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Old 04-12-2004, 09:54 AM
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Using Wheel Scales To Set Preload

As you all know, equal rear tire loading is very important, when drag racing, for two reasons: It provides the best elapsed times and it keeps you in your lane. You can achieve equal loading (with a RWD live axle car) by using either...or a combination of...two methods: Static loading and dynamic loading. I prefer dynamic loading, but, since most of you use some form of preload (which is a static loading), I thought I'd point out how you can use a set of wheel scales to get you very close to optimum performance.

First, you're either faced with the cost of a set of wheel scales or the need to become a buddy with someone who's into oval track racing. Such a buddy either owns a set of wheel scales himself or knows where he can borrow some. Then, some simple calculations will get you going.

Suppose you have a 2500 pound car (with driver) that accelerates at 2 g's on launch. (Where did I get the "2 g's"? Well, the 60 foot times being recorded indicate that modern wrinkle-walls are commonly capable of accelerations between 2 and 2.5 g's, with some going even beyond that range. If you stage deep, your 60 foot time can be used to calculate the acceleration. If you stage shallow, the calculations become a bit shaky, but a value of "2" will get us pretty close.) That means the total rear tire thrust is 5000 pounds. With a loaded rear tire radius of 14 inches and an axle ratio of 4.11:1, the driveshaft torque would be about 17000 inch-pounds. Assuming 60% of the total roll stiffness of the car is at the front [This is an assumption, but it's not too far off for the average car. If extensive efforts have been made to reduce the front roll stiffness and increase the rear, these efforts should be taken into account and the percentages changed accordingly.], 40% of that 17000 inch-pounds would be cancelled out, leaving 60% of 17000 inch-pounds, or 10200 inch-pounds to unload the right rear tire. If the rear track is 64 inches, that means the driveshaft torque is trying to make that right rear about 160 pounds lighter than the left rear. (Even if you're not familiar with these calculations, a little playing around with a calculator will quickly indicate how I came up with the numbers.)

So, if you're going to achieve equal rear tire loading with preload, those wheel scales will have to indicate 160 pounds more on the right rear than on the left rear. I suspect that most of you will find that you don't have nearly enough preload.
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Old 04-12-2004, 02:52 PM
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A really good quality bathroom scales that read up to 800 pounds will also suffice.
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Old 04-13-2004, 04:59 AM
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Quote:
Originally posted by johnsongrass1
A really good quality bathroom scales that read up to 800 pounds will also suffice.
Don't tease us like that. Is this what you're using? Do you have a manufacturer's name? I poked around on the Internet and found a heavy duty physician's scale that would cost $210 per corner. (That might sound expensive to some of you, but that's really pretty cheap for wheel scales.) Have you found something cheaper than that? There are a lot of scales with limits of about 400 pounds. Best price was 50 bucks, but that's not enough capacity. I suppose you could fixture 2 of them together. That would bring the cost down to $100 per corner. Now, that sounds pretty good!

I didn't realize bathroom scales ever went up that high. Years ago, I bought a set of cheap scales to set up a microsprint I had built, but I never considered them for a full size car. There are outfits that make lever systems for use with bathroom scales, but I don't know how well they work.
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Old 04-13-2004, 09:26 AM
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A buddy of mine uses two per corner by placing the each scale in front of the tire as well as behind the tire and doing the extra math. So if you were to purchase four scales and only do front or rear at one time you could get away for as little as $80. I know of a guy who has bathroom scales that read 800 but I don't know where he got them at.

Weight watchers maybe?!




cheap scales
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Old 04-13-2004, 09:41 AM
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Willys36 posted a method using ordinary wood planks and a bathroom scale that involves very simple math. Maybe if you PM'ed him, he could show you. It might be what you're looking for, dirt cheap too!
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Old 04-15-2004, 04:05 PM
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Thought I'd mention that, with a 4link, the wheel scales don't reveal the dynamic loading that is available. If the rear mounting points for the lower links are ahead of the rear axle, you will see changes in wheel scale readings as you change, say, the length of the upper right bar, but this is only "the tip of the iceberg." As the upper right bar is lengthened, the axle housing is twisted like a big torsion bar. Then, when you launch, the center section rotates and tends to dynamically load the right side (which is, of course, what you want) by that extra bit of torque which was stored in that twisted axle housing. The more forward of the axle centerline the lower rear mounting points, the more pronounced the effect.

What's the magnitude of this extra loading of the right rear tire? Beats me! With enough information, I could estimate it, but it wouldn't be easy. And, that's why I believe the procedure used by the roundy-round boys is to be preferred. If you use an adjustable coilover at the left front, you can use the wheel scales to crank in exactly what you want and you have a pretty good idea that you're close to correct from the get-go!

Besides, using the axle housing as a torsion bar can lead to problems if you're trying to use weld-on brackets and a stock housing. This is why people like G-Force Racing Cars in Tonawanda, and others, are in the business of supplying heavy duty housings and center sections to the pros.
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Old 04-15-2004, 04:09 PM
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Here's how to use the typical 300# bathroom scales to weigh a heavier load. I sketched this up for guys wanting to size front springs on MII front ends. For your rear end problem, measure each wheel and multiply the reading by 10 instead of 20 to get the weight on each wheel instead of the whole rear end.
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Old 04-15-2004, 05:46 PM
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Thanks for the sketch. As I said, there are outfits that offer this sort of thing for sale, but they're much more "close-coupled" and, as a result, probably aren't as accurate as that which you suggest.
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Old 04-20-2004, 06:35 AM
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Quote:
Originally posted by BillyShope
As the upper right bar is lengthened....
Hey, guys! When I say something stupid, please tell me it's stupid. Jump all over me. Don't let me get away with it.

I would never use a 4link, as I prefer an asymmetric 3link or some other form of dynamic driveshaft torque cancellation, so I have a bit of an excuse for my ignorance. I saw, in another thread, where somebody commented about lengthening the upper right link to increase preload, so I assumed that's how it's done. I tried to make sense out of it, which is why I made the comments about the rear bottom pivot being ahead of the rear axle. But, I now realize this other fellow must have meant "shortened," rather than "lengthened."

The usual dragstrip setup, with a 4link, is to have the upper links angled down from the rear. When, on launch, the pinion tries to climb up the ring gear, those upper links would both go into tension. So, due to their angle, they'd be trying to lift up the rear of the car. When the upper right link is shortened, it is put in tension statically. Since every force requires a reaction, the upper left link is put into compression by this adjustment. (And, this will be seen on wheel scales as the adjustment is made, BUT it is still, as I said in the earlier post, only the tip of the iceberg.) Then, on launch, the upper right link is stretched even further, but that upper left link, which had been in compression, must be "decompressed" before it can carry a tension load. This is the 4link preload which results in additional loading of the right rear during launch and which does not show up on the wheel scales.

(The lower links are usually angled up from the rear, so a similar mental exercise will yield the same result.)

Note that, if the links remain parallel to themselves and parallel to the ground, you can shorten or lengthen those links until you strip the threads and you won't change the wheel loadings. This is why there are "street" and "strip" settings for 4link angles.

Last edited by BillyShope; 04-20-2004 at 09:07 AM.
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