Obviously to eliminate bumpsteer, does the centres of the inner tie-rod joints of the book's choice of Escort rack lie directly in the line that bisects the upper and lower suspension arm mounts?

I've been reading up in a few books dealing with kit-car suspension and steering and its mentioned that the rack can be moved out of line (higher-lower from horizontal, and/or further forward or back) to achieve that proper line, but that it shouldn't be at an extreme. Has anybody paid attention to that and know where the ideal location for the rack would be?

Thanks...

Check Dave's comments in here:
http://locostbuilders.co.uk/viewthread.php?tid=3285

Very iseful info

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Originally posted by Berwyn
Check Dave's comments in here:
http://locostbuilders.co.uk/viewthread.php?tid=3285

Very iseful info

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If you can let me know the distance between the inner pivots on the rack and the height and distance from the center line of the tie rod end center on the steering arm, I can give you a suggestion on what height you should set the rack to start. Then you can use something like Dave's method to tweak it.

I figure out the location using the method I outline at http://www.7builder.com/SuspensionGeometry/BumpSteerDiscussion.html

I don't know if it was mentioned in the other thread, but you can remove the spring and move the suspension through it's range with a jack and mark the location of the laser on a card set on the wall - might be easier than bouncing it up and down.

Good stuff Mike, I'm bookmarking your site for future reference.

As it is, I can't provide you measurements as I don't have the parts (I'm trying to do as much of the homework before starting the project!), but when I do, I'll get back to your site and get in touch.

You can locally eliminate bump steer with an escort rack, but I reckon it's about 10 to 15cm too long (distance between track rod inners) for a book chassis to dial it out properly. I'm aiming for the softest suspension in the locost world (!!!) so I'm going to shorten my rack. I'll be thereabouts in a month or so - be interesting to see how it all pans out. (don't worry - the backup plan to retrofit hard springs is cheap & easy if it all goes pear shaped!)
cheers
Bob

Yes I agree with Bob C,
I have built a McSorley +4" wider chassis and the standard escort Mk 2 rack is almost perfect for my purpose. Therefore it must be about 4" (100mm) too wide for a book chassis.

Be careful if you mess with the height of the rack as you can introduce some scary handling characteristics if you go too far...

Cheers,
Craig.

Hi Craig, I hadn't heard that before about topo much height - other than if it introduces too much bump steer, which would miss the point .

What kind of nasty handling does too high of a rack cause?

So if the book's choice of rack is ideally too long inbetween inner rod bearing centres and without having to go through the effort of shortening it, is there some online resource/supplier of steering racks that provide dimensions of possible racks that might fit?...

Mike & Craig, I suppose if its too high, it could cause severe toe-in, plus it could also reduce lock (not necessarily at the steering wheel, but at the wheels due to the extreme angle of the tie-rods).

Ooops, if its too high, and the suspension goes into full bump, because of the steep angle of the tie-rod, it would then flatten out, effectively lengthen and cause severe toe-out... That's the scary part!

pete - I asked the same question about a month ago. Answer seems to be no. It's like wheel weights - we all need to know but nobody's telling...
cheers
Bob

Yes that's what I was meaning!
It is widely regarded as good practice to ensure that the track rods remain as close to parallel to the lower wishbones as possible throughout normal suspension movement.

(Note that when I say parallel to the wishbones I mean parallel to the plane intersecting the lower balljoint and lower suspension pivots which is usually slightly different to the wishbone itself.)

I suppose that bump toe-IN (rack too low) would be even worse as it would make the car very twitchy indeed on bumpy corners but ideally you don't really want ANY significant tow change on bump or droop.

It's not a major problem to have a rack shortened so don't discount the idea without getting some more information on the subject.

Cheers,
Craig.

When I have a go at rack shortening I'll post some photies for general interest. Then if I die you'll know what NOT to do.....
Bob C

Hehehe, so now we have advice to avoid both toe in and toe out .

Don't forget that the upright is moving in and out as the control arms move too, so it's often tricky to guess whether it's more or less that the effective length change of the steering tie rods in bump or droop.

Mike,
Toe-in and toe-out as fine and most cars have one or the either to a certain extent as I'm sure you are aware.

What you don't want is large changes to the toe due to bump or droop of the suspension as this will have an effect on the steering of the front wheels. This could be the result if you try to "lose" the extra 4 inches of rack width simply by moving the rack higher or lower. I agree that the uprights will tend to move inwards at the top during bump and thus it is perhaps desirable to have a small amount of effective shortening of the track rods to match this. This would require the rack to be fitted slightly lower than normal to achieve this I think.

Anyway, I still think that either shortening the rack or widening the chassis as I have done is the "correct" solution although I wouldn't recommend widening the chassis JUST to avoid shortening the rack...

Cheers,
Craig.

