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Set Up Assistance Please


CC57

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Car: Thunder Roadster

Coilovers at all 4 corners; parnard bar (tied into left axle tube on driver's side, chassis on passenger side); 3-link rear suspension.

 

When setting up your car, is the order:

 

1. Set ride heights

2. Set percentages (cross %, left side %, rear%)

 

And if so, I set my ride heights where i wanted themt (LF 3.25, RF 3.5, LR 3.5, RR 4.25) and my cross was 60%! (I'm looking for around 50 to start.)

 

So I went back, reset the coil-overs at each corner, (3 rounds off LR & RF, 3 rounds more on RR & LF) to where my cross was now 50.5% (better correct?).

 

But then I rechecked my ride heights and they were way off (LF 4, RF 3.5, LR 3.75, R 3.6).

 

Now what? help please

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First off, contact your chassis builder to see what ride heights he/she recommends. Most chassis builders air for certain susupension geometry and then build the ride heights around those geometry settings. If you are shooting for different ride heights than those recommended, you probably will end up with geometry problems.

 

If your desired ride heights are similar to those recommended, you will need plenty of ballast to move around the car in order to get your desired weight distribution.

 

Let's look at the "plenty of ballast" scenario.

1) Set the desired ride heights.

2) Move lead around until you get your desired weights.

3) Re-set ride heights.

4) Move lead around until you get your desired weights.

5) Re-set the desired ride heights.

6) Move lead around until you get your desired weights.

7) etc. etc. until it's right.

 

There really isn't much you can do in the "no or very little ballast" scenario .

1) Run the ride heights even in the rear to go in the desired weight direction a bit...

2) Compromise all your desired weight goals as well as all the ride heights equally so that your not taking it all from either side of the equation.

 

Nick

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LOL at Johnny..

 

It's my way of saying that most chassis builders have geometry settings and an optimal weight distribution in mind when they build their chassis.

 

In most racing divisions these days, both the geometry and weight distribution have to be right to run up front. Changing the ride height changes the front end geometry a bunch. In a well-designed chassis, ride heights should only be changed to make minor cross weight changes ( a round here, a round there...) and should not be used to obtain several percentage points of cross weight change.

 

But, bottom line is you gotta do what you gotta do to get the car to handle, even if it's not 100% right...

 

Nick

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First off, contact your chassis builder to see what ride heights he/she recommends.
I did and they are the ones I quoted as "where I wanted them"

 

If your desired ride heights are similar to those recommended, you will need plenty of ballast to move around the car in order to get your desired weight distribution.

"plenty of ballast " That I don't have. I only have (2) 10# pieces of lead on the car. And those put me right where I want to be at the end of the feature. The lead is currently bolted inside the frame rail behind the LF & in front of the cockpit. And with them ,I currently have 51.5% left-side (rules allow up to 52%) and 52.5% rear. Any suggestions?

 

1) Run the ride heights even in the rear to go in the desired weight direction a bit...
I'll bring that up with the chassis builder and get their take.

 

2) Compromise all your desired weight goals as well as all the ride heights equally so that your not taking it all from either side of the equation.
Could you elaborate, you lost me here.

 

And Nick, I appreciate the guideance. While I don't enjoy this (settig nthe car up) as much as the driving, it does give one a sense of accomplishment when it comes together.

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First off, contact your chassis builder to see what ride heights he/she recommends.

I did and they are the ones I quoted as "where I wanted them"

 

If the rides heights are the ones suggested, then I'd have to say that the chassis was designed to run with a lot of cross weight because when you use the weight jackers to get the much lower cross weight the ride heights are no longer at the recommended heights. OR.... the chassis has been bent so that the RF and LR suspension pick up points are lower than they were when the car was unbent...

 

 

"plenty of ballast " That I don't have. I only have (2) 10# pieces of lead on the car. And those put me right where I want to be at the end of the feature. The lead is currently bolted inside the frame rail behind the LF & in front of the cockpit. And with them ,I currently have 51.5% left-side (rules allow up to 52%) and 52.5% rear. Any suggestions?

 

With only 20 pounds of lead to move around, you really don't have a lot of options weight wise. And you can't change the left side percentage or the rear percentage by using jackers (unless you practically tip the car on its side or on it's nose or tail). Obviously, you want as much left side weight as possible if you're running pavement.

 

And Nick, I appreciate the guideance. While I don't enjoy this (settig nthe car up) as much as the driving, it does give one a sense of accomplishment when it comes together.

 

Without knowing many more of the details about any chassis, it's really difficult to provide meaningful advice. I really hesitate responding to handling issues except in the most general terms that would apply to virtually any chassis on any track.

 

Nick

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I should know a bit more tomorrow after I contact the chassis builder.

