Getting to Grips with Bump Steer...

[Mark]

Hi guys.

I have reached the point where I am attempting to correctly mount my steering rack. I have statically set both Camber Toe to Zero. Using a Longacre Bumpsteer gauge, I am altering the height of the rack before finally making up the rack mounts.

From what I have read and been told, I am given to understand that in a car designed for use on the track (such as mine), any Toe-In is BAD, but a certain amount of Toe-Out is desireable and can assist the cars' stability under braking. I intend to dial in small amount of Toe Out when the car is finally set up.

I am hoping that you more knowledgable folks can look at my readings so far and put them into some kind of context. Frankly I do not know how to interpret my readings. In other words, what is a lot of toe out and what is acceptable? What I can say is that at no point can I get no Toe Change. Closest to Zero change results in both Toe-In and Out at various points throughout the travel and the readings become very erratic.

I stopped last night at the following readings to take advice here before proceeding any further. Incidentally, the langacre gauge reads 0.001" to 1.00" in increments marked out 10, 20, 30 etc up to 100/0. I assume that one complete turn of the needle equals one inch? (no explanation of readings came with the gauge).

Here are my results for your comment/consideration........

All readings are TOE-OUT

BUMP Travel / Dial Reading
2 = 53
1.5 = 35
1 = 12
0.5 = 4
0 = 0 (Ride Height)
DROOP Travel / Dial Reading
0.5 = 6
1 = 23
1.5 = 70

The full suspension travel ranges from 1.5 Droop to 2.5 Bump. All measurements are in inches.

Any help, comments or suggestions are welcome. TIA
Mark

 

[Trevor Booth]

Not familiar with longacre gauge, however, would not the graduations be 0.2, 0.3, 0.4 etc
which would make the readings 0.04,0.12,0.35,0.53 ?? (bump)
If this is corrrect I would suggest toe out in bump is excessive. Any static toe out would be added to these figures. Try also moving rack fore and aft.
There is no hard and fast rule as to how much bump steer, it depends on the car, tyres and drivers preference/style.
Personally I aim for mimimum/slight toe out change with rack location and set/adjust static toe out as required and or raise /lower the steering arm end of the rack tie rod.

 

[Mark]

Thanks for the reply.

I think you are right regarding the the readings. I've spent a lot of time today reading until my eyes burned! LOL I had trouble finding anything that said just how much is bad, vs what is acceptable. Plenty of quotes to say that perfect is probabably unatainable though.

I had also thought that the track rods should be as horizontal as possible. It looks as though that assumption is very wrong.

I'm back up to the car tomorrow evening to re-try. I will also try your advice and move the rack fore and aft. Hopefully tomorrows readings will be better. It's a black art this, but very much fun.

Cheers.
Mark

 

[Jac Mac]

A pic from in front with clip removed would help, but this should get you pointed in the right direction.


Extra... There is a quick & nasty way to check this is a move in the right direction, just shorten either tie rod by say 0.250 inch or thereabouts, turn that wheel to the straight ahead position & re-run your bumpsteer check, if its improved ( ie-amount decreased ) we are on the right track.

 

[Phil]

Mark:
From your readings it looks like your rack is still a bit on the low side, or your tie rod is too long. If your tie rods are level I would try raising the rack slightly with shims of a known thickness, and if you can, move the tie rod up at the upright also.
Not seeing your suspension layout I would also say check alignment of the inner rack pivot with your inner upper and lower wishbone mounts.
I looks like your tie rod is swinging in a broader radius than your upright which usually means it may be longer than it needs to be.
Jac Mac hit the nail on the head, and I might add keep track of all changes or you will be chasing your tail all the while.
Good Luck
Phil

 

[Tim]

you might find it easier if you chart your movements.

if when you chart your readings you get an "S" looking curve ( toe in through toe out, or vice versa. ) it is an indication to start looking at rack or tie rod length or both.

If those lengths are right then you will get either no bump steer or toe through toe, or opposite toe out etc. this will mean that the rack position is in need of adjustment.

This is my understanding.

 

[Dean Lampe]

I wrote an article for KitCar magazine last year on bump steer but i can't find the article on line. Basically you don't want more than 0.020 of movement per inch of travel. Here is the data on my car. This is very difficult due to the short control arms. Toe out is more stable and will allow for more control during turn in. You would never want to cross neutral. For example, you wouldn't want to set up for toe in then under braking have the car move to toe out. http://www.gt40s.com/forum/rcr-forum-rcr40-rcr70-p4-mkiv/23042-rcr-33-track-test-chassis-setup.html

 

[Mark]

Thanks Dean. I had read threads here mentioning your work and aerticle on the subject but couldn't find the actual text. Your data certainly gives me something to aim for.

