, 11:13 AM

quote
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.