Roll centers, weight distribution, caster...

Roll centers, weight distribution, caster...

Postby DennisRacing3 » Mon Jul 16, 2012 9:56 pm

Alright. My car has handled funky from the get go and unfortunately everything that I read to fix it has two sides with valid points. I honestly do not know what to do. I would not like to rebuild the car like a fool. Which with us garage builder guys is quite easy. I know for sure that I am going to cut the rear perches off and move them down about 3/4" to raise the rear of the car. Now this will make the rear roll center higher than the front which in most cases of my reading is good. But, my car is not a regular daily driven street car done over and over again. Being said, the car has awesome turn in(minus pebbles and balding tires) and really poor from then on out until straight line. And this is what a rear higher roll center is suppose to do. But is it too much? When getting into higher speeds the car becomes completely unstable, to me it feels like the front is lifting off the ground and it becomes a boat. At Okemo I had to lift through the straight at the kink because the car felt like it was going to slide off the road. I should be able to keep my foot planted there. Even 3/4 throttle I did slide way right and was panicking. Is this resulting in the rear roll center being too high because it is mid engine and creating immediate weight transfer with more rear weight and lifting the front tires off the ground? I mean there are probably so many different variables, weight, aero, spring rates, shocks, caster? While reading I had also found that some positive caster is good for low and high speed stability. Right now Im fairly sure set the front and rear about as zero I could make it in U Daves garage. I just feel like I am trying far too hard to drive my car fast. Some input? And I would like to race the car comfortably and im sure the hillclimb family has the little quirks specific to the hills. After I fix what is bent on the car and rebuild suspension components I think Sherms house is the place to go. Especially since Kevin said his car was easy to drive @ Okemo. That means Sherm must know what he is doing. :geek:
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Re: Roll centers, weight distribution, caster...

Postby sachilles » Mon Jul 16, 2012 10:35 pm

As custom as that car is, are you keeping track of changes? Have you had it on an alignment rack and corner scales to get a baseline? If so what do you have for numbers?
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Re: Roll centers, weight distribution, caster...

Postby sdwarf36 » Tue Jul 17, 2012 6:27 am

Well hurry up and fix it. Lets get it straight sooner than the Thurs before the next Okemo. ;) Plan on a long Sat. on the scale pads with alot of stringing + tape measure work. Any chance of putting some coil-overs on it?
One thing to think about-espcially with a light car-your tires are a big part of your suspension. I had zero luck trying normal car street tires on my car-the sidewalls are just way too stiff. I only run 10 psi air in my slicks. (1200 lb car). And with my 43/57% weight dist, I have 200lb ft-140rear springs.
Translating road racing to hillclimbing:
Proper tire selection== nothing hooks up on moss or wet leaves.
Staying on the racing line==anything paved is considered good.
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Re: Roll centers, weight distribution, caster...

Postby Chief Geek » Tue Jul 17, 2012 7:29 am

Josh

This is one of the things I love about NEHA. You need suspension that actually moves, none of this "crank it down until it's dead rigid stuff" that works on smooth roads. Since none of the NEHA cars has much aero, there is no reason for the suspension to be hard. Simon, a guy I used to work with taught me a few of his ride/handling principals:

* You know the suspension engineer has thrown in the towel when the springs are stiff.
* Make the tires work, everything else is BS.
* The ratio of front/rear spring rates, roll center movements, etc is more important than the actual values.
* Unless Johnny Herbert is in the car, making it easy to drive is faster than raising the limits. (Johnny Herbert was Lotus's best driver when I worked with Simon. Herbert's rookie teammate, Mika Hakkinen turned out OK too.)

You described the turn-in response as good but the handling is otherwise weird. Do you mean that it's unpredictable from run to run, or that, while it responds differently than "normal" cars, it is consistent to itself?

If the car is reasonaby consistent, fast turn in sounds to me like the front is nice & soft & grippy when the car is square to the road, but as soon as you get some roll the grip goes away. Could it be that the front jounce/rebound stiffness is OK but the front roll stiffness is way too high? I don't remember if you have a front anti-roll bar on the car. Is it one from a front heavy car with a much higher center of mass that is twice the weight of yours? If you're car needs a front bar at all, it won't be big. I'll bet that, once you're happy with the car's balance, you either won't have any anti-roll bars or just a 3/8" dia front one.

