I was talking with another Fiero owner who had put 18x8 wheels on the back of his 88, and was told that by using the offset that he used (38mm IIRC) that he was doing nothing to improve his scrub radius.
OK, first off, I drive an 86. Secondly, I don't autocross and I rarely push my car up to 9/10ths. Do I really need to be concerned about scrub radius, and did that "expert advice" that my friend received mean anything?
Educate me- What is Scrub Radius, and does it matter to me?
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Doug Chase
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This seems like a good time to jump into this forum...
The spindle that the front wheel is mounted on pivots. If this pivot point is directly inline with the center line of the tread then when you turn the steering wheel, the front wheel will pivot on the ground.
If the center of the tread is, say, 4" out from the spindle pivot, then when you turn the steering wheel, the front wheel will move in an arc.
In the first case your scrub radius is 0. In the second case your scrub radius is 4".
Smaller scrub radius is better because it has better steering feel and less kick-back. One of the improvements made to the '88 front suspension was reducing the scrub radius. This improved the steering feel and let GM get rid of the steering damper.
Make any sense?
If the "expert advice" given to your friend was in relation to the rear wheels, then the expert doesn't know what he's talking about. Scrub radius only exists if the wheels turn.
The spindle that the front wheel is mounted on pivots. If this pivot point is directly inline with the center line of the tread then when you turn the steering wheel, the front wheel will pivot on the ground.
If the center of the tread is, say, 4" out from the spindle pivot, then when you turn the steering wheel, the front wheel will move in an arc.
In the first case your scrub radius is 0. In the second case your scrub radius is 4".
Smaller scrub radius is better because it has better steering feel and less kick-back. One of the improvements made to the '88 front suspension was reducing the scrub radius. This improved the steering feel and let GM get rid of the steering damper.
Make any sense?
If the "expert advice" given to your friend was in relation to the rear wheels, then the expert doesn't know what he's talking about. Scrub radius only exists if the wheels turn.
Howard Definition:
Scrub is the amount of distance the wheel gets pushed out or in during changes of wheel height.
If you think about suspension geometry and reduce it to a simple model,the wheel travels on an arc and that arc is what causes the wheel to change position on that axis.
When you increase the offset you are increasing the diameter of the circle that the wheel would travel on.
ICP Definition:
King Pin Trail is also called "scrub radius" or "king pin offset." It s the distance, perpendicular to the centerline of the chassis, between the center of the tire contact patch and where that line intersects the ground. The contact point of the king pin line is usually inboard of the contact patch center, though many passenger cars design it to be outboard of the patch center for control purposes when a front tire goes flat. That distance can vary dramatically from manufacturer to manufacturer, but on modern racecars is usually kept down around 1 to 1 1/2 inches. ON some stock cars it may increase up to 6 inchers or more.
King pin trail is varied to either lessen or increase steering effort and weight jacking effects when high caster is used. The shorter the offset, the less the steering effort , and the less the weight jacking for any given combination of caster and king pin inclination.
It should be noted that if your wheels have varying backspacing from set to set, you are changine the scrub radius and therefore the handling characteristics. Make sure that your wheels all have the same backspacing.
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Is it good or bad. . . depends who you talk to like anything else.
Claud says in some situations it can help heat up tires which is a good thing. Others say it gives feedback to the drivers and is a good thing on a racecar.
Milliken writes that large amounts will introduce lateral velocity components which will laterally disturb the car. HE also writes that for high speed stability. . . . zero or slight negative is desirable.
My opinion is you want to minimize it.without compromising other areas. I don't think you need to worry about it too much.
Scrub is the amount of distance the wheel gets pushed out or in during changes of wheel height.
If you think about suspension geometry and reduce it to a simple model,the wheel travels on an arc and that arc is what causes the wheel to change position on that axis.
When you increase the offset you are increasing the diameter of the circle that the wheel would travel on.
ICP Definition:
King Pin Trail is also called "scrub radius" or "king pin offset." It s the distance, perpendicular to the centerline of the chassis, between the center of the tire contact patch and where that line intersects the ground. The contact point of the king pin line is usually inboard of the contact patch center, though many passenger cars design it to be outboard of the patch center for control purposes when a front tire goes flat. That distance can vary dramatically from manufacturer to manufacturer, but on modern racecars is usually kept down around 1 to 1 1/2 inches. ON some stock cars it may increase up to 6 inchers or more.
