Nitrogeno25
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| posted on 16/8/05 at 09:33 PM |
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Suspention and steering, I'm afraid
Well I finished my chassis with IRS. I used a suspension software to calculate the roll center and to optimize the top wishbone for near 0 roll center
offset in roll and good camber gain.
I also use the software to calculate the length and height of the rack to get almost 0 bump steer.
That said, I see that very very small variations in the chassis and small variations in rack height make big differences.
I measure my chassis and I get a 2mm error (diagonal). then, my rack is about 2mm higher than it should be.
According to the software this would make near 2.5mm toe in in bump.
Now, maybe I'm getting a little obsessed about this as it's imposible to get the exact dimensions (as specified in the software), or
should I continue caring about this???   
About bump steer, how good is to have 1-2mm toe in in drop so as to get some Ackerman effect?
Thanks a ton!
[Edited on 16/8/05 by Nitrogeno25]
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Russ-Turner
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| posted on 16/8/05 at 09:39 PM |
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I can't wait for this part of the build..... about as much as the wiring!
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andylancaster3000
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| posted on 16/8/05 at 09:57 PM |
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What we will do, as most do I think, is not worry too much during the chassis build stage. When you weld the chassis you can assume that there will
quite large amounts of heat distortion from welding and there is not a lot you can do about it. But once the chassis is fully welded you can try to
dial these out by building an accurate jig for the suspension bracket points.
If it is a book chassis you are building, we found with some quick drawings on autocad that the standard geometry is suprisingly good. Th roll centre
doesn't move around too much at all.
HTH,
Andy
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pbura
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| posted on 16/8/05 at 11:35 PM |
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It would be better to have a little toe out in bump. Hmmm, got a large hammer?
You can either ignore it (it's not so bad) or re-do the rack mounts. If you do, leave some room so you can use shims and experiment a
little.
2mm difference on the diagonal measurements is outstanding! You should be proud of yourself.
Pete
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Nitrogeno25
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| posted on 17/8/05 at 02:06 AM |
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quote:
Originally posted by pbura
It would be better to have a little toe out in bump. Hmmm, got a large hammer?
You can either ignore it (it's not so bad) or re-do the rack mounts. If you do, leave some room so you can use shims and experiment a
little.
2mm difference on the diagonal measurements is outstanding! You should be proud of yourself.
You really put a smile on my face 
About the rack, what i see with the software is that 2mm up or down makes the toe from almost 0 to 2mm toe-in or toe.
Thanks all!
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Nitrogeno25
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| posted on 17/8/05 at 02:11 AM |
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quote:
Originally posted by andylancaster3000
What we will do, as most do I think, is not worry too much during the chassis build stage. When you weld the chassis you can assume that there will
quite large amounts of heat distortion from welding and there is not a lot you can do about it. But once the chassis is fully welded you can try to
dial these out by building an accurate jig for the suspension bracket points.
If it is a book chassis you are building, we found with some quick drawings on autocad that the standard geometry is suprisingly good. Th roll centre
doesn't move around too much at all.
HTH,
Andy
What I'm doing is use a jig at the front and other at the rear and try to align all the brackets.
I'm a little surprised to read Des Hammill "How to build & modify sportscar & kitcar" where he state that 1.5mm is the max
error it should be in the brackets, but first he state that a garage floor is good to build the chassis... I have measure more than one garages and
all of them have from 2 to 10mm difference...
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Cita
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| posted on 17/8/05 at 06:19 AM |
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quote:
Originally posted by Nitrogeno25
quote:
I'm a little surprised to read Des Hammill "How to build & modify sportscar & kitcar" where he state that 1.5mm is the max
error it should be in the brackets, but first he state that a garage floor is good to build the chassis... I have measure more than one garages and
all of them have from 2 to 10mm difference...
That sure is a contradiction of terms!
I have a polished concrete floor in my shop and aldo I have not measured it yet I'm sure there will be more than 2 mm difference in heigth.
I would not worry about 2mm.If you take in account how many yards of steel tube that are in the chassis than it's impossible to keep to mm
standards even on a hot summersday(that's for the Aussy's amongst us!) the chassis will "grow" and deform slightly.
As been mentioned in earlier threads,measure your daily factory made driver and you'll be surprised to find out that almost nothing is
correct.
The famous SR-1 spyplane leaked like a sift after a flight due to heat distortion and it took several hours of cooling down to get everything back in
place.
Not that you will be driving at those speeds but just to let you know that 2 mm is possibly the difference between a cool and a hot tire.
Job well done I would say mate!
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britishtrident
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| posted on 17/8/05 at 08:29 AM |
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Almost impossible to get bump steer right on the drawing board, caster angle throws the situation out as soon as the steering wheel is turned. My take
on rack position is to design it slightly low for two reasons firstly it is easier to adjust the rack height up rather than down and the second being
I regard effects toe-in on bump as being easier for the driver to counter.
Unless you rose jointed outer track rod ends the only way to partially correct for the rack being to high is hot(!) bend the steering arms upwards
-- see the Fred Phun book "How to make your car Handle" on this subject --- it needs a lot of heat in the right place.
Also did you allow for any ackerman offset built into the steering arms when calculating the rack width ?
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VinceGledhill
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| posted on 17/8/05 at 08:32 AM |
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Worry not. Take a chill pill. Adjust it as best you can when you get the steering rack fittted.
Stay off the computer....
Note to self... stay off the computer... get in the garage.
Regards
Vince Gledhill
Time Served Auto Electrician
Lucas Leeds 1979-1983
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Nitrogeno25
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| posted on 17/8/05 at 01:17 PM |
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quote:
Originally posted by VinceGledhill
Worry not. Take a chill pill. Adjust it as best you can when you get the steering rack fittted.
Stay off the computer....
Note to self... stay off the computer... get in the garage.
Maybe I should do that 
As for ackerman my pretty software  calculates the best place for the rack and then you can adjust (for real world fit) and see how bad it gets.
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pbura
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| posted on 17/8/05 at 02:33 PM |
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Earlier on I suggested toe-out in bump, which is what a lot of people do to provide a sort of fake Ackerman effect. On further reflection, I think
this needs to be reasoned through a little better.
In a nose-down situation (braking into a slow turn), what's needed? More Ackerman or less? I think the answer depends on how much Ackerman
there is to begin with, which in a production car would be 'not enough' if the system were designed for parking lot maneuvers. The
outside tire needs to be at a higher slip angle than the inside.
On the other hand, if a car were designed with the correct Ackerman for steady-state curves, a bit less Ackerman under braking might be needed. My
reasoning is that with more load on both front tires because of braking, the disparity in slip angles between the two tires would be smaller.
Bottom line: Toe-in in bump is good if steering will provide sufficient Ackerman effect. For straight-ahead braking, toe-in is better for
stability.
[Edit] I am not certain about this synopsis, just throwing it out for discussion.
Sorry for the nerdy post, Vince! I promise to build a car eventually! 
[Edited on 17/8/05 by pbura]
Pete
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NS Dev
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| posted on 17/8/05 at 02:57 PM |
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haven't read all this post, but for my contribution, toe change in pitch is the tip of the iceberg.
The big issue is toe change in roll (and in combination of pitch and roll) as this will affect the car during cornering, where pitch should be
reasonably constant.
How you achieve this is another thing altogether!!
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