Anyone know the scrub radius of an 88 Formula??

All I know is that it is significantly less than the scrub radius of the 84-87 cars.

Whay do you need to know? What are you trying to do?

I've heard 1.3" but can't verify it.

Will, I'm in the process of selecting new wheels and tires. These could significantly change the vehicle dynamics. I had a earlier post looking for opinions on my choices and you had mentioned scrub radius which I had not considered. Since wheel offset and tire size affect scrub radius (among other things) I'm trying to find an offset and size to give me a negitive scrub radius in addition to reducing front ride height with respect to the rear.

Wannabe, thanks for the input and if I don't verify the value I may try to measure it ugg!

To keep the scrub radius the same you want to keep the centerline of the tire in the same location laterally. To decrease scrub radius you need to move the centerline of the tire inboard. You can also decrease the scrub radius by going to a taller tire. Because of kingpin inclination, going to a taller tire moves the spot where the kingpin axis intersects the pavement outward, closer to the tire centerline. I went from stock to 235/60-15 Dunlop G/T Qualifiers on my '87 and noticed a definite decrease in steering effort -> reduction in scrub radius.

I'm not sure there is a 15" equivalent, but there are some 14" Grand-Am wheels (don't recall what year) which look like the 14" Fiero wheels but have more offset, reducing the scrub radius.

Why would you want a negative scrub? Wouldn't that make the car act wierd?

Negative scrub radius wouldn't make it act weird because most new cars have that. it makes steering easier and gives it less of a kickback on bumps. However I think that in order to give a fiero a negative scrub radius you'd have to put wheels with such an offset that they would rub on the inside of the wheel well while turning.

Lower steering effort and less kickback (not to mention outstanding contact patch feel) are effects of having a very small positive scrub radius. I don't know how the car would act with a negative scrub radius. It might be unstable.

I don't think I can achieve a negitive scrub radius as was mentioned by posthumane can't find an large offset (so far). I measured the clearance on with the stock tires and I have 3" to work with. However, I think I can reduce the positive scrub radius. I really can't find much info on negitive scrub radius but I do know all FWD cars have this otherwise torque steer could ripe the wheel out of your hands. I think the only bad handling situation would be a 0 scrub. A positive scrub makes your tires toe out so I would think a negitve scrub would make them toe in a bit. A large positive scrub makes the car follow grooves in the road and increases steering effort. A lower positive scrub would reduce the shock from bumps (but is reduces road feel). The reason I was interested is a study by Dana Corp on the effects of scrub radius on vehicle dynamics. In short they found that the lateral cornering g's increase as scrub radius goes from positive to negitive (+2" 0.68g's, -0.80" 0.79g's). So if I reduce the scrub this would reduce steering effort (nice with wider tires) and may increase lateral cornering g's a bit. In addition they found that brake forces on the wheel are reduced with scrub radius and are much lower for an equal amount of negitive scrub as positive scrub. Lower positive scrub reduced steering wheel effort in transient maneuvers. However, reducing scrub radius farther into the negative region created an even greater detrimental increase in steering wheel effort during the transient maneuvers. I'm still trying to find out more about handling with a negitive scrub.

Check this out with regards to how much offset you CAN use http://fiero.cc/fiero-tdc/members/mws/Boxster/index.html

As far as scrub radius, zero scrub IS desireable and is often a design criteria for suspensions, including the '88 and up Corvette. Assuming the figure of 1.3" scrub radius is accurate, you would need about 70mm offset to get zero scrub. Not practical, but you can still reduce it with around 50mm offset which is what I'm aiming for. There is a wheel dealer here locally who will be setting up a custom set of 3 piece Boyd's wheels for me, 16x7's front and 16x8 rear with just the right offsets. We'll see.

As I said before, scrub radius approaching zero is good. I don't give any credence to that "increasing lateral g's from negative scrub" idea until I see a trial which uses the same absolute value of scrub radius for positive and negative scrub. I'm not going to put any faith in that study anyway until either I read it or I know more about the testing procedure and modifications to the vehicle.