King pin trail is varied to either lessen or increase steering effort and weight jacking effects when high caster is used. The shorter the offset, the less the steering effort , and the less the weight jacking for any given combination of caster and king pin inclination.
It should be noted that if your wheels have varying backspacing from set to set, you are changine the scrub radius and therefore the handling characteristics. Make sure that your wheels all have the same backspacing.
------
Is it good or bad. . . depends who you talk to like anything else.
Claud says in some situations it can help heat up tires which is a good thing. Others say it gives feedback to the drivers and is a good thing on a racecar.
Milliken writes that large amounts will introduce lateral velocity components which will laterally disturb the car. HE also writes that for high speed stability. . . . zero or slight negative is desirable.
My opinion is you want to minimize it.without compromising other areas. I don't think you need to worry about it too much.
My '88 has 17x7 with 40mm backspacing in the front, while in the rear, it has 18x8 with 38mm back spacing. Is this 2mm an issue? I really don't think it is since I am able to pull a .97 g (G-tech measured) and have peaked at 1.13g's. I would think alignment settings are more of a concern than scrub radius.eHoward wrote:
It should be noted that if your wheels have varying backspacing from set to set, you are changine the scrub radius and therefore the handling characteristics. Make sure that your wheels all have the same backspacing.
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'88 Fiero GT- 3800 Turbo Best E.T. 11.36 Best MPH 121.50 (Sold and gone)
2021 Hyundai Veloster-N (SCCA Solo D-Street)
2004 Mazda RX-8 (SCCA Solo STX)
WNY SCCA-Region Auto-X Program Chair
2021 Hyundai Veloster-N (SCCA Solo D-Street)
2004 Mazda RX-8 (SCCA Solo STX)
WNY SCCA-Region Auto-X Program Chair
Scrub radius and static alignment are both settings in a vehicle's set up.
If the car feels good, I wouldn't worry about. If you drive another 88 with similar set up with the exception of backspacing, and like that a whole lot better, then it's a different story. If you have money, buy three piece wheels and
I had the opportunity to go to a seminar given by ICP, http://www.icpcitation.com (these are people who someone like Chip Ganasi would go to for advice), a couple of years ago. The single most important thing I got out of it is change settings and experiment. Try something different every time out during testing, then go back and compare your results. Scrub radius is just a setting and there is no right answer.
If the car feels good, I wouldn't worry about. If you drive another 88 with similar set up with the exception of backspacing, and like that a whole lot better, then it's a different story. If you have money, buy three piece wheels and
I had the opportunity to go to a seminar given by ICP, http://www.icpcitation.com (these are people who someone like Chip Ganasi would go to for advice), a couple of years ago. The single most important thing I got out of it is change settings and experiment. Try something different every time out during testing, then go back and compare your results. Scrub radius is just a setting and there is no right answer.
GoD wrote:
My '88 has 17x7 with 40mm backspacing in the front, while in the rear, it has 18x8 with 38mm back spacing. Is this 2mm an issue? I really don't think it is since I am able to pull a .97 g (G-tech measured) and have peaked at 1.13g's. I would think alignment settings are more of a concern than scrub radius.
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Doug Chase
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Ahhh, if only life were that simple. It would be ideal if you had the time, money, and parts to do testing on a course that was representative of (or the same as) the course you'll be racing on. The unfortunate reality for most amateur racers is that the only chance to get out on a course is in a points event. Especially something that has a different course every time like autocross or rally.eHoward wrote:Try something different every time out during testing, then go back and compare your results.
ICP is 100% right, of course. I'm just whining because the forms of racing I chose (and my wallet) make it very difficult for me to do this.
Thanks for providing the link to ICP. Until that I was wondering how Insane Clown Posse was involved in racing.
Howdy,
first post here.
I'm the same Gixxer (PRICK) I was back at Old Europe.
To get to the heart of what (I think) your question was.........
I believe the Fiero has a fair amount of scrub radius effect in the front end. This makes the car steer easily at slow speeds and gives the car a large amount of steering self centering when rocketing around onramps. If you have the dumbass Old Europe recommended amounts of positive caster--as much as you can get, your cars steering (in corners)will be heavier yet.
