sgraber
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| posted on 7/2/07 at 02:35 PM |
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quote:
Originally posted by kb58
It sounds like the pads and brake disks haven't be properly bedded in. Were they?
About the oversteer, it sounds like rear toe-out in bump (bumpsteer). Check using a string and floor jack. While you're at it, check to front
too.
hmmm. quite honestly I don't recall. The chassis portion of the build was completed so long ago (3 years) that I don't remember if the
procedure was followed. If it helps, I have had the pads off, and they don't appear glazed? Can one tell by looking?
edit: deDion rear, 0 static toe, 1/2 degree camber.
[Edited on 2/7/07 by sgraber]
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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kb58
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| posted on 7/2/07 at 02:43 PM |
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Forgot it was de dion. Depending where the trailing links attach, it is still possible to have toe-out for a one-wheel bump... Check with a string.
Is wheel rate approximately 1/2 the sprung weight at all four corners?
[Edited on 2/7/07 by kb58]
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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Ivan
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| posted on 7/2/07 at 03:18 PM |
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An obvious question is whether the trailing arms at the back are parellel with the road at normal ride height - if not they will cause rear over or
under steering on roll depending on which way they are pointing.
Another quick and ugly check of the brakes is to check the disc temperature (carefully  if you are using a wet finger and not a thermometer) after
braking hard a few times to see if they are equally hot.
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tadltd
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| posted on 7/2/07 at 10:32 PM |
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Congrats Steve - I know how much of an effort it is to get to that stage AND how much effort is still required to get half-decent performance! 
Stick at it though, as you cannot beat the feeling of achievement when you start to get the results you want!
Best Regards,
Steve.
www.turnerautosport.com
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Doug68
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| posted on 8/2/07 at 03:50 AM |
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sgraber, whilst searching posts I came across the fact you bought a copy of susprog3d but were yet to use it?
If thats still the case this might be a good time to break it out to find out what your suspension is doing.
The spring rate calcs if nothing would make it worthwhile.
U2U me if you need any help getting the thing going.
Gratuitous screen grab attached:
 
Rescued attachment Susprog3D.jpg
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JC
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| posted on 8/2/07 at 07:47 PM |
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Hi there.
First of all, congratulations on what you have achieved so far! An amazing job - don't be too downbeat!
As far as I remember, when Sylva first launched the Mojo, it had a De Dion rear end and suffered from excessive understeer caused by rear end roll. At
first they tried stiffer springs but ended up back on the soft springs but with an anti-roll bar at the back. This I believe worked, although
developement was halted when race-series rules allowed them to use wishbones (I think). I realise that this is probably like comparing apples and
bananas but hope it gives encouragement! (I expect reciprocation in several years when my project finally gets on the road!!!!!!)
JC
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sgraber
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| posted on 8/2/07 at 08:13 PM |
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Seems like a lot of those gone before me have been in the same place. That's nice to know. I agree that at least I have someplace to start from
because I have actually built something and started testing it. Sometimes it's better to just get something built to have a place to start the
inevitable changes from. Plus, as was said, if I can get some positive changes made, it will feel like I've done something right! 
Doug, you have u2u.
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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Ratman
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| posted on 9/2/07 at 11:27 AM |
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Hi Steve. Congrats. Are you sure you really do have an oversteer problem? A taught midi is likely to feel that way to a driver who has just stepped
out of a regular car. Did it actually spin on corners when pushed? Maybe it doesn't need much changed at all. Just needs a bit of getting used
to. Did it feel stable at speed in a straight line? Can you turn in under hard braking without it spinning? These are the real tests. The fun is just
begining.
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suparuss
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| posted on 9/2/07 at 04:32 PM |
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i agree with you there steve, i think its the only way really. i already want to change my rear setup and havent even finished the body work yet. can
you still pick up the front end by the way? i can still get the front wheels an inch off the floor and im not exactly worlds strongest man!
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scuzzer23
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| posted on 9/2/07 at 09:47 PM |
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Steve:
Let me start off by saying congratulations on such an important milestone! It's great to see just how far you've come!