I will report back to you all here when my second bump steer session is completed tonight. (I wish I could do this all day instead of my real job!)

Mark

 

[Mark]

Well work got in the way again! What's a guy to do?

I have been unable to get back to the car since I last posted. In the meantime however, I have purchased a pair of rose-jointed track rod ends to replace the standard ball joint jobbies, so these should help with fine-tuning the adjustments, (plus they look way cool!!!)

Hopefully I will have something to report before the weekend!

Thanks in the meantime for all the kind feedback, both here and via email. GT40s.com ROCKS!!!

 

[John P]

Here is a page from Longacre's web site dealing with bump steer and setup. My apologies if you have already seen it.
Longacre Racing Online -- Tech Article "Bump Steer "

 

[Chris Duncan]

the langacre gauge reads 0.001" to 1.00" in increments marked out 10, 20, 30 etc up to 100/0. I assume that one complete turn of the needle equals one inch?
BUMP Travel / Dial Reading
2 = 53
1.5 = 35
1 = 12
0.5 = 4
0 = 0 (Ride Height)
DROOP Travel / Dial Reading
0.5 = 6
1 = 23
1.5 = 70

The full suspension travel ranges from 1.5 Droop to 2.5 Bump. All measurements are in inches.

One full revolution of the needle is 1/10". Set it down on the bench alongside a ruler and push the plunger till it does one turn and look at the travel distance.

So those are thousandths readings which are in the ballpark. Like 2" = .050", 1" = .012"

That's kind of a lot of travel for a front shock. You need to determine what the usual travel is of the shock and try to get less bump in the usual travel range and not worry so much about the bumbsteer that happens in the unused range especially at full droop.

you can put a wire tie around the shock shaft push it down on the shock body, do a hard lap then see where it ends up. This will tell your normal bump but not droop.

 

[Mark]

Thanks to everyone for your kind input.

I apologize for the delay in any updates from my end. The chasis has just returned from having a full cage fitted and in the meantime I managed to grab a set of Penske 3-way adjustable shocks with remote canisters (sexy).

I have built in an adjustable mounting system for the steering rack, so coupled with the adjustable rose-jointed tie-rod ends, I should have adequate scope for fine-tuning.

The shocks have a two inch travel from full droop to full bump, so I will now work to fine-tune the rack position and tie-rod lengths to get the best readings within the shock travel parameteres as pointed out above.

I will post my next set of results for your expert opinions in the next day or so.

Once again, many thanks your advice everyone.

Mark

 

[Chuck]

Mark:

The nuances of bump steer are way above my pay grade, but the following excerpt from Carol Smith's book, Tune to Win, page 62 struck a chord when I saw your initial post:

I described the geometric causes of bump steer and detailed the procedures used in adjusting it in Prepare to Win, which means that if you don’t have a copy, you will now have to buy one. At that time I basically stated that the front bump steer should be adjusted to as close to zero toe change as could be arranged but that toe-out in bump should be avoided at all costs. (emphasis added)

 

[EGLITOM]

Here we are again Chuck ;

I´ve read that link to longacre above and they recommand never toe in in bump and say that bump out can enhance stability at corner entry.

Who knows ?

TOM

 

[Trevor Booth]

check the date when "prepare to win" was written

a lot of things have changed since then , and in particular, the construction and the knowledge of tyres

 

[Chris Duncan]

The shocks have a two inch travel from full droop to full bump,

I'm in error then, was confusing suspension travel with shock travel

2" shock travel isn't excessive and is more for racing, 3 would be more for street. It's going to be a stiff ride but should minimize the bump steer.

Another place to adjust bump is where the tie end connects to the upright, spacers to adjust it up and down.

 

[EGLITOM]

Hi

everyone is talking about how much inch the max toe change should be (0,020 - 0,025" per inch of travel).
But this value also changes depending on which horizontal distance you measure it away from your hub center.
So what the reco on this ?
I would assume to measure it at the dimension the wheel rim would set. So for a 15" wheel application at 7,5" away from the center and for a 17" application 8,5" away.

Any recos on that ?

TOM

 

[Mark Maybury]

Tom,
The bump steer for street use is the opposite from track use as you need a push on the street( more stable ) and in racing need the inside front tire to to out in a racing application to create more tire bite.
The distance between the indicators on the Longacre gauge is preset. If you would like I can measure the distance after work.
Also, people need to remember about the Acherman affect as that is just as critical as bump steer.
Just my 2 cents worth from a former stock car racer and crew chief/setup man
Mark