 

Forgot to post this: The reason I'm doing this is to tighten the car up on exit. Car feels fine going in and through the middle, but loose off.

 

I was running 1.5" stagger (LF to RF, LR to RR), but I took out some in the rear (now, 0.75" LR to RR) in ana effort to tighten the car up. I was checking out my ride heights and percentages after the change (reduction in stargger) when I discovered the high cross percetange.

 

Any other particulars about the chassis you would like to hear Nick?

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A car that is fine on entry and in the middle but loose of doesn't need a 10% change of anything. I don't know about thunder roadsters so I can't help but 10% is drastic.

 

Thanks for repl;ying poorboy.

 

10% of what? I'm not folllowing you?

 

Thanks in advance.

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A car that is fine on entry and in the middle but loose of doesn't need a 10% change of anything. I don't know about thunder roadsters so I can't help but 10% is drastic.

 

Thanks for repl;ying poorboy.

 

10% of what? I'm not folllowing you?

 

Thanks in advance.

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I think he was referring to your initial statement about 50% target cross and 60% actual.

Got ya!. Thanks.

 

 

Its edumakashu-nul :rolleyes:

"edumakshu-nul" I see yu tu ar a gradiate of Texas A&M liek me. Cool.

 

And I don't see you as a "cynical and negative" individual, just a sum beyatch :P:rolleyes: (wink smiley inserted here)

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I think he was referring to your initial statement about 50% target cross and 60% actual.

Got ya!. Thanks.

 

 

Its edumakashu-nul :rolleyes:

"edumakshu-nul" I see yu tu ar a gradiate of Texas A&M liek me. Cool.

 

And I don't see you as a "cynical and negative" individual, just a sum beyatch :P:rolleyes: (wink smiley inserted here)

come on guys i believe he's a retired mayor ..forgive him for his short comeings .[.uh me to if your in the forgiven mood ]....that should be a good enough excuse ..sorry 49 .just trying to help at no charge ...lol

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I think he was referring to your initial statement about 50% target cross and 60% actual.

Got ya!. Thanks.

 

 

Its edumakashu-nul :rolleyes:

"edumakshu-nul" I see yu tu ar a gradiate of Texas A&M liek me. Cool.

 

And I don't see you as a "cynical and negative" individual, just a sum beyatch :P:rolleyes: (wink smiley inserted here)

come on guys i believe he's a retired mayor ..forgive him for his short comeings .[.uh me to if your in the forgiven mood ]....that should be a good enough excuse ..sorry 49 .just trying to help at no charge ...lol

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I should know a bit more tomorrow after I contact the chassis builder.

 

Forgot to post this: The reason I'm doing this is to tighten the car up on exit. Car feels fine going in and through the middle, but loose off.

 

I was running 1.5" stagger (LF to RF, LR to RR), but I took out some in the rear (now, 0.75" LR to RR) in ana effort to tighten the car up. I was checking out my ride heights and percentages after the change (reduction in stargger) when I discovered the high cross percetange.

 

Any other particulars about the chassis you would like to hear Nick?

Now you have me really confused. If your car is fine going in and fine in the middle and loose on exit, why would you want to loosen your car up even more by reducing the cross weight????

 

Now if your car was tight going in and tight in the center and loose coming off, I'd tell you to loosen the car up until your entry and center were perfect. Tightening a car on exit is probably the easiest to accomplish unless you have a 2000 HP monster engine or something.

 

Nick

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Now you have me really confused. If your car is fine going in and fine in the middle and loose on exit, why would you want to loosen your car up even more by reducing the cross weight????

 

Sorry for causing the confusion. Let me see if I can eliminate some if not all of it.

 

The car had these characteristics: “fine going in and fine in the middle and loose on exit” when I had 1.5” of stagger in the front and rear. After talking to the chassis builder, he recommended reducing the rear stagger to 0.75” max.

 

Next, I’m also aware that a number of people driving these cars like to have around 50 to 52 % cross.

 

So, once I changed the stagger and rescaled the car, I discovered the measured cross (60%) was higher than recommended and set about reducing it to the recommended range. With the cross at 60% I was anticipating an overly tight condition and was trying to bring it in the recommended range.

 

I had not driven the car post stagger reduction high cross condition.

 

Does this clear it up? Some?

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Robert,

 

What was your cross when you had more stagger? As for loose off, we have all been loose off because the track has been so slick. I have been trying to find more traction off the corner, or forward bite, whatever they wanna call it. Nick, you can see his car in the video at

 

http://myspacetv.com/index.cfm?fuseaction=...ideoid=15078080

 

you may be able to understand what he is looking for.

