Does anyone know the scrub radius of the 1984-87 front suspension with the stock 14" HiTech alloy wheels and 215/60X14 tires?

I am trying to figure out how much gain there would be by using the Grand Am wheels which have 12mm more offset which means, with the same tires, there would be 12mm less scrub radius.

Notice that on the 1988 performance models they used more offset and less width on the front wheels, possibly for the same reason. MrPBody got me started on this, now I want details.

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Ira Crummey
1984 econo coupe (soon to depart)
1985 coupe (work in progress)
The Iron Duke Resource Site

BTW for those who are not familiar with the term "scrub radius" this diagram may help:

and here it is reduced by a wheel with more positive offset:

Thanks in advance.

Ira

What does scrub radius affect?

Scrub radius, amongst other things, affects the amount of effort that is needed to turn the wheel, as well as the 'feel' and amount of feedback that you get from the steering. I think alot of new cars have a negative scrub radius, which means the center of the tire is further INSIDE than the steering axis. The 84-87 fiero has a fairly large positive scrub radius (sorry, don't actually know the amount) which gives you heavy steering with alot of feedback (feel every bump in the road).

Exactly Posthumane that is why there is a great deal of kickback and the need for a steering damper (stabilizer). I am hoping that the 12mm will reduce it enough to make the stabilizer unnecessary.

Ira

Modern cars still have positive scrub radius. Negative scrub radius causes strange effects that would cause the average driver to crash into something.

Try it and see if you like it. What price are you getting for those wheels? It's not going to hurt anything, and any reduction in scrub radius is going to help.

Will
All VW and Audi FWD have had negative scrub radius since the Audi Fox of the mid 1970s, many other FWDs also use negative scrub since it reduces torque effects on oneven traction surfaces. Mercedes Benz uses a near zero scrub radius to reduce steering effort.

Ira

quote Originally posted by Monza76: Will All VW and Audi FWD have had negative scrub radius since the Audi Fox of the mid 1970s, many other FWDs also use negative scrub since it reduces torque effects on oneven traction surfaces. Mercedes Benz uses a near zero scrub radius to reduce steering effort. Ira

Positive or negative doesn't matter for torque effects. It's all about magnitude. A negative scrub vehicle can have strange behavior under braking with differential braking left/right. If the vehicle pulls left under braking, then you'll have to steer left to get it to go straight. That's a bit much to ask of the average driver.

Zero to ever-so-slightly negative I can believe, but no production car has significant negative scrub for the above reason.

SAI (steering axis inclination) angle == Kingpin angle

[This message has been edited by Will (edited 03-07-2003).]

I'm still looking for an answer guys so I'm sending this to the top again.

Will... I think if you check it out just about every FWD chassis with MacPherson struts has at least some negative scrub axis (or zero). The only definite exception to this is the Saab 900 series (and possibly all Saabs), they reasoned that rather than making the car self correcting during uneven braking (which the negative scrub axis tends to do) they would not confuse the driver (as you have said). The self correcting nature of the negative scrub axis on FWD cars keeps alot of drivers out of trouble. I tested it out with my 1980 VW Rabbit, you could brake heavily with no hands on the wheel with one wheel on the pavement on one on the dirt from about 50km/h and stop very straight.

BTW I am not suggesting negative scrub axis for a RWD car like the Fiero, I just want to reduce it (as the diagram above shows) in order to lighten the steering effort and reduce kickback.)

Ira

[This message has been edited by Monza76 (edited 03-08-2003).]

Please read your E-mail.

To muddy the waters a bit . . .

Most sources say that Grand Am wheels have 47mm offset (compared to Fiero's 35mm) however Murphy led me to buy a pair of Grand Am wheels that I believe have 48mm offset ("14X6 48JJ" cast into the wheel). If I read this correctly, these wheels reduce scrub radius by 13mm. Because the steering still feels like it has positive scrub radius after installing these wheels, I infer that the stock scrub radius is significantly larger than 13mm. Beyond that empiric and obvious answer, I have no clue.

In lieu of data, you get a free bump.

Ira, I just threw together a winter beater using the 87 suspension with a set of 88 14 " wheels with the greater offset.

I threw on some meaty 195/70R 14 snow tires, and I find it does steer noticably easier than it would with the proper wheels/tire size.

It does seem to have a sharper turning radius also.

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1988GT T-Tops, 5spd, Loaded

Just received some information from someone who had calculated the scrub radius for the 1988 suspension. He wasn't sure if all of the measurements were totally accurate but with his numbers a suspension design program came up with a scrub radius of 3.3" , I can only assume that the earlier cars had an even larger figure than that. 3.3" is more than 80mm so a difference of 12-13mm will not make any significant improvement. Still... every little change is a help.

Ira

quote Originally posted by Will: ...and any reduction in scrub radius is going to help.

quote Originally posted by Monza76 : Still... every little change is a help.

Hmm...

I took some measurements off a 2000 Grand Prix. It's strut tops are ~5" further apart than the Fiero's with the same track. This means less kingpin angle and more positive scrub radius.

There are a lot of measurements of the hub carrier that have to be very accurate to get a good estimate of scrub radius. I'm not too worried about exactly what the scrub radius is, as I know that a reduction helps. I've gone to 16x6 LeBaron wheels on the front which have more offset than the stock Fiero wheels. The steering was noticeably lightened. Why are you concerned about exactly what the scrub is if the mod has the effect you want? You're probably better off measuring steering wheel torque before and after than trying to calculate anything.

quote Will wrote: There are a lot of measurements of the hub carrier that have to be very accurate to get a good estimate of scrub radius. I'm not too worried about exactly what the scrub radius is, as I know that a reduction helps. I've gone to 16x6 LeBaron wheels on the front which have more offset than the stock Fiero wheels. The steering was noticeably lightened. Why are you concerned about exactly what the scrub is if the mod has the effect you want? You're probably better off measuring steering wheel torque before and after than trying to calculate anything.