About this toe vs scrub junk:
1) if your suspension deforms enough for the minute force of pushing the wheel down the road exerted at the end of a scrub length lever arm to change your toe, you have more immediate things to worry about than theoretical advantages of different scrub radii.
2) if your suspension deforms enough under braking for toe to change noticeably, you need to go to poly or delrin bushings before you worry about theroetical advantages of different scrub radii.

FWD cars in general have questionable geometry at best. Most just don't have enough power to bring out their bad behaviour. Case in point: I know a guy locally who did a custom turbo setup on his base model Stealth. The extra power, probably about 300 total at the wheels, added considerable of torque steer.

A zero scrub suspension is the ideal for handling. The low scrub radius means that the percentage change in effective scrub as the tire looses adhesion is larger and esier to feel than it would be with a large scrub suspension. This means that it is much easier to take the car to the limit and hold it there because the suspension is much more communicative about where the limit lies.

A "zero scrub" suspension isn't relly zero scrub, it's only zero lateral scrub. There is still some longitudinal scrub radius owing to the existence and magnitude of the castor angle. This however does not contribute to torque steer, only contact patch feel vice high speed stability.

Wannabe - Thanks for the infro you certianly did a lot of work and looks great. I was looking at wheels with a 47 mm offset on stock ride height so do you think those would not rub?

Will - I'm no expert and I am in the process of learning all I can about the suspension geometry. In regards to toe in and toe out from scrub radius what I meant was that the forces then to push the tires in that direction is doesn't change your toe in or toe out if you suspension is not worn. There are a lot of complicated forces on a tire that push against the suspension system, if it is worn then you will get movement that is bad. What I found out is the 0 scrub is really not good (it is great because there are no steering forces so it steers easy) because there are no forces on the tires (or little) and it will wander. I look at it like having 0" toe in-out, you want toe in so your tires track straight if it is 0" they will wander. Here is the link on the article I found on scrub test. http://www.sae.org/automag/scrubradius/
My findings are that a 0" scrub gives you no feedback from the road and without this input I would guess that you would not know when things are breaking loose. Most things I found say that as scrub goes to 0" you loose road feel and this is a problem so you don't want 0" scrub. The way I look at scrub is that it is a lever arm on the steering axis and as long as you on one side or the other of 0 scrub this lever keeps a constant force on the steering and holds in the direction you point it in. If the scrub is too large then the road puts forces on the steering wheel and causes the road to influence steering. Maybe I'm being too simplistic. The other thing I found out is how important this is when selecting wheels and how most people ignore the effects. I appreciate all the dialog on this subject, I'm sure learing a lot.

One thing that has come to mind for me is what effect does changing thet scrub have on the alignment specs?? My first guess would be camber adjustment might need to be reduced but I'm not sure at this point.

Reread what I said above about zero scrub.

What you're talking about is a suspension with zero scrub and zero castor. Take with a few pounds of salt the words of anyone who says that zero castor is good.

Here's the way it works:

Zero lateral scrub is good. (period)

Even with zero lateral scrub, the point at which the kingpin axis intersects the pavement will still be in front of the traction center of the contact patch. The distance which the kingpin axis is in front of the traction center is longitudinal scrub radius. The traction center of the contact patch is the point at which the tire's tractive forces are exerted. Under normal conditions, this point is actually somewhat behind the geometric center of the contact patch. When the tire begins to lose adhesion, the traction center moves forward. This reduces longitudinal scrub radius. Since the cornering forces are exerted on the steering through a lever arm the length of the longitudinal scrub radius, reducing the longitudinal scrub radius reduces the force on the steering and causes the steering wheel to "lighten up" as the limit of the tire's adhesion is approached.
Longitudinal scrub is a direct result of castor angle, which is generally quite adjustable. The smaller the longitudinal scrub and smaller the castor angle, the greater the percentage change in steering effort as the limits of adhesion are reached, and the easier it is to tell what your contact patches are doing.

Setting castor and longitudinal scrub is a compromise between at the limit steering feel and straight line stability. That is why race cars have at best mediocre straight line stability, but excellent feel at the limits of traction, while a luxury touring car might have marvelous straight line stability and be completely numb to loss of front traction.

The early Fiero suspension is fairly numb to loss of adhesion in the front, while the '88 suspension is marvelously communicative about what the contact patches are doing.

[This message has been edited by Will (edited 06-28-2001).]