PeterGT4
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| posted on 6/6/04 at 08:37 PM |
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The book's steering rack... Correct size?
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...
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Berwyn
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| posted on 6/6/04 at 09:52 PM |
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Check Dave's comments in here:
http://locostbuilders.co.uk/viewthread.php?tid=3285
Very iseful info
24 hours in a day, 24 beers in a case. Coincidence? I think not
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PeterGT4
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| posted on 6/6/04 at 10:14 PM |
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quote:
Originally posted by Berwyn
Check Dave's comments in here:
http://locostbuilders.co.uk/viewthread.php?tid=3285
Very iseful info
Yes. Although, it seems like its a hit-or-miss and can only be determined with the parts in hand... Fair enough.
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MikeP
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| posted on 7/6/04 at 03:19 AM |
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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.
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PeterGT4
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| posted on 7/6/04 at 03:53 AM |
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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.
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Bob C
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| posted on 7/6/04 at 09:18 AM |
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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
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craig1410
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| posted on 7/6/04 at 12:02 PM |
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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.
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MikeP
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| posted on 7/6/04 at 12:24 PM |
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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?
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PeterGT4
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| posted on 8/6/04 at 05:15 AM |
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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).
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PeterGT4
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| posted on 8/6/04 at 05:18 AM |
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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!
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Bob C
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| posted on 8/6/04 at 11:28 AM |
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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
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craig1410
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| posted on 8/6/04 at 11:39 AM |
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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.
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Bob C
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| posted on 8/6/04 at 11:39 AM |
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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
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MikeP
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| posted on 8/6/04 at 02:29 PM |
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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.
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craig1410
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| posted on 9/6/04 at 09:16 AM |
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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.
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MikeP
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| posted on 9/6/04 at 12:47 PM |
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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 !
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craig1410
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| posted on 9/6/04 at 03:52 PM |
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quote:
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.
Mike,
I agree totally with most of what you have said but the bit I've picked out above seems back to front to me... Typical eh?
My logic goes like this:
Having toe-in(static for now) will generally give the front end better turn-in grip because as soon as the weight starts to transfer to the
loaded(outside) tyre, it will already have attained a greater turning angle and thus will give more bite. Correct?
If so then surely having extra toe-in under bump conditions, bearing in mind that the loaded (outside) wheel will be more heavily loaded and
thus under compression, will enhance the turn-in grip even more! Just like having a couple of degrees of extra steering lock on. Even better is the
fact that the inside wheel will be in droop with respect to static ride height and will toe itself out. This will give an effect like enhanced
ackerman and help the turn in even more. Obviously most grip comes from the outside tyre so this effect will be less significant.
So I would conclude that if you can't get rid of bump steer completely then toe-in bump steer is probably better for the track as it will make
the car a bit more "loose" provided this isn't taken to extreme's as mentioned earlier.
For the road, probably the reverse is true with toe-out bump being preferrable but I'm not sure that is a good thing on a sports car to be
honest...kinda misses the point
At the end of the day I think that we should all try to get rid of bump steer as much as possible and control over and under steer using more
traditional methods such as springs, shocks and other geometrical methods.
Cheers,
Craig.
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MikeP
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| posted on 9/6/04 at 05:09 PM |
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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
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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
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craig1410
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| posted on 9/6/04 at 08:23 PM |
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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.
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Mark Allanson
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| posted on 9/6/04 at 08:43 PM |
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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.
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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MikeP
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| posted on 10/6/04 at 12:26 AM |
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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 .
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PeterGT4
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| posted on 10/6/04 at 04:54 AM |
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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...)
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pbura
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| posted on 10/6/04 at 12:58 PM |
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quote:
Originally posted by MikeP
But this is a whole different subject, and I refuse to discuss it without Pete Bura in the room .
Too funny!  To make this a certified nerdy discussion, I suppose 
Toe-out under bump makes sense for autocrossing, because under heavy braking the vertical loads on the front tires will be more equal, and the inner
tire will tolerate less slip angle than in a steady-state turn. So in a tight turn under braking, the bump steer provides an Ackermann effect.
Carroll Smith recommends having bump steer shims that can be used to tune this characteristic for different tracks. Sounds like a good idea to me, as
would be adjustable Ackermann as is available in some (custom) uprights.
Tuning slip angles vs. load is some very serious geekery, but I'm sure it's what the Big Boys do. They're armed with some very good
tire data, of course.
An autocrosser could have some fun fooling around with this.
Pete
Pete
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