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Robert,

 

What was your cross when you had more stagger? As for loose off, we have all been loose off because the track has been so slick. I have been trying to find more traction off the corner, or forward bite, whatever they wanna call it. Nick, you can see his car in the video at

 

http://myspacetv.com/index.cfm?fuseaction=...ideoid=15078080

 

you may be able to understand what he is looking for.

I took a look at the video. Who knows what was going on in the cockpit or what sort of stuff was on the track at the time, but I'll take a stab at what I think I see going on.

 

Looks to me like at least two of those cars have too much rear weight, or are too stiffly sprung in the rear, or both. With as much cross weight and as little stagger as you say you were running, those cars should be pushing coming off the turns. I would be very interested in seeing the tire pyrometer readings after that race to put it together with what I saw and what you are telling me...

 

It's very tempting and, unfortunately, quite intuitive to increase the spring rate on the heavy end of the car. When a car is rear heavy, like a rear engineered VW for instance, the trick is to increase the front spring rate and decrease the rear spring rate. I know, I know. It sounds backwards, but roll couple distribution theory says that's the way to go.

 

Rear stagger should be set at what the track physically calls for and not as a handling crutch. There are several very good rear stagger computational tools readily available on the internet if you go check. Once a car has the proper springs, geometry and roll centers, stagger can be used to make minor handling adjustments in the heat of battle, but not as a primary handling adjustment tool. Springs, shocks, sway bar and panhard bar should be the primary adjustment tools. Too much rear roll out and your will be loose coming off the turns. Not enough rear roll out and you're scrubbing off speed in the turns.

 

Since you asked what other information I might need to get a better picture of your chassis and suspension, for starters what are the wheel rates for all four springs? Not the spring rates, the wheel rates. And do you know the vertical height of both the front and rear roll center? What about bump steer settings? Do you run Ackerman steering? What are the camber, caster and toe settings?

 

Once again I have to throw in a disclaimer here. I'm just talking off the top of my head based on my own prior experience and with no real insight into your car.

 

Nick

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Who knows what was going on in the cockpit or what sort of stuff was on the track at the time,...

 

Geesh, I hope I know what was going on in the cockpit :blink: Anyway, I wasn't all that busy, with the steering wheel that is, until corner exit. Car felt fairly neutral going in. It's on exit, where I have to either baby the throttle or risk having to dirt track the steering to catch it before the rear end starts to step out on me. Also, that race day was very hot, and a couple of heavy afternoon thunderstorms had just rolled through. Plus we were the 1st group to hit the track after the rains swept through (green track essentially)

 

I would be very interested in seeing the tire pyrometer readings after that race to put it together with what I saw and what you are telling me...
No readings available for this venture onto the track, but here are some readings after a 10 or 12 lap shake down session on a day of similar ambient temps (keep in mind, these are tire surface mount temps, not using a probe that penetrates the surface)

 

(all temps read left to right, as if from the rear of the car)

 

LF RF

Outside Middle Inner Outside Middle Inner

129 130 126 150 145 138 (based on these readings I dropped a pound in the LF)

 

LR RR

Outside Middle Inner Outside Middle Inner

140 139 138 157 152 150

 

... what are the wheel rates for all four springs? And do you know the vertical height of both the front and rear roll center? What about bump steer settings? Do you run Ackerman steering? What are the camber, caster and toe settings?

Nick

 

Tell me how to compute/determine the wheel rates/roll center/ackerman, I'll see what I can do. Bump steer, I have already wondered how I couold go about measurnig it without investing in the rather expensive tool required (or is there a Saturday night low bucks alternative).

 

LF RF

Camber 2.0 4.25 (both camber & caster measured with a Longacre bubble guage)

Caster 0.5 2.0

Toe LF straight, RF 3/8"

 

Corner weights from the scales, with driver weight in car, with end of feature fuel level in fuel cell

LF: 365#s RF: 348#s

LR: 421#s RR: 385#s

 

And just in case, spring rates:

LF 225 RF 250

LR 175 RR 205

 

And according to the car designer, who was at the track for our 1st race of the year, he said those were good for the track.

 

Once again, I appreciate all the attention and guideance.

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I noticed you car is just a little over 1500 lbs but you run the same springs in the rear that I run on my midified. Seems a little stiff.

 

If I can ever reach the chassis builder, I'll pose that question to him. Or I'll ask around at the track next race weekend.

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Poorboy has picked up on the same issue I did... I suspect that at least the rear of your car is oversprung - and perhaps the entire car depending on what the motion ratios are for the four springs.

 

But I am going to gracefully bow out of this thread since your chassis builder is directly involved in your setup issues. I hope no one takes this wrong, but I have no desire to contradict what your chassis builder is telling you -- he makes his living doing this and doesn't need someone like me quibbling over his setups.

 

Thanks.

 

Nick

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