So unless you like a car that steers like the proverbial tuna boat in corners, get as much of thy caster out forthwith if not sooner.
A sneaky way to ease up on the cornering steering effort is to remove some of the scrub radius (relative to the stock rims) by changing rims as decribed in prior posts. There are some drawbacks to this-- If you were to reduce the scrub radii to a very low value, the steering will get very heavy at slow speeds. The tire will no linger roll freely around the kingpin while tuning, the center of the contact patch will sit stationary below the kingpin axis and turning the tire will simply try to rip the tread off.
The steering WILL get lighter as the car speeds up 'tho. It may also be less susceptable to steering kickback on bad roads. A real goofy result of too little scrub radius combined with small or no amount of Ackermann effect will mean that in very tight corners at a high tire slip angle the steering forces received to you thru the steering may REVERSE. Meaning- the tires will twist to the inside of the corner of their own accord and the steering wheel will follow suit. The tires are doing what comes natural to them but the steering geometry is not masking this effect out. I have an English mid-engine sports car, and it does this (because of the above factors) and it is very---------interesting when it occurs
but in reality, gives little trouble.
Unless you're from Old Europe, then you'd wet your knickers!
first post here.
I'm the same Gixxer (PRICK) I was back at Old Europe.
To get to the heart of what (I think) your question was.........
I believe the Fiero has a fair amount of scrub radius effect in the front end. This makes the car steer easily at slow speeds and gives the car a large amount of steering self centering when rocketing around onramps. If you have the dumbass Old Europe recommended amounts of positive caster--as much as you can get, your cars steering (in corners)will be heavier yet.
So unless you like a car that steers like the proverbial tuna boat in corners, get as much of thy caster out forthwith if not sooner.
A sneaky way to ease up on the cornering steering effort is to remove some of the scrub radius (relative to the stock rims) by changing rims as decribed in prior posts. There are some drawbacks to this-- If you were to reduce the scrub radii to a very low value, the steering will get very heavy at slow speeds. The tire will no linger roll freely around the kingpin while tuning, the center of the contact patch will sit stationary below the kingpin axis and turning the tire will simply try to rip the tread off.
The steering WILL get lighter as the car speeds up 'tho. It may also be less susceptable to steering kickback on bad roads. A real goofy result of too little scrub radius combined with small or no amount of Ackermann effect will mean that in very tight corners at a high tire slip angle the steering forces received to you thru the steering may REVERSE. Meaning- the tires will twist to the inside of the corner of their own accord and the steering wheel will follow suit. The tires are doing what comes natural to them but the steering geometry is not masking this effect out. I have an English mid-engine sports car, and it does this (because of the above factors) and it is very---------interesting when it occurs
but in reality, gives little trouble. Unless you're from Old Europe, then you'd wet your knickers!
I did a sanity check and re-read what I wrote yesterday today and realized I forgot to clarify something.
I think what ICP meant was to make sure that all of your front wheels have the same backspacing. So if both of your front wheels have 40mm, that's fine. The rears should have equal backspacing too.
What I think they were addressing is where you might run into problems as a racer if you had multiple sets of wheels and didn't realize that they had different back spacing and paired a 38 with a 40 up front. Or if you changed from practice rubber to race rubber for a race and introduced new geometry without realizing it.
I hope that made sense.
I think what ICP meant was to make sure that all of your front wheels have the same backspacing. So if both of your front wheels have 40mm, that's fine. The rears should have equal backspacing too.
What I think they were addressing is where you might run into problems as a racer if you had multiple sets of wheels and didn't realize that they had different back spacing and paired a 38 with a 40 up front. Or if you changed from practice rubber to race rubber for a race and introduced new geometry without realizing it.
I hope that made sense.
GoD wrote: My '88 has 17x7 with 40mm backspacing in the front, while in the rear, it has 18x8 with 38mm back spacing. Is this 2mm an issue?
My local SCCA has a test and tune at least once a year where they have a slalom and skid pad along with measuring equipment set up for solo2 racers to test on.
It would probably be tough to do something similar for rally. I was going to suggest setting up something pretty similar to setting up a rallycross course, but it would probably be easier just to go to a rallycross.