I guess I am just curious about the numbers, right now I also have Dodge wheels, 14X5.5 styled steel from a Shadow, with 205/70X14 tires. The wheels may actually have less offset, I am not sure, but the combination works fine. Taller tires actually decrease the scrub radius slightly in this design (check it out on the diagram above). The wheels I will be using are shown below:

Notice the difference in offset.

Ira

Interesting thread.
As you know, increasing the scrub radius makes the car steer easier at parked/slow speeds. Decreasing scrub radius makes it steer better over rough roads but makes slow steering harder.
Using the tire rolling around a point (scrub radius) theory, I measured my '88s tire rotation as I steered from straight ahead to full right lock when the car was stationary.
I got 1.5" of tire rotation with ~25* of steering angle which TENDS (sorta maybe) to support 3.3" scrub radius on MY car-with MY tires that showed up on the computer program.
But even finding out how many degrees the tire is supposed to steer is difficult. Nothing in the shop manual, (only some info in a magazine road test).Think I'll buy an angle guage and double check this.
I'm still taking all this with a grain of salt until I get a chance to actually take the front upright out and measure it later in the year.
But, helpful hint, if you put a wheel on your car and turn it as I did on mine this morning and it rotates fwd, it has a positive scrub radius.

quote gixxer wrote: helpful hint, if you put a wheel on your car and turn it as I did on mine this morning and it rotates fwd, it has a positive scrub radius.

Very good point.

BTW My interest in scrub radius is based on my car's current handling. I have just replaced the entire front subframe and have not yet gotten the wheels aligned. Since both my car and the parts car were in proper alignment before the transplant, the alignment is fairly close. I am just using the car in town and the streets are often icy or snow covered, the only handling problem is a very mild tendency to hunt from side to side and an overly light steering feel at low speeds (possibly both due to the street conditions). The steering damper is probably shot as well, will check as soon as the weather improves. I have driven my old 1984 without a steering damper before and I must say I was not very happy with it because there was more road feel but a general feeling of twitchyness, I put the stabilizer back after I cleaned it up a little. Maybe the greater offset would reduce this, Herb Adams felt that with more roll stiffness at the front (larger front sway bar) you could do away with the damper and have better road feel.

Obviously I will be getting the alignment but I was just curious as to how much impact a small change in scrub radius would have on steering feel.

MrPBody, did you notice any change when you changed to the GA front wheels?

Ira

[This message has been edited by Monza76 (edited 03-09-2003).]

quote Originally posted by Monza76: BTW My interest in scrub radius is based on my car's current handling. I have just replaced the entire front subframe and have not yet gotten the wheels aligned. Since both my car and the parts car were in proper alignment before the transplant, the alignment is fairly close. I am just using the car in town and the streets are often icy or snow covered, the only handling problem is a very mild tendency to hunt from side to side and an overly light steering feel at low speeds (possibly both due to the street conditions).

Also possibly due to insufficient toe and/or caster. One thing to be aware of is that the rear pivot of the lower control arm is attached to the chassis. The production tolerance of the location of that pivot is only accurate +/- 5 mm or so. Possibly your chassis could be off from the donor chassis by 10 mm, which will definitely affect caster and toe.

quote Obviously I will be getting the alignment but I was just curious as to how much impact a small change in scrub radius would have on steering feel.

It's definitely noticeable. I've run my current LeBaron wheels on my current '87 chassis and lightened the steering effort noticeably. I've also run early 15x7 wheels with tall tires on an '88 coupe front suspension. That was a very light steering setup. I'd like to try the LeBaron wheels on an '88 some day.

The 1985 steering damper was leaking oil, when I mounted the good one from the 1984 the steering got heavier and somewhat numb BUT it feels more controlled. (the darty, twitchy feeling is gone) Since it only takes a couple of minutes to remove and replace this (no jack needed) if I ran the car in solo events I would to remove it for racing and then replace it for the ride home (if the rules allow).

quote Originally posted by Monza76: MrPBody, did you notice any change when you changed to the GA front wheels?

Definitely.

Before I installed GA wheels, I removed the steering damper and drove the car for a few weeks. Transient steering was lighter, but I felt lots of kickback. When I installed GA front wheels, transient and steady-state steering became lighter, and kickback was greatly reduced.

Well I didn't get any numbers but it seems that the people like Will and MrPBody who tried greater offset wheels noticed some improvement so I look forward to the end of winter so I can mount the alloys.

Thanks\
Ira

Well I have been driving with the GA front wheels and Fiero rear wheels for a couple of weeks now, definitely a noticable improvement.

I can now understand GM's reasoning with the GT an Formula wheels in 1988.

My car now has far less kickback and the steering feels a little lighter. I also like the look since the rear wheels "look" wider than the fronts (I know thay are both 6" wide but many people who notice the difference do not, if they ask I tell them but if they don't....

Here is a picture from a couple of weeks ago, sorry it was taken with a lo-res digital, I'll get some good 35mm shots scanned as soon as the weather here allows for a good cleaning and a decent location. (I may even have the spoiler mounted by then.)

Ira

[This message has been edited by Monza76 (edited 04-22-2003).]