Travel Limiters

[LS14gezr]

I am bracket racing my 2001 Z28 with rear lower control arms relocated and adjustable shocks F/R, 250 lbs. drag front springs and front sway bar disconnected. Judging from event photos I appear to be getting good chassis separation in the rear but extreme front end travel without wheel stand. For a picture of the vehicle in question: www.dragway.com ,photos, Oct 28, page 4, upper left hand corner, dark blue. Should I be using travel limiters at this point and if so how would this affect the vehicle on the street as I do drive it back and forth to the track although it is not my everyday driver. If so, what can I expect them to do from a tuning standpoint? Are travel limiters even considered a good idea? Please offer your opinions on the limiters or anything else you can see from the photo

[F1Fever]

this car?
http://photosbyjim.smugmug.com/gallery/ ... 84204-A-LB

[BillyShope]

I am impressed by the picture! It actually appears that the right front is being held higher than the left front. I'd like to see the fronts remain on the ground, but that certainly is not excessive air. If you have wheel scales available, I'd really be interested in some traction dyno readings.

[LS14gezr]

F1 – yes that is the picture.
Mr Shope. I’m quite excited that you have taken the time to respond to my inquiry and more so that you don’t think there is anything wrong. I went to your website and reviewed the traction dyno chapters. I do have access to scales and the measurements could be taken. So far I have relied on event photos and time slips to make adjustments. This is my first year bracket racing (one event win, one runner up). Looking at competitors photos, I’ve noticed that, even during a wheel stand their wheels do not appear to hang or show a lot of air between the top of the tire and the wheel well. This led me to believe that I may have a lot of wasted energy driving the car up instead of powering out of the hole. Is this a correct assumption? I have my front shock (12 position single adjustable) settings as shown in the photo as follows: Passenger side (right) = full soft, Driver side (left) = 2 clicks up from full soft. Although I didn’t try it this past season, it occurred to me recently that I could increase the front spring pressure with the adjusting rings on the front shocks with perhaps more bias to the passenger side to help even things up on launch. Although I played around with shock settings and kept careful notes, I really have no prior experience with suspension tuning. I feel very fortunate that a man with your extensive background would provide direction. Thank you

[BillyShope]

This led me to believe that I may have a lot of wasted energy....

I don't know who came up with this idea of "wasted energy." The front end is always going to rise. It will simply rise higher if the front spring rates are decreased. The increased front end rise provides, for a given acceleration, more weight transfer, but that is obviously not a problem with your car.

If you wish to reduce the air that you're presently pulling, simply moving the battery back into the engine compartment might be all that's required.

[Bubstr]

Wasted energy. I always believed it took energy to lift the front wheels off the ground. When I jack it up, my arm says it does. My thinking says this energy has to come from the instant center pushing under the center of gravity, causing it to rise. Even if the rise is a byproduct of going forward, wouldn't any rise more that it takes to transfer enough weight to the rear to get traction, be a waste? As far as the waste goes, Wouldn't it be a waste to raise the front farther than you can maintain and let down smoothly? I'm talking the bing bang thank you Mam wheelies. The up and back down so fast it unloads rear tires before the wheel speed comes up. Wouldn't it be better to only raise the front as far as you can maintain until wheel speed comes up, keeping a more constant weight on the rear tires?

I believe on the higher HP cars, it may be more important how they end the wheelie, than how high it goes. I look at compression in the front, coming down. Some of them up to 4 inches. If you take 4 times spring rate times 2 springs, some have about70 or 80% front weight about the time right before the shift when torque is up there, then wonder why they get out of shape between 60 and 330 mark. The reason being they only have 20% of the car weight to get traction.

Am I over thinking this?

[BillyShope]

Am I over thinking this?

It's pretty much impossible to over think anything. You just went down the wrong rabbit trail. The energy to raise the front of the car comes from the stored energy in the suspension springs, NOT from the engine. The stored energy in a spring is proportional to the product of the spring rate and the square of the deflection. So, a given amount of energy change involves a larger deflection change for a lower spring rate.

[Bubstr]

You just went down the wrong rabbit trail. The energy to raise the front of the car comes from the stored energy in the suspension springs, NOT from the engine. The stored energy in a spring is proportional to the product of the spring rate and the square of the deflection. So, a given amount of energy change involves a larger deflection change for a lower spring rate.

