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For the suspension Gurus.

Old 01-30-09, 10:41 PM
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For the suspension Gurus.

I have a question for you guy's. I know that allot of circle track guys use lite spring rates with the use of bumpstops making up the difference. I was told that they could use a 200lbs spring with the use of various compound bumpstops to make the equivalent of say a 8 or 900lbs spring rate.

What would be the disadvantage of using this type of setup on a road racing application? No one that I know of uses this other than round track guys. It seems that buy using that light of a spring you could save quite a bit of weight at each corner of the car. The difference in a 200lbs spring and a 700lbs spring at 8" is about 2lbs per corner according to eibach.

This is strictly for track or racing cars. I did speak with one of the suspension guys for one of the mazdas at the rolex 24hr race. Want to know what some of you guys think why this would or would not work. Will be interested in hearing your thoughts.
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Old 01-30-09, 11:34 PM
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With road racing, it's best to use the bumpstops as they come on the car, to prevent shock damage and to provide a soft stop to prevent any over travel that could cause the wheel and tire to hit the fender, cause control arms to contact the body or whatever else would happen if the wheel traveled too far.

There is some merit to bump stop tuning if you're running really low though, to try and get the softest, most progressive deceleration out of it as possible, without having it being used all the time.
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Old 01-30-09, 11:35 PM
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I would avoid if at all possible. You are going to sacrifice a TON of suspension travel if you use a ride height low enough to use the bump stops as springs. The bump stops are also progress in their 'spring rates', much more than a progressive coil spring. Once the bump stops bottom out, your spring rate will approach infinity, since you have no suspension travel left.
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Old 01-31-09, 12:30 AM
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There was an article in RCE about this a month or 2 ago. The following is lifted from the article.

NASCAR (and I assume oval racers in general) need to pass a static ride height check, yet they need to get the car as low as possible in dynamic situations, especially for the aero benefit. To do this, they run extremely soft springs so that the spring compresses even under light aero loads and lowers the ride height. Since they still need to need a high rate for cornering, they use the bump rubbers to provided a rapidly rising spring rate, or just have the spring coils compress to bind height. This allows a high roll stiffness, while retaining the desired ride height. These rubbers act in both roll and ride, and hence are always a compromise.

To sum it up: they use the bump rubbers because they need a rising rate suspension and it is the only way they can do it by the rules. If you are looking for a rising rate suspension, just get a progressive spring. The NASCAR guys can get away with it because they basically take 4 of the same corner, which is nothing like a road race. Even then, the bump rubbers are poor at reacting to varying grip levels and are difficult to set up well even with advanced datalogging. Ask the front and backrunner teams this year how much time they spent on their bump rubbers, and the result will probably be very telling.

Also, on your initial paragraph. They don't have the equivalent of a 900lb spring, they have a 200lb spring that then steps up (very quickly) to an effective 900lb spring.

Again, this is all from RCE. Go read it.
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Old 01-31-09, 01:24 AM
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Roundy-round guys do some really wierd things that aren't applicable for road racing, some because of rules, some because of the nature of the track and the racing. They only have to worry about turning in one direction, so sometimes they'll set the car up to "crab" sideways on the straights, they run very uneven cross weights, very uneven weights side to side and so on.

They're basically just turning in, taking a set and carrying it through 180 degrees, small straight, then doing it again (on a true oval, where I'd call it 2 corners, not 4). Road racers have to worry about transitions between left and right corners, bumps, off camber corners, flat corners, cambered corners, fast corners, slow corners, long corners, small corners, chicanes, up hills, down hills, etc. Makes for a very different set of needs from the suspension.
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Old 01-31-09, 10:06 AM
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Let's not forget the front noses that look like they've been hit by a left cross (ie they point to the left). The entire car is set up to turn in only one direction.

This is why teams in the past have had a completely different car (or two) specifically for road racing.

Now how that is going to play out with the whole Car of the Future thing should be interesting.

-b
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Old 01-31-09, 10:31 AM
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If you're speaking of what the NASCAR guys are doing with the COT, the bumpstops are coming into use only because of how they're having to work within the current rule set. Not because it's the best way and what they want to do. The COT only has about 3" of suspension travel as mandated by the rules. The old car had around 7". Since the car has to maintain a relatively high static ride height all of the travel is used up at speed when aero loads push the car down. The car needs to be soft enough so that the ride drops at speed in order to lower ride height so that the front splitter works as well as it can. So what happens is that the car ends up on the bumpstops and springs are no longer a tuning tool and they're replaced by bumpstops and tires. Since the tires are spec tires tire pressure becomes critical. And that leaves bumpstops as the big variable and what Cup teams have spent many, many, many, many hours trying to understand and optimize.

In short, riding on the bumpstops is not THE way to setup a car.
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Old 01-31-09, 11:28 AM
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That is what the guy I talked to said at the Rolex. He said that he would not recommend it. That it would not be the ideal setup for road racing. He was saying that he would be affraid of it becoming too rigid off a curb and loosing control of the car.
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Old 02-02-09, 06:39 PM
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I'd be most worried about a hard suspension hit, which is going to bottom you out and then launch the car up in the air. Not a good scenario mid-corner. I like to run the curbs pretty hard and this kind of launch would really make for a disaster. If you want to run low enough the bump stops are becoming a factor it's time to either change cars or start radical suspension surgery. (I'm debating both) My car is occasionally bottoming the wheel in the wheelwell right now.

-Trent
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Old 02-03-09, 07:29 AM
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X number of Spec Miata folks are on the stops. Set up with Spec Miata mainly to get more negative camber. When the chassis is on the stops hard ya better have a VERY QUICK set of hands or
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Old 02-03-09, 09:33 AM
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Using bump rubbers to pump up spring rate was a popular trick in the Firehawk series, as well. In alot of cases, solutions racers come up with are based on limitations imposed by a rule set. Making lemonade and all that. If you're just building a track day car, it's probably best to focus on fundamentals and ignore this off the wall the stuff.
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Old 02-03-09, 05:56 PM
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I got interested in the idea because the guy's that are helping me build the car are dedicated circle track guy's. They build suspension dyno it, do it all. They run the bump stop setup and are very successful with it. They hold the track record in the late model division here, and led allot of laps at the Snow ball derby. They told me to go out with what is known to be a safe setup and we could go from there with experimentation. The car is being built for the unlimited class. It has custom 8300 penske shocks. That were based off Jack Mardikians Rx7. In fact he got some after mine were made for his car. Thanks TWINS TURBO (Eric) Shamless plug. The car has been in the build process for a year. I will post pic's when it is done. It should be a pretty formidable opponent for anyone in the class.
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