THSCC Forums

anybody had their steering rack modified to eliminate bump steer? who did it, and how much did it cost? huge difference for autox and track SCHOOL, or not worth the money?

Kevin

Alright, I'll bite and show my ignorance...

What is "bump steer"?

I have a couple of guesses but I'll wait for a more experienced answer.

Simon

Kevin,
You can't actually eliminate bump steer, just minimize it (or maximize it). I guess you can raise or lower your rack to make a difference, but I can't see how modifiying the rack itself will help. Bump steer is the change in toe settings as the wheel travels up and down.

If your steering rack and track rod are dead horizontal at normal suspension deflection, then as the wheel moves up or down the track rod doesn't lenthen so it must pull the wheel. If the rod is front, toe in.

If the steering rack and track rod are not horizontal at rest, then just do the geometry to determine what will happen to the wheel.

If you're interested in how toe might impact performance, dial in some additional toe-in or toe-out and run an event (once you understand your car's geometry). When I arrive at an event I go from about a degree toe-in to a degree toe-out. Makes a big difference as my inside wheel now wants to drag me in the direction of the turn instead of fighting the outside wheel!

Phil

I'll argue with my own posting.

If your steering rack mod moved the inner pivot point of tie rod it would definitely impact the bump (good or bad depending on the design). I thought of that after I hit submit!

Phil

Depending where in that arc of travel things start the bump steer can be minimized. The problem is, with a lowered car, you have left the "ideal" range.

I do have spacers on the old car that correct for the lowering.

Scott

Interesting...

Is this SP legal? Not questioning Kevin, actually considering looking into doing it myself.

_________________
Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

All I'm going on is several glowing reviews from some guys over in UK who've had it done to their Imprezas by local race shops there. They don't actually alter the steering rack, they just measure the bump steer (how they do that, I have no idea) and then shim the rack until the bump steer is "gone."

I'm assuming that this would be a SM thing, but I don't know.

Kevin

Bump steer in less technical terms, is how much the car turns without steering input when the suspension loads up (usually from uneven pavement, or bumps...hence "bump steer").

Article on it:
http://www.longacreracing.com/articles/art.asp?ARTID=13

Devices for measuring bump steer:
http://www.longacreracing.com/catalog/i ... 55&catid=5
http://www.brunnhoelzl.com/BUMPSTEERGAUGE.htm

As for the toe-in/toe-out that Phil mentions, race alignment for a track car, you usually want toe out on both wheels. The idea is that the car is "already turning". Might not be the best for tire wear, but it makes it easier to turn.

Here are some alignment specs for one of my cars for track, street, and autocross, and you can see the recommended differences:
http://www.pettitracing.com/faq/index.html#handling

You can buy a bump steer gauge that will measure the bump throughout the typical range of travel for your suspension.

Or you could do it yourself, I suppose, by deflecting the suspension through compression and droop and measuring the toe changes.

Phil

I bent my steering arms some to reduce it. Then they broke. It was very scary, I'm not gunna do that again. The gain in stability was quite nice while it lasted though.

_________________
2006 Civic Si - #24 HS for 2015

2005 GMC Sierra
1991 318is Garage Ornament

I think you actually changed your Ackerman, which influences bump steer in all situations except when going straight ahead. I have a very cool article (hard to understand) on the subject from RaceTech Magazine. If anyone wants a copy, PM me and I'll bring it to the next meeting.

Oh, shimming the rack is most definitely not SP legal. Unfortunately, thanks to the clarifications of the last few years, even replacing the mounts with polyurethane is illegal.

--Kevin H.

_________________
2003 WRX (again!)

Now, that's a favorate topic of mine.

First off, I would visit the Longacre
Racing web site and read the very excellent "white paper" one of their
engineers wrote to seperate myth from fact.

Second, I would determine if you have a "problem" that needs to
be worried about. I have a bump steer gague and would be willing to
lend it out, but you MUST remove the sway bar & springs in order to
test. It's an all day job.

These first two steps are key, because you need to understand
the geometry and be able to figure out what needs to be moved where.
You also need to be able to determine which end of the car, front, back
or both has a problem. Toe changes in the front from ride height change
is called bumpsteer and toe changes in the rear is called roll steer.
Toe chages from steering angle is ackerman. These are all compelty
different things. Some are part of the design of the car and some
are the result of drastic changes in the ride height ie lowering.