LOL I like the rabbit. My rabbit told me he never seen a 3,000lbs car with 1500 lbs of stored energy in the front springs jump up in the air and stay there for a while, with out moving forward. My rabbit says that stored energy is much the same as the stored energy in a garage door spring. The door will stay closed till you lift it, then the stored energy will help you till it is spent. Then there will be some inertia force left from raising combined with the acceleration force,(rear tires driving forward), to raise it higher, or maintain it or spend the force and let it down. On cars that carry the front for long distances, I suggest the inertia force has been spent long before front comes down. Me and the rabbit have a brain trust going on. lol

[F1Fever]

any energy that it takes to raise the front will be transfered into forward motion when the front comes down if it doesn't unload the rear suspension and spin. For cars that carry the front for long distances the downfall is in wind resistance.

[BillyShope]

No, it just doesn't work that way. As far as the engine is concerned, there is no such thing as "wasted" energy. Also, there is no "conversion" of such energy into useful performance.

The engine provides a horizontal force at the rear tire patch. Since the center of gravity is at some height from the track surface, vertical loadings at the front and rear are affected. These vertical force changes affect the attitude of the chassis. But, whatever is happening with the chassis, the horizontal force at the rear tire patch...and, hence, the forward acceleration of the car...remains unaffected.

This is not to say that front end rise is of no importance. It affects weight transfer and aero drag and...since it can affect link angles...it can change the anti-squat and introduce undesirable oscillatory loading. But, you needn't concern yourself with trying to avoid "wasted" energy. It simply doesn't exist.

[Bubstr]

I don't want to appear as a hard head, but I am really having trouble grasping this. Possibly instead of over thinking it's over simplification.

I realize that the line of force being lower than the center of mass causes that mass to want to rotate. As we have track under the rear it can not. So this rotation is seen in front lift. Then I'm thinking if the front comes up 1 foot and it weighs 1500lbs, we did some work. Work takes energy. Some of that energy came from stored energy in front springs. If they where 300lbs springs, they would supply energy to lift cars front about 3 inches, giving the benefit of doubt to some inertia. the rest of the energy came from engine. My reasoning is that the engine supplied X amount of energy. Energy is spent doing work, we can't use it more than once. So some of the supplied energy was not used to propel car forward.

As F1 pointed out, the energy is stored as mass hight and will give it back. Then all we would loose is the time it took.

I think we are looking at it similar in the fact that the real energy waster is areo drag, time and unloading the rear tires when the front drops. Plus it makes it hard to steer.

[F1Fever]

If the front comes up 12" (front tire) and the car is 110" long (wheelbase) with 28" tires:

28 * 3.14= 87.92

(110 * 2) * 3.14 = 690.8

690.8 - 12 = 678.8

X / 87.92 = 678.8/690.8

X/ 87.92 = 0.9826

X = .9826 * 87.92

X = 86.3927
86.3927 - 87.92 = lost ground

-1.527" = lost ground

[Alan Roehrich]

Imagine how much ground I'm losing with a 108" wheelbase, the front tire 36" off the ground, and not touching down until about 65 feet or so out.

[BillyShope]

Here's another way to look at it. Suppose you had a car that loses 1000 pounds from the front on launch. (That 1000 pounds shows up at the rear wheels, of course.) Now, suppose you had another car, similar with respect to suspension rates, but with a 1000 pound weight that is movable, on frictionless bearings, between the front and rear of the car. In other words, with no effort, it can be moved from over the front wheels to over the rear wheels. The change in chassis attitude would be the same for both cars.

In the first case, the change in chassis attitude is due to the dynamic loading experienced during launch. In the second case, it is due to a change in static loading. The point is, the suspension reacts exactly the same. The suspension doesn't know why. All it knows is that it must react to the change in vertical loading by an extension of the front springs.

And, of course, engine torque is not involved in the second case.

I would like to think that the "Getting Started" section, on the first page of my site, would make all of this very clear.

[Bubstr]

I would concede that there would be no energy spent if the center of mass wasn't lifted. we are moving weight, not just rotating it on a fixed axis at the center of mass. Aren't we rotating from an axis point at the rear tire contact patch. It all comes from forward force, but that don't mean that it comes without cost. If there was no cost there would be no point in moving weight forward and lower for higher horsepower cars, or back and high for lower horsepower cars. If it didn't cost anything to rotate weight, it wouldn't make a difference how much harder it is to do the job. I may not be thinking right but this has always worked for me.