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Old 01-08-2008, 08:41 AM
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Bumpsteer limits

A couple of weeks ago I helped a guy install a bumpsteer "correction" kit in his Mustang and a couple of questions came up during the process. He was installing this kit even though he had no idea what the term "bumpsteer " even means and after trying to explain as best I could it got me to thinking about my own Mustangs so to make a long story short I now have a bumpsteer gauge. I am about to set it up and take some measurements and although I understand WHAT I am looking for I am somewhat in the dark about how much is too much. Starting with the control arms in a normal sitting position what range of compression/rebound travel should I check? What would be acceptable limits? Both of these cars (64 1/2 Mustangs) have Steeroids type R&P steering and if I did my homework right the bumpsteer error should be better than factory but that is what I am determined to find out.

This gauge set-up uses two dial indicators so the readings I get will be in fractions of an inch.
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Old 01-08-2008, 10:54 AM
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Some bumpsteer IMHO is good, as long as it creates understeer on roll. This will make the car more forgiving and stable.

Any oversteer is unacceptable, again IMHO, as it make the car hard to control.

Front wheel toe out on bump will make the car understeer as the car will turn less than expected when thrown into a turn.

Then it depends on who is driving the car and what they wil be doing with it. A daily driver should have more understeer than an autocross car.

Perfectly neutral is not possible due to changing road conditions and loads, therefore I shoot for a little understeer.

How much? I look at the static toe settings as a guide. more than that ( 1/8 inch for example) is enough to feel.

and thats the bottom line, make it feel good. Some people like bumpsteer due to the understeer characteristics, some like a really neutral car.

Pick a number and try it!

Keith
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Old 01-08-2008, 11:54 AM
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Bumpsteer is the toe in or toe out of the front wheels as the suspension moves up and down over bumps or causing the steering angle to move..For most street cars I think it best to limit bumpsteer to the minimum possible..Excessive bumpsteer leads to the car feeling "twitchy" and hard to steer.

Roll steer is an entirely different animal..adjusting roll steer is done in competition cars to get the front rear balance in steering to suit your drivers style..some guys like them more on the loose side (oversteer) and some guys like the car "tighter"..

Factory cars come with a great deal of push or understeer as this is safer for the average driver..For example my daily driver is set up that way which makes it very stable and results in the car being relatively easy to control and keep straight on icy snowy roads..

On factory cars all we get to work with is camber caster and toe..The bumpsteer and roll steer is built in and not adjustable very easily..On competition and modified cars we can relocate the suspension links to affect the steering characteristics of the car.

This all takes a good bit of study to understand what to move and how much in order to meet the required handling requirements of a particular car..For most of the hot rods as long as the car is comfortable for the owner to drive they are happy..For those who participate in competition the subject is a very near and dear one..

One thing to remember when setting up a car is that we do not need to be perfect..we just need to be a tenth better than the other guy..

Hope this has not confused you too much..

Sam
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Old 01-08-2008, 12:02 PM
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The smallest amount of toe change from ride height to compressed, ( bump steer) is what you want. If you have excess, it will drive like crap, (that is the nice spelling).
Dave Tallant Rod Shop
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Old 01-08-2008, 01:16 PM
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i know what bumpsteer is and what causes it on the Mustang but it is not practical to completely eliminate it in these early Mustangs and I was curious as to what would be considered too much. From the factory they are terrible but the rack&pinion set-up I have relocates the inner tie rod to a more favorable location and (hopefully) eliminates part of it. Still don't have any figures yet since this is going to involve removing the spring and I need to remove the UCAs on one of the cars anyway so it will be a couple of days before I get it back together and get it tested, just wanted to know what I should look for once I get it set-up.
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Old 01-08-2008, 02:43 PM
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To do the test set the car up so you can get 3" up and 3" down from static ride height..I look for no more than 8-10 thousands of an inch in bump..Generally that will work OK on most cars..However lets not stress out too much just get the bumpsteer as good as you can get it and do a test hop to see if the handling is acceptable..Zero bumpsteer is nirvana but then if you have some and the car is working well it is one of those "it ain't broke don't fix it" deals..