If the car isn't very consistent, Simon always looked at the chassis and corner stiffnesses first. He had little patience with floppy chassis. Have you ever done a roll stiffness test on your cars chassis? It looks like it's stout enough for it's weight, but it's an easy test to do if you have a scale, some big weights, a jack, and 4 water levels.

Have you found tires that you want to use, long term?

Paul
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Re: Roll centers, weight distribution, caster...

Postby KevinGale » Tue Jul 17, 2012 11:08 am

John Reed knows far more about this than me. I try to get the cross weight percentages near 50%. I'm running 125 lbs springs in the rear and 300 in the front. Of course those are raw numbers you have to take into account mounting points and angles to figure out the actual wheel rates.

The biggest problem I had with the modfied was too much rear spring. This would transfer far to much weight to the rear tires during cornering.

Here are the current numbers off my car and I've been really happy with the way it is handling. This is from a speadsheet I can send you if you give me your email address. The rear motion rate calcs are based upon a live axle so you would have to change those.

The weight is with me in the car (or my weight equivalent) and a normal fuel load. The car is about 2250 without me in it. Looking at these made me realize I was telling people the wrong percentages. I know I have told a few people lately that my car is 55% rear but actually it's only 53.4% rear and 54% left. Either I'm just getting forgetful or I'm remembering the numbers before I moved some stuff around on the car.

Car Setup Stuff


Weights

LF 616 ---- 520 RF

LR 700 ---- 600 RR


RF+LR 1220 50.1%
LF+RR 1216 49.9%
Front 1136 46.6%
Rear 1300 53.4%
Left 1316 54.0%
Right 1120 46.0%

Total 2436


Caster Camber Toe

RF Caster 4
RF Camber -0.5
LF Caster 4
LF Camber -0.5
Toe -0.125

Front Suspension
11 Lower control pivot to shock mount (Inches)
13 Lower control arm length (pivot to center of ball joint) (Inches)
59 Ball Join to Instant center (Inches)
70 Center of tire to instant center (Inches)
7.5 Upper control arm length (Inches)
19 Angle of Shocks (Degrees)
0.677 Motion Ratio (Includes angle adjustment for shock)
203 Wheel Rate

0.716 Sway bar motion ratio
107 Sway bar Wheel Rate

514 Front total rates (includes sway bar)

Rear Suspension
49.6875 Distance between coilover mounts (Inches)
69.5 Distance between tire centers (Inches)
10 Angle of right coil over (Degrees)
0.704 Motion ratio including 10 degree coil over
88 Wheel rate
176 Total rear wheel rates

Current Spring and Roll Bar Rates
300 Front spring Rate
150 Sway bar rate
125 Rear spring rate

74.48% Front roll couple
28.31% Total percentage (Total Spring Rates / Car Weight)
52.88% Roll Bar percentage of right front rate
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Re: Roll centers, weight distribution, caster...

Postby walterclark » Tue Jul 17, 2012 2:25 pm

First piece of advice. Only change one thing at a time, then try it.

Right off the bat I would say that 0 degree front caster is not what you want. Can you easily crank in caster to that? If so start with 5 degrees. That will help with the darting you feel on straights.

The rear end is probably doing a lot of steering too. Maybe the best thing you can do right now is understand how changing tire height (bumps) and camber (leaning around corners) affects tow. Once you have something of a map, then figure out what changes will make tow stay closest to a constant near-zero number. Once you have that set in - drive it to determine how you want to modify that behavior. It should be possible to change the roll center, camber. caster and (what used to be) steering tie-rod length to make it do most of what you want such as a tad bit of steady toe-in in a straight line for stability and a little zero, toe-in or toe-out in corners (depending on how tail happy or tight it feels).
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Re: Roll centers, weight distribution, caster...

Postby Mopar 151w2 » Tue Jul 17, 2012 3:08 pm

If it is darting, toe (in or out) is off - and the car is following whichever wheel is heaviest at that instant.

In your case my advice is to start drinking - no, wait a minnit, that,s how to get in a mess like this!!!!!

