15 mm spacers on rear - effects on handling?

[Elombard]

This is a 84' car

Running 7/8x 15 fuchs. Lower and bigger t bars.

I purchased hub centric 15mm spacers and longer studs. Its a bit of work to install. And this is pretty much just for looks....

What are the effects on handling? Positive negative?

Since the rear is changing relative to the front...

I imagine they are pretty negligible.

I did some searching but could not find anything.

It seems like with a wider track and longer moment you are :

- making the rear sway bar effectively smaller - mine is adjustable
- lowering the C/G in the rear
- changing the roll axis as a result of the C/G change

Taken independently the changes could be good but they may have a negative effect on the overall handling and balance.

Got me thinking - perhaps changing the track on the rear only is NOT a good idea since there is no room to match the change in the front.

I know there are several members here who could write or have writen a thesis on this stuff....

or maybe there is a thread discussing it already that I did not find. If that is the case I would a appreciate a point in the right direction.

[Steve@Rennsport]

Widening the rear track relative to the front results in increased understeer (push) and generally speaking, never a good thing to do.

As a matter of fact, we normally try to increase the FRONT track relative to the front since we are always fighting an understeer issue with these cars.

Hopefully, you have front & rear adjustable swaybars.

[Elombard]

Thanks Steve

Yeah front is adjustable as well. Hmmmm well understeer would probably be good for my mediocre pilot skills I want to think that I can handle a well balanced car. Maybe I need to reconsider installing them or at least removing them at the track.

Why does it increase understeer?

Is it the less weight transfer with the wider track or something with the roll axis changing? Or something else entirely?

[Walt Fricke]

Eric

The effect of adding spacers is principally of reducing the effective spring rate, same as a small decrease in torsion bar diameter. This is a purely geometric, lever type effect - the load is now imposed farther out on the lever arm. Typically the sway bars play less of an effect.

I don't think these effectively softer torsion bars will result in a measureable change in CG height - and none in its plan view location.

If I recall rightly, one of the terms in roll center calculation is the center of the tire. Moving that out will change things, though I can't do the mental math on that one other than to suspect is isn't a large effect.

The was to deal with all of this is simple - your front and rear adjustable sway bars. Tighten up on the rears and you can get the same front/rear roll resistance balance you had before. Or loosen the fronts. Or both. me, I'd hold off until I tracked the car and felt or measured some adverse effect.

The next answer has to do with the front/rear distribution of total roll resistance - which includes springs, sways, and even shocks. Reducing the contribution to roll resistance at one end of the car, or increasing it at the other, will reduce increase the grip at the first end, and reduce it at the other. That is the simple rule of thumb we all use to try to dial in our car's handling to where we like it. Why doesn't really matter.

The why has to do with the non-linear relationship of pneumatic rubber tires between the weight pressing them into the pavement and individual tire grip. If you could corner dead flat (as go karts almost do) the outside tire would not be weighted more (roughly) than the inside tire, and the combined grip at each end would be the same as the static weight distribution, more or less. However, as the car rolls the increase in weight/grip on the outside is less than the loss of weight/grip on the inside. I think I have that right, and not backward. Think of a 911 cornering with the inside front wheel up in the air. The outside front has way more grip than it did going straight with only half the front weight on it, but double the weight didn't double the grip.

So at most you might want to stiffen your rear sway bar. You do know that positioning of the drop link on the bar is non-linear? Moving an inch in from the softest position produces much less of a change in rate than moving (discounting the direction of change) an inch out from the stiffest position. Plus if the front feels like more push, just move that link out. Or, if that makes the car roll more than you like, move the rear sway drop link in.

I suggest you leave this all alone, but have the equipment to adjust the sways with you at the track. See how it feels. If, indeed, your piloting skills are actually just mediocre, I suspect you won't feel a thing. As those skills progress (haven't you been at this quite a while?), if you start feeling the car is pushing more than you'd like (if it seems to take forever before you feel you can get the right foot all the way down to the floor coming out of corners lest you drive off the track at track out), then start adjusting. Or find a way to use a skid pad and see if tightening this or loosening that helps going clockwise, and then counter clockwise. Nothing bad will happen if you get too much front grip compared with the rear - you will just spin out and know you have gone too far in that direction.

[tirwin]

Originally Posted by Elombard

Got me thinking - perhaps changing the track on the rear only is NOT a good idea since there is no room to match the change in the front.

Maybe others understood what you meant, but when you say there is no room to change in the front, I'm not sure what you mean. Do you mean you don't have room to widen the track in the front? I think you should have room if that's the case.

My understanding is that the preferred approach is to use wider wheels instead of spacers though because of the negative effect on scrub radius of spacers and the advantage of wider contact patch with wider wheels. I'm running 7.5x17 front and 9x17 rear. I had 21mm spacers front and rear with the 16x6/7 Fuchs and they did fit but I had to roll the fenders. But the feel to me is much better with the wider 17's and no spacers than 16's with spacers. Car is lowered.

This suspension stuff is very complicated stuff. You know, a lotta ins, lotta outs, lotta what-have-you's.