Just my take on this..
Sam
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Old 01-08-2008, 03:27 PM
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Bumpsteer limits

Something that may help you and others setting things up and taking bump steer measurements. I bought a bump steer gage that had one dial indicator witch worked fine but was hard to (zero out). So I got a digital one that you just position at your starting point and press the zero button. So with the gage mounted at the back of the wheel when it toes in I get a positive number like + .010 and a toes out condition gives me a negative number right on the gage. No math or guessing involved, also a small digital level setting on top of my bump steer plate is easer than trying to see and adjust a bubble level built into my gage.
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Old 01-10-2008, 09:47 PM
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8-10 thousandths of an inch I did'nt know it needed to be that close, is that reading of 8-10 thousandths taken at tread width? I should have this thing back together in a day or so and I want to check it so I can make any changes while I have it in the shop. If the bumpsteer is off too much it can be corrected on this set-up pretty easy since the tie rod length can be changed as well as relocating the inner pivot point but I did not want to change anything until I know exactly what I am looking for..
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Old 01-10-2008, 10:11 PM
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In the interests of accuracy setup so you can work off of the rim..tires are notorious for being out of round enough to cause fits when doing this..

Also give the wheel a spin to check for runout and measure on the same spot each time..Sorry if I am a bit anal about this sort of thing..

Like I said tho is just get as close as you can and do a test hop to see how it is handling..Bet it turns out waay better than stock..

Sam
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Old 01-10-2008, 10:29 PM
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Quote:
Originally Posted by OneMoreTime
IBet it turns out waay better than stock..Sam

If I get it within a 1/2" or so it would probably be better than stock!



Joking aside those things were just awful from the factory and those so called "correction" kits don't do much to help the problem. After I installed the R&P (a cloned Steeroids) the improvement in handling was dramatic but still I would like to know if I can do even better, just never satisfied I guess.
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Old 01-11-2008, 12:20 AM
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re: Bumpsteer limits

If you go here http://www.longacreracing.com/articles/art.asp?ARTID=13 they have information and equipment on there web sight. And if you want to rely get information and understanding there is a small book called Advanced Race Car Suspension Development, By Steve Smith. Your not building a racecar but the chapter on bump steer is grate.
Good luck.

Iceman
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Old 01-11-2008, 06:46 AM
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Quote:
Originally Posted by OneMoreTime
To do the test set the car up so you can get 3" up and 3" down from static ride height..I look for no more than 8-10 thousands of an inch in bump..Generally that will work OK on most cars..However lets not stress out too much just get the bumpsteer as good as you can get it and do a test hop to see if the handling is acceptable..Zero bumpsteer is nirvana but then if you have some and the car is working well it is one of those "it ain't broke don't fix it" deals..

Just my take on this..
Sam
Unless you are building an F1 car and have an F1 level driver, trying for .008 - .010" is crazy.

In fact I'd say it's actually counterproductive, a little toe out on bump is desired.
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Old 01-11-2008, 07:41 AM
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Quote:
Originally Posted by 427v8
Unless you are building an F1 car and have an F1 level driver, trying for .008 - .010" is crazy.

In fact I'd say it's actually counterproductive, a little toe out on bump is desired.
Well some of the folks here strive for excellence and others are satisfied with something else..

Sam
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Old 01-11-2008, 08:01 AM
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Quote:
Originally Posted by OneMoreTime
Well some of the folks here strive for excellence and others are satisfied with something else..

Sam
Well if you consider .008 excellent fine, but you are not seeing the forest for the trees.

The problem with being that close to zero is that changes in vehicle loading can put you into toe in on bump. That is an undesirable situation as it makes the car hard to handle. ( Herb Adams)

Dynamic forces on a suspension are hard to measure in your garage. A little toe out on bump will prevent toe in when the suspension deflects during that evasive maneuver on the freeway.


I'm not the only one that thinks this. The longacre article mentions it as does every suspension book I've read.

Last edited by 427v8; 01-11-2008 at 08:14 AM.
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Old 01-11-2008, 08:17 AM
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Quote:
Originally Posted by OneMoreTime
In the interests of accuracy setup so you can work off of the rim..

Sam

You should be using a rigid toe plate bolted to the hub.

Otherwise as the wheel moves you will be indicating off of a different part of the rim. Rims are not flat.

And runout is immaterial unless you are spinning the tire.
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