Thanks Craig, I know - I think I've got a pretty good understanding of bump steer and what causes it. If you read back above on what to do if bump steer can't be eliminated, Peter says to avoid toe-out on bump, you say to avoid toe-in. I laughed because that's pretty typical, everyone has a different opinion on the right way to approach it.

Think about it like this - the upper arms, lower arms and tie rods all describe separate size circles. Perfect bump steer would be if the circles all lined up, but that can't happen. The trick is to find a location that has them work relative to each other over the range of suspension travel. Interesting thing is that none of the rules of thumb I've heard actually give the best rack location, though one is usually better.

Some thumb rules I know to be clearly wrong for minimizing bump steer - like the advice to use a horizontal tie rod. Have a look at my web page to see the geometry behind why. What I don't know is whether the bump steer that results is intentional on the part of the designers - may cars appear to be built that way (toe-in on bump BTW).

From the handling books I've read, at the rear toe out under droop and especially under bump (squat) should be avoided, as it makes the rear unstable under power.

At the front the experts seem to argue, like us here, for both sides. I think the fronts should toe-in on bump for the street (stability under braking/dive) and toe-out in bump on the track (better turn in), if bump steer is unavoidable.

I agree, don't take drastic action on the chassis or the rack to avoid bump steer at least until you either draw it out or try the rack at different heights in the chassis. And don't think that the theoretical locations are the best - start with them and move things around till the steer is minimized and any remaining is in the direction you think is best.

Good luck all !

quote:

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Originally posted by MikeP
I think the fronts should toe-in on bump for the street (stability under braking/dive) and toe-out in bump on the track (better turn in), if bump steer is unavoidable.

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LOL, no problem Craig, what fun is it if you don't pick out my back to front stuff ?

My understanding and experience for toe-in vs toe-out came from a description very much like the one I found here (no access to my books right now): http://www.ozebiz.com.au/racetech/theory/align.html.

With toe-in the car feels to me to be very stable at speed, and I would think more so with more toe-in from bump steer/dive under braking. This is with the steering wheel straight ahead (this is important, read on). Better for the street IMO.

With toe-out the car seems to wander at speed. Under braking approaching a corner it almost turns itself with virtually no steering input - again, I picture even more so with toe-out from bump steer/dive. Better for the track IMO.

As soon as the steering wheel is turned, the inner rack pivots have moved relative to the inner control arm pivots. So all of our talk about controlling bump steer with rack length and height have been blown out of the water - the straight ahead measurements we've taken don't work. I want to play with the bump geometry of an offset rack and a vehicle under roll some day, but probably not till it gets cold outside again .

We're in complete agreement on reducing bump steer - might as well do it now while building, it's much harder after... And absolutely, balance the car with shocks and springs not bump steer geometry.

I'm still trying to sort out my thoughts on ackerman, toe-out, etc in the turn. It's pretty obvious at slower speeds, but I'm not so sure at the limit of traction - slip angles and weight transfer complicate things more than I can handle. Smith didn't think ackerman mattered at all on the track for the same reason you mention - it's the outside tire doing most of the work. Des Hammill agrees with you about the toe-out on droop. But this is a whole different subject, and I refuse to discuss it without Pete Bura in the room .

Mike

Mike,
Interesting stuff but I think I'm going to have to read it a few times before I can either accept or reject it to be honest... You know how it is when someone asks you to take your current understanding and turn it upside down!!

One thing that did occur to me is whether the scrub radius of the car has a bearing on the effects of toe on stability. I think(?) that race cars tend to have zero scrub or perhaps even negative scrub whereas road cars often seem to have positive scrub. This will, as far as I can see, reverse the effects of a transient disturbance. Do you see what I'm getting at?

Anyway, right now I'm off out to the garage to do some car building!

Cheers,
Craig.

I am not sure about all the science, but setting toe in and toe out statically is only to ensure zero toe when in motion.

Usually front wheel drive cars need toe out, because the torque draws the wheels in, and drag in rear wheel cars does the opposite so you set toe in statically.

Yep, we know Mark - for the road that is, it's different on the track. We're talking about what to do with bump steer if you can't get rid of it, primarily under hard braking where both front wheels are in bump.

Hi Craig, yeah, I see what you're getting at. Sounds reasonable, but I dunno much about what it does. It's cheap and easy to play with toe, some of this other stuff is real money and work .

I hate to stir things up, but after reading all of this, I'm reminded also of Ackermann angles on steering geometry and I suppose it'll have its affect in respect to this too, to whatever rack and/or location is chosen.

Maybe the Ackermann angle could cancel out a dramatic affect due to the size and position of this rack?

The Cortina uprights, do the steering links point towards the disc, or towards the car? The pictures in the Champion books don't really give a good view of this. Or, are they really well-suited to this long rack and it shouldn't be of a concern?

(sorry for all of these questions...)

quote:

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Originally posted by MikeP
But this is a whole different subject, and I refuse to discuss it without Pete Bura in the room .

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