In your case, I'd get ride height and roll centers straightened out first - 4" is enough ride height if the car will stand an occasional scrape - and if it never scrapes at speed, it ain't low enough.
Front roll center needs to be low - well below the C/G (center of gravity). Usually, this means 4" or less. Rear should be somewhat higher, like around 1/2 the tire diameter.

Get that stuff straightened out, and you need to get bumpsteer (also affects toe ) right - like less than .030"/inch of travel.

Once you get the wheels going in the right direction most of the time, you can worry about springs and shocks.
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Re: Roll centers, weight distribution, caster...

Postby DennisRacing3 » Tue Jul 17, 2012 9:25 pm

Jeeze I opened a monstrous can. Seth. Yes, I have kept track of changes. Front to rear ratio a year ago was around 40F 60R. I changed front suspension springs from progressive 190lb to 125lb springs before Burke and did a re alignment. Honestly, I dont know if this made it better or worse. You can not tell much from Ascutney to Burke. The rear springs are around 212lb. It seemed to handle better, but was that just good hopes? Over the weekend at Burke I raised the front because it was scraping. Raised it about an inch until it didnt scrape. And handled good enough to do the 2:12 I think. But the car was maxed out. Then after Burke I changed the rear tube arms to a much closer length as the lower a arms. It passed the visual garage jump up and down test. No more rear steer.Check. Then car was resquared after Burke because my dad thought the car was not straight. So we re strung it again and again and again. Then at Okemo all day sat the car handled awful and we couldnt figure out why. Rear knuckle was loose. So this did not help at all in the handling department. It handled much better in the morning of Sundays fam run after tightening the knuckle down. About the same as it was at Burke. Then I pushed the car with serious lack of tire from destroying them at Burke from lack of air pressure and bent the right side of the car. So Okemo was beyond a flop for getting the car to "handle" lol. O well. Sherm. The car may get back together before Ascutney but that is doubtful. I'm going to get a new turbo set up since the one I have is fried. Re wire. Try to do all metal lines for oil and fix a whole slew of other things that you find need fixing on a new car. Then re build the suspension. I think that I am going to try coil overs in the rear. See if someone on the forums is selling them cheap and get a rebuild. Then camber plates so I can do more adjusting in the rear. So, I have to do a lot of replacing and find the money fairy to replace tires wheels and other suspension components. Paul. The turn in response is very consistent. Car is very consistent. Brakes well good turn in and after that the rear is starving for grip. There is a Sway bar on the front of the car. The neon bar that is on it is HD compared to what I may need. The front bounces nicely the rear about the same but I honestly dont know if I am in the right direction. By both Kevin and Sherms car being rear heavy like mine I am completely opposite on spring rates. They both have the front with heavier springs and the rear with lighter. Mine Vice Versa. The tires I dont know if im keen on. They are the softest hoosier compound tire but the side walls are wicked squishy. In the Pictures from Burke the roll makes me really nervous with 16lbs of air. Plus I destroyed them. They were peeling and literally garbage at Okemo. So I may go with a different tire. Walter and John. Im going to concentrate on roll centers and caster when rebuilding suspension. And do I want Caster in the rear as well?
Thanks guys
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Re: Roll centers, weight distribution, caster...

Postby Mopar 151w2 » Wed Jul 18, 2012 3:22 pm

No rear caster =no rear steer. Without an axis of steering, you cannot have caster. Yes, you are upside down on spring rates, and have likely been running on the oversize front swaybar . You need most of the roll stiffness on the steering end, but the "Big Bar, Soft Spring" setups popular in NASCAR are (A) mostly for "aero", and (B) quite difficult to get right.
My guess - and it's only a guess - is that you want rates somewhere around 250# front, 100# rear, and we can figure out which front swaybar to start with from there
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Re: Roll centers, weight distribution, caster...

Postby 3rdgendennis » Thu Jul 19, 2012 9:17 am

John,
Don't forget the car is about 40/60% with the 60% in the rear. At roughly 1500#, that's only 600# on the front tires. We had stiffer springs in the front, but the car was too bouncy.
Also, I believe the front sway bar is just a factory base model Neon sway bar (as in, the smallest one made for that setup). If it is an ACR sway bar, I have a base model one we can try
The youngest David Dennis
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