[tobluforu]

This depends on what you use the car for imo. Lots and lots of people space their cars out for looks only, and most of these people are not carving canyon roads. If its for looks, have at it, if you are carving canyons, auto-x, track events, then don't. I run spacers in the front of my 72 because of understeer, but that's only because I could get the car to understeer all the time at auto-x, etc. I also run spacers on my viggen, but for that car its for looks only as I don't drive the piss out of it around the twisties. Some people will never push their cars hard enough to induce understeer, but like you state, its better to under that to over, most of the times.

[Bill Verburg]

Originally Posted by elombard

thanks steve

yeah front is adjustable as well. Hmmmm well understeer would probably be good for my mediocre pilot skills i want to think that i can handle a well balanced car. Maybe i need to reconsider installing them or at least removing them at the track.

Why does it increase understeer?

Is it the less weight transfer with the wider track or something with the roll axis changing? Or something else entirely?

+1

[Bill Verburg]

Originally Posted by Walt Fricke

Eric

The effect of adding spacers is principally of reducing the effective spring rate, same as a small decrease in torsion bar diameter. This is a purely geometric, lever type effect - the load is now imposed farther out on the lever arm. Typically the sway bars play less of an effect.

I don't think these effectively softer torsion bars will result in a measureable change in CG height - and none in its plan view location.

If I recall rightly, one of the terms in roll center calculation is the center of the tire. Moving that out will change things, though I can't do the mental math on that one other than to suspect is isn't a large effect.

The was to deal with all of this is simple - your front and rear adjustable sway bars. Tighten up on the rears and you can get the same front/rear roll resistance balance you had before. Or loosen the fronts. Or both. me, I'd hold off until I tracked the car and felt or measured some adverse effect.

The next answer has to do with the front/rear distribution of total roll resistance - which includes springs, sways, and even shocks. Reducing the contribution to roll resistance at one end of the car, or increasing it at the other, will reduce increase the grip at the first end, and reduce it at the other. That is the simple rule of thumb we all use to try to dial in our car's handling to where we like it. Why doesn't really matter.

The why has to do with the non-linear relationship of pneumatic rubber tires between the weight pressing them into the pavement and individual tire grip. If you could corner dead flat (as go karts almost do) the outside tire would not be weighted more (roughly) than the inside tire, and the combined grip at each end would be the same as the static weight distribution, more or less. However, as the car rolls the increase in weight/grip on the outside is less than the loss of weight/grip on the inside. I think I have that right, and not backward. Think of a 911 cornering with the inside front wheel up in the air. The outside front has way more grip than it did going straight with only half the front weight on it, but double the weight didn't double the grip.

So at most you might want to stiffen your rear sway bar. You do know that positioning of the drop link on the bar is non-linear? Moving an inch in from the softest position produces much less of a change in rate than moving (discounting the direction of change) an inch out from the stiffest position. Plus if the front feels like more push, just move that link out. Or, if that makes the car roll more than you like, move the rear sway drop link in.

I suggest you leave this all alone, but have the equipment to adjust the sways with you at the track. See how it feels. If, indeed, your piloting skills are actually just mediocre, I suspect you won't feel a thing. As those skills progress (haven't you been at this quite a while?), if you start feeling the car is pushing more than you'd like (if it seems to take forever before you feel you can get the right foot all the way down to the floor coming out of corners lest you drive off the track at track out), then start adjusting. Or find a way to use a skid pad and see if tightening this or loosening that helps going clockwise, and then counter clockwise. Nothing bad will happen if you get too much front grip compared with the rear - you will just spin out and know you have gone too far in that direction.

another +1

just like to add that yes, the center of the tire contact patch is used to find the instant center, which in turn is used to find the roll center.


moving the tire out w/ spacers has no effect on the center of mass.
it does lower the instant center a bit, this increases the length of the roll couple which causes the rear to roll more for the same lateral force. other effect is more understeer. This can be countered w/ stiffer rear springs or sway bar setting.

when I ran 7 & 8 x15 205/50 & 225/50 on my C3 the best handling came from lowering the car. You can get these cars very low w/ this setup. The only other thing to address is the bump steer that lowering exacerbates.

[Elombard]

Thanks guys

currently running 225/50/15 RA1s all around. the front is lowered so the lower a arm is parrallel to ground and I have bump steer spacer and a mono ball tie rod with a spacer (bump steer kit) commonly available. There is no room to add spacer in the front but there is a gap in the rear, hence desire to be 'hella flush' i guess is the term :-)

The tie rod and lower a arm are pretty close to both being level at static.

Now I remember - as Bill says the IC goes down (tire contact patch moves out slightly), longer roll couple (longer lever) and more torque for same roll force so more roll. More roll in the rear better bite potentially as compared to the front so grip is higher in rear that front = under steer.
assuming all else is equal ;-)

I might try it just for kicks, I want to run longer studs with the open end lugs any way.

[RichardNew]

If this is a street car you probably won't notice any actual difference as you drive around town. It might look cooler though.

If this is a track car then be ready to get into some serious work. In no time at all you'll be on a first name basis with Steve.

Richard Newton
Porsche Concours College