I recently had an encounter with oversteer. I drive a Toyota MR2 Spyder (which has roughly the same weight distribution as La Bala). It was raining
and I was accelerating in a turn and the back end lost control until the speed dropped. Apparently staggering the tires by 20mm isn't enough to
keep oversteer in check. I was in an empty parking lot, so luckily nothing happened. I've since upgraded to a 3-setting adjustable solid
1" ARB up front and kept the stock rear ARB (3/4" me thinks). It now corners flatter and has understeer without power, and slight
oversteer with power (140HP engine). I have never experienced oversteer in dry conditions. Overall I'd say it handles much better/safer. Also I
read somewhere than a toe-in condition at the rear will tend to understeer/stabilize the car. I imagine you could adjust the toe on a de dion susp
with application of generous amounts of heat!
Advice from people more knowledgeable than I say it may take you several tries to get the spring/shock settings right. Of course you can try the
progressive spring avenue but I'm sure that can have a snag too, as you still have to get the rates right.
As far as the scrub radius goes, as long as you keep the same offset and outside tire diameter, it doesn't matter what the width of the
tire/wheel is. The scrub radius will not change. However if the offset is different you will have to change the KPI to reach the desired scrub radius.
If the tire radius is different, so will be your scrub radius. Here are some helpful links:
http://www.miata.net/garage/tirecalc.html
http://marksink.com/tire_wheel_offset/tires.html
http://www.1010tires.com/WheelOffsetCalculator.asp
All of this IMHO, of course.
Fred W B:
Where can I get a copy of those spreadsheets? Can you send them to me?
- Justin Peterson
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CGILL
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| posted on 13/2/07 at 07:09 PM |
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Re: Bump steer
Hi Steve, looking at your rear end setup, with the long trailing arms you have, there shouldn't be too much change in toe with the dedion. Did
it feel like the outside front corner was dropping before it swung, or did it feel like the inside rear lifted for the swing? Usually you can dial it
out with softer springs on the rear, or stiffer springs on the front but choice between the two will depend on what it 'feels' like. Good
luck!! It looks great.
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locost_bryan
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| posted on 14/2/07 at 03:13 AM |
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Do you have a tyre pyrometer? Measuring the temps across the tyres should give an indication whether the rears are being worked evenly - could
indicate an unexpected camber change or some such.
Check out Keith Tanner's site www.cheapsportscar.com - he's documented all the things he did sorting the handling and brakes on his
Miata-based 7.
Do you have access to a brake dyno? Our vehicle testing stations have drive on brake dynos that measure the actual brake performance - that could
give you an indication whether the problem is at all 4 wheels, or just the front or rear.
Are MCs bottoming at full pedal travel - I had the clevis pin fall out of a Mini brake pedal and it wouldn't quick push the rod all the way into
the MC. Gave about 75% braking - very sweaty palms! And I had a professional brake shop connect a dual-circuit MC back to front, so all the effort
went on the rears - every brake application led to a 270* spin
Mightily impressed by what you've achieved - very inspirational. 
Bryan Miller
Auckland NZ
Bruce McLaren - "Where's my F1 car?"
John Cooper - "In that rack of tubes, son"
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sgraber
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| posted on 14/2/07 at 05:07 PM |
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Based on all the comments received here and by local sources, here is my list of fixes before the next track event.
Steering:
-Reduce KPI from 16d to 12d by moving the upper bj mount outward. re-adjust length of upper a-arm to achieve 2 or 3d camber.
-Install lock stops on steering rack to prevent overtravel condition.
-Increase caster to 7d.
Braking:
Step 1 - line sand pad faces to eliminate glazing as a possible cause.
Step 2 - increase rotor size
Step 3 - replace caliper with larger unit from Supra
Oversteer:
-Replace undersized brass bushings in rockers with sealed bearing units to eliminate binding.
-Change to softer springs.
-Adjust damping/rebound settings on shock dyno to better match car.
-Corner weight the car correctly with driver weight included.
Anything I am missing?
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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DIY Si
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| posted on 14/2/07 at 09:18 PM |
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That should do it. You don't want to do too much at once in case you mess something else up. If possible, it's best to change only a
few/couple of things at a time, to see what actually fixes the problem/was causing it in the first place. A lot of it may be suck it and see with a
new car like yours, so you don't want to fix one thing, just to go cause another.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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sgraber
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| posted on 14/2/07 at 09:35 PM |
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For at least the braking and oversteer categories I am going to do one change at a time. For the steering category I am certain that I need a decrease
in KPI and an increase in Caster, and the rack is taking the steering arms over center so I have to limit it.
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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DIY Si
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| posted on 14/2/07 at 09:52 PM |
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Fair enough then! If changing one things alters an other, then it is best to try and out it back where it was to start with.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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