Hi Guys!
This looks like a great forum, and I look forward to contributing.
I'm currently working with some friends in designing the car. I'm working on the front suspension/steering. I've hit a problem with bump steer.
All my calculations thus far have been done using excel spreadsheets, yet I can't figure out the appropriate equations to work out bump steer. I've searched the net, but haven' found equations.
I guess mines is a maths problem - can anyone help me with this?

Welcome to the site

The attached file is an excel worksheet to calculate bumpsteer. I think it came from:

www.locost7.info

Cheers,
David

Thanks for that!!

I've been playing around with it for a while, yet i think it might be wrong. I'm not sure about equations involved. If you look here you'll see that equations can be quite complex,

www.me.udel.edu/meeg467/Vehicles/SLACamberBump.doc

After reading so much about how complex bump steering calculations are, it's surprising to find such a simple excel program. Don't you have to take into account Ackerman?

Your main problem with theoretical bumpsteer issues is the very practical one of deciding exactly where the centre of a mass produced ball joint is! For this reason it makes more sense to build the front susp with approximate values & move the rack (shims) to get it spot on. To get it right over full suspension travel involves slicing about 3.5" off the middle of an escort rack on a book front end. The +4 wide plans work pretty well using the stock escort rack.
cheers
Bob C

What affect would ackerman have on bump steer?

You will never dial out bump steer completely in all situations even with an ideal width rack, caster, ackerman, kpi (even camber) have an effect.

quote:

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Originally posted by MikeP
What affect would ackerman have on bump steer?

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From what I can tell, the first reference shows the formula for calculating the amount of bump steer in a given design.

The spreadsheet I believe solves for rack length using the assumption that the inner and outer rack pivots should fall on the lines between the top and bottom suspension pivot points.

Neither one (unless I'm mistaken, which is very possible) looks at what happens when the steering wheel is turned and the rack pivots are offset from ideal. Like BT says, add in ackerman, caster, etc and there's some serious non ideal toe changes going on in a corner.

It was my understanding from the way we've talked about bump steer here, that it's for maximizing straight ahead stability. I didn't think ackerman, caster, etc was an issue.

Yeah I've only considered the straight on case. Hitting bumps in corners will alter your line for reasons other than bump steer!!!!
cheers
Bob

Normally under normal straight line running bump steer isn't a major problem unless it is really bad ---- but lets face it the book Locost has a quite lot of bump steer.

At the end of a straight the brakes are applied for a corner bump steer will cause the car to feel imprecise -- not so noticeable if The brakes are applied firmly and the weight transfer quickly settles the load on the front.
Around the point where the driver comes off the brakes and turns into a corner a car with a lot of bump steer will waver and not feel sure footed, difficult to precisely position on the track. When settled into corner the effects tend to be swamped by other cornering forces.

[Edited on 5/4/05 by britishtrident]

Then I'm a bit confused about it . Reading Smith and the like, I understood there were 3 different situations. Most of the focus seemed to be on straight line running (despite calling it bump *steer*, which was confusing too).

1) Front straight line (and corner approach and exit). Single wheel bumps will make the car darty, especially if there's toe-out under bump. Toe change under braking (bump) can stabilize (toe-in) or destablize (toe-out). With acceleration (droop, less weight) on corner exit toe-out could destablize.

2) Rear straight or cornering - toe-out on acceleration (bump - rear squat) is destablizing on corner exit, very bad, neutral is best.

3) Front cornering - like Smith says about ackerman - you steer the outside tire that has most of the weight & traction, the inside doesn't matter as much. As soon as the steering rack moves, you'll have some serious bump steer - the driver adjusts intuitively to the bumps.

From drawings and such, I've found a few things: Small changes in rack pivot locations make for big changes in bump steer - made me think it must be for straight line only. Roll (with wheels straight) has almost no effect no matter how bad the "static" bump steer. Bump steer with turned wheels (including caster & KPI, etc) is too hard for me to draw or calculate .

I don't suppose anyone here as some drawings or analysis of bump steer during cornering they'd like to share (he asked hopefully)?

Keith Tanner (on TOL-NA) is planning to try some different settings on the track for an article, I'm looking foward to that.

Re-reading BT, I may not be so far off after all. I think all that's different is what you've said about corner entry.

I really need to draw this stuff out to understand what happens with the wheel turned and the car starting to roll. And maybe even try it for myself in real life...

Normally under normal straight line running bump steer isn't a major problem unless it is really bad ---- but lets face it the book Locost has a quite lot of bump steer.

fairy nuff, but also to be fair the effect is toe in on bump and droop (if you choose to locate the rack thus) which should be the less unpleasant of the possibilities!
Actually I don't recall the book being at all specific about rack location, I do remember a sentence along the lines of "don't worry about bump steer - they all do it" - stretching the truth (a lot) in my opinion!
cheers
Bob