It would probably be tough to do something similar for rally. I was going to suggest setting up something pretty similar to setting up a rallycross course, but it would probably be easier just to go to a rallycross.
Doug Chase wrote: The unfortunate reality for most amateur racers is that the only chance to get out on a course is in a points event. Especially something that has a different course every time like autocross or rally.
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The Dark Side of Will
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To bring some of this together...
There are two components to what is generally considered "scrub radius".
The first is steering offset, which falls under the ICP definition that Howard posted above. This is a purely lateral distance.
The second is caster trail which is the longitudinal distance between the "dave point" and the geometric center of the contact patch.
Steering offset and caster trail are important for all the above reasons... in addition to a few related to the way tires work.
At zero slip angle, the point of application of the tires force is far back on the contact patch. As slip angle rises, the center of force moves forward. As the tire slides, the center of force coincides with the geometric center of the contact patch.
Most importantly, at max effective slip angle, the largest slip angle at which no part of the contact patch is sliding, the center of force is root(2)/2 times the length of the contact patch back from the front edge... which can also be expressed as 0.207 times the length of the contact patch behind the geometric center of contact patch. As the tire progresses from maximum effective slip angle to full slide, the center of force moves forward to coincide with the geometric center.
This effect reduces castor trail. On a well set up suspension, it is one of the primary means of good contact patch feedback to the driver.
So a good suspension setup will have very close to zero steering offset, and a castor trail relative to geometric center of 0.207 times the length of the contact patch. This will result in a linear loading of the steering as the lateral G increases, followed by a reduction of steering effort of approximately 50% as the tires start to slide.
Zero steering offset is possible with the early Fiero suspension, but I think that it would require an offset of over 60, maybe even 70 for the front wheels. Then a bit of custom fab to set the castor at 1.5 degrees or so...
With that much offset the tires are going to be well back in the fenders... time for a coil over conversion with +1" track width on each side.
There are two components to what is generally considered "scrub radius".
The first is steering offset, which falls under the ICP definition that Howard posted above. This is a purely lateral distance.
The second is caster trail which is the longitudinal distance between the "dave point" and the geometric center of the contact patch.
Steering offset and caster trail are important for all the above reasons... in addition to a few related to the way tires work.
At zero slip angle, the point of application of the tires force is far back on the contact patch. As slip angle rises, the center of force moves forward. As the tire slides, the center of force coincides with the geometric center of the contact patch.
Most importantly, at max effective slip angle, the largest slip angle at which no part of the contact patch is sliding, the center of force is root(2)/2 times the length of the contact patch back from the front edge... which can also be expressed as 0.207 times the length of the contact patch behind the geometric center of contact patch. As the tire progresses from maximum effective slip angle to full slide, the center of force moves forward to coincide with the geometric center.
This effect reduces castor trail. On a well set up suspension, it is one of the primary means of good contact patch feedback to the driver.
So a good suspension setup will have very close to zero steering offset, and a castor trail relative to geometric center of 0.207 times the length of the contact patch. This will result in a linear loading of the steering as the lateral G increases, followed by a reduction of steering effort of approximately 50% as the tires start to slide.
Zero steering offset is possible with the early Fiero suspension, but I think that it would require an offset of over 60, maybe even 70 for the front wheels. Then a bit of custom fab to set the castor at 1.5 degrees or so...
With that much offset the tires are going to be well back in the fenders... time for a coil over conversion with +1" track width on each side.
I KNOW you didnt just bring a term into this discussion from SCC.
For those of us that dont read Sport Compact Car, Caster Trail is the distance betweent the center of the tire s contact patch and where the line through the ball joints intersects the ground.
I am curious where you are getting data for the contact patch information you have posted.
For those of us that dont read Sport Compact Car, Caster Trail is the distance betweent the center of the tire s contact patch and where the line through the ball joints intersects the ground.
I am curious where you are getting data for the contact patch information you have posted.
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The Dark Side of Will
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When you change the offset of the wheel, you are moving that green line that intersects the ground either in or out. That is what people talk about when they talk about changing their scrub radius.
I figured I should draw caster trail while I had the crayons out. Similar concept, different plane. Caster trail should not change with change in offset.
I hope that helps
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The Dark Side of Will
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