Equalized tire load on four wheel drive pull truck

[rpmisking]

Okay, here is my problem. I am trying to equalize tire load (weight) on the two back tires and the two front tires of a four-wheel drive pulling truck going down the track. I understand that the back tires will have a much greater load than the front (due to the weight of the sled), but I want the left to right load on the front and rear tires equalized. The rear suspension is solid and the front suspension is leaf (with a sway bar). I have a data recorder in the truck and observe that during a pull the left front corner of the chassis (due to engine torque) is coming up a lot more than the right. My first reaction was to put shift weight to that side of the weight bar, (it is 60 inches in front of the front axle) but after further thought I am not sure this is correct since the front drive shaft torque is pushing down on that left front tire already. Correct me if I am wrong but I believe that the rear drive shaft torque is putting more load on the left rear tire (even if the rear suspension is solid and the differential is locked?) and the front drive shaft torque is putting more load on the left front tire. (Both front and rear shafts turn in the same direction, which is clockwise viewed from the front). One of the problems that I have is understanding how each axle affect the other and how all these forces relate. How can I figure out when these left/right tire loads are equal? I understand that a lot of factors become involved (chassis roll, chassis stiffness, engine torque, etc.) in trying to calculate this but if someone could give me some direction and formulas I would greatly appreciate it. Thanks.

 

[BillyShope]

Although you don't exceed 300 mph, what you have is essentially a dragster setup. Well, not quite, in that you do have a suspension at the front, but the similarity is still strong.

So, allow me a few comments on the dragster before proceeding to your problem. With the dragster, the reaction to the driveshaft torque is distributed, front-to-rear, in proportion to the relative roll stiffness. Ideally, they want that reaction to feed back to the rear wheels in order to equalize rear tire loading. The low torsional stiffness of the long dragster chassis acts to "decouple" the front end and aid in this effort. The left front wheel will, however, still be seen to lift before the right, indicating that some of the reaction torque is being "lost" to the front end.

Now, with your puller, the relative roll stiffness still influences the distribution of driveshaft torque reaction. And, your observations indicate the effect. Your options for a "fix" fall into either the "static" or "dynamic" category. A static fix would be some kind of preload which would give you equal tire loading while pulling. This would include a static weight redistribution or a torsional preload of the chassis. The torsional preload would amount to a "droop" of the left front when the front is lifted with a centrally located jack.

The ideal fix would be dynamic, which would mean some form of asymmetric suspension linkage or asymmetric spring rates. Specifically, you might consider a right front spring with a rate much higher than that of the left front.

As to quantifying the solution, I would suggest use of a "traction dyno," as described on one of the pages (forgot which one) of the following site:

http://home.earthlink.net/~whshope

Whoops! Just recognized a possible problem. You might have to disconnect the front wheel drive while using the traction dyno or risk damaging your wheel scales.

 

[Fabrico]

If the vehicle is really getting tweaked it likely means you are getting a lot of power to the ground. Evening things out might not increase pulling ability.

 

[Warpspeed]

Another, (probably impractical) approach to the problem is independent front and rear suspension.

Articulated driveshafts will not convert tailshaft torque into wheel lifting forces. The whole chassis may still twist somewhat, but the problem would be greatly reduced.

If chassis twist can be minimised, and the suspension rates made softer, the whole effect of diagonal weight transfer could be reduced to insignificance.

 

[Fabrico]

Re: "Another, (probably impractical) approach to the problem is independent front and rear suspension.

Articulated driveshafts will not convert tailshaft torque into wheel lifting forces. The whole chassis may still twist somewhat, but the problem would be greatly reduced.

If chassis twist can be minimised, and the suspension rates made softer, the whole effect of diagonal weight transfer could be reduced to insignificance."

That sounds like a good recipe for letting the rear wheels do the real pulling! All things optimized, two will outpull 4 anyway

 

[GregLocock]

proof? Given real world tire characteristics?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warpspeed]

Chassis twisting loads could be eliminated if the tailshaft twisting reaction of the diff centre is coupled directly back to the engine/gearbox directly, and not through the chassis.

The rear diff housing could be coupled by a torque tube back to the rear of the gearbox casing. It could even be made into a single rigid structure. The front diff might be made part of the engine sump structure.

The whole diff/engine/gearbox/diff assembly could then be rubber mounted and there could be absolutely no chassis twist generated from tailshaft torque.




Even if you do nothing else, a rear torque tube, (even with a live rear axle) might be well worth thinking about.