Circle track racers play
with ackerman but generally thats not a problem that you have to
correct its "design intent".

What happens is when you lower the car you can introduce
bump steer, however if the car was designed with no bumpsteer( like
a Corvette) over 4.5 inches of suspension travel you don't get bumpsteer
even at a moderatly lowered ride height, however with someting llike
a VW GOLF you end up with tons of bumpsteer. Numbers like .025-.030
should not be too much cause of concern, but when you get in the .050-
.100+ range you will feel the car dart under braking and over bumps.

After you read the Lonacre Racing paper and any of the scores of
books on suspension design, you can almost tell by just looking at
the relationship of the LCA and the tie rods if there is a problem.
Like on the VW, when the LCA is pointing up and the tie rod is
pointing down- there is an obvioius problem.

I can safely say getting rid of bump steer in the front is a definate
advantage. However in the rear, roll steer can be built into the suspension
to ballance it and unless you have a 5 or 6 link syspension with tie rods
in the back, its going to be a big re-engineering job to correct.

Some cars like older Ford Torino's and Fox platform (mustang/fairmount)
are noterious for large amounts of roll steer in the back that caused
the car to make a sudden transistion from terminal understeer to
terminal oversteer
Some cars like a C4 corvette, have just a small
amount of roll undertsteer to help you accelerate out of corners.
The rear uprights can get tweeked or tollerances in the frame and
dog bones can result in more or less roll steer than what was indended
or different valuse from one side to the other.
You have to move the rear tie rods up or down to correct it or
change the lengths of the dog bones to rotate the rear upright..
This would require replacing the dog bones with heim jointed struts.
( Dog bones are the rear laterlal links, its a alloy casting that, well,
looks exacly like a dog bone)

I've got .000 bumpsteer in the Corvette over 2.5 inches of compression
and 2.5 inches of rebound (from the factory, impressed thehell out of me)
and I got the read down to .003 over the same amount of travel and
I can increase it to .025-.030 if I need to tighten up the car a little
by removing two washers.

However, all these catagories of modificatioins are gemotery changes
that are not allowed by just about every catagory of stock/ prepaired
type classes. Generally you can bolt stuff on but you can't move
suspension location points and this is exaclty what we are doing here.


Depending on what car you have and how the steering rack is mounted
and what sort of kits are avilable or what can be used from the Circle
track catalogs correcting bumpsteer can range between a bolt in job
to some moderate frabracation, however someone like Linwood at
Hamaliton Machine can make all the parts you will need.

HOwever, after you do all this if you got soft bushings that are
going to move everthing around, then you are not going to accomplish
much, again these type of mods are more suited to a Production or
Mod type class where you would also have all solid busings and
sperical end links. OF course, forumla type cars will have adjustible
end links everywhere to facilitate these adjustments.

_________________
Mark Vitacco
THSCC TT Chairman
mvitacco@bellsouth.net

an update...

I drove my wife's Civic around a bumpy on-ramp recently and realized that I have practically NO bumpsteer. Her car, on the other hand, needs to be traded for a Subaru.

Every bump I hit made the steering wheel jerk violently counterclockwise. The Impreza's steering wheel doesn't even react under the same conditions. Thanks, Subaru!

And thanks for all the responses, folks. I'm going to check out that website now, Mark.

Don't confuse bump steer with the steering wheel moving around in your hands. They don't necessarily relate. You can have no bump steer in your suspension design, but the wheel can still jerk in your hand as you hit bumps. Suspension compliance, type of steering, power steering design, etc can all lessen/increase that effect.

I've always associated the bump steer "symptom" more as a difficulty to maintain the intended line. As the toe moves in/out with suspension deflection you find your car taking a different line than you anticipated. There may be no detectable tugging at the wheel at all.

Then there is that other type of "bump steer" common to solid axle cars and trucks (and stiffly sprung irs cars) that has nothing to do with toe in/out changes.

_________________
Dick Rasmussen

FS 50 2018 Mustang GT