McRat
09-16-2005, 12:54 AM
One of the greatest problems we face is wheelhop. We can certainly suppress it by going down in tire pressure or attempt to restrain it with traction devices. I have noticed that some trucks hop with traction devices, and others without them might not, so I want to discuss tire pressure. A truck with a solid suspension can get hop, so I'm not sure hop can be addressed there alone.
I made a stupid mistake when I was first playing with Blue by assuming that more tire pressure might supress hop (less flex in the tire, less wrap?). Nope. While that might work with some car tires, it doesn't work on our trucks. You will get more hop at higher pressures. Sorry Trippin, bad advice on my part.;)
The less tire pressure you have, the slower you go. So you do not want to go lower than necessary. But do you want more pressure in the front or rear? Ideally we want identical traction at all 4 tires, because we are a "locked" transfer case. We must spin both ends of the truck or nothing. I believe (with no proof) that a poor balance of front/rear traction increases hop, and a balanced truck it much harder to make hop.
When you launch, the torque from the front wheels pushes the rear of the truck down, and the torque of the rear wheels pulls the front end up. This is important because your "contact patch" is a function of tire pressure and weight. A tire at 20PSI has twice the contact patch as a tire at 40PSI, and bigger the contact patch, the greater the traction.
I want the contact patch on launch to be the same for all 4 tires. Well if there was no launch torque, it would be an easy calculation. Say you have a 7000lb truck, with 4000lb on the front, and 3000lb on the rear. You would run 40PSI Front, and 30PSI Rear. That would yield 100sqin of contact at each end. But torque screws that up, especially with a 4WD. First we need to know what the torque is at the axles. Assuming we are launching at 1000ftlb of engine torque, first we divide the torque in 2 (two ends of the truck), so that's 500ftlb. Then multiply it by the first gear, say 3:1, is 1500ftlb, then multiply by the axle gears, say 4:1 (yes, I'm making up numbers), so we end up with 6000ftlb at the axle. Now we need to take into account tire radius. The bigger the tire, the less the torque. For simplicity, say tire radius of 1.5 feet = 4000 ftlb at each end.
So 4000 ftlb at each contact patch is trying to rotate the truck. Now the wheelbase comes in. Say it's 12 ft. You take the 4000/12 and get 333lb of "lift" on the front of the truck from the rear axle, and the same amount of "push" downward on the rear from the front axle. So now at launch, your front weighs 3666lb, and the rear weighs 3333lb. As axle torque goes up, weight bias shifts to the rear. At low power levels, you will need more air in the front than the rear, and moderate power levels it will be equal, and at high power it you would want less air in the front than the rear.
Or am I way off base? I need to get actual numbers to figure this out right. If you figure a 1.70 60' time and assume constant acceleration, and know the weight of the truck, you can calculate effective axle torque, and from there with the wheelbase and weight bias, you can figure the ratio of tire pressures.
Any math wizards out there want to take up the challenge? Come up with a formula that takes:
Weight front
Weight rear
60' time
Wheelbase
And come up with a F/R tire pressure bias number?
I made a stupid mistake when I was first playing with Blue by assuming that more tire pressure might supress hop (less flex in the tire, less wrap?). Nope. While that might work with some car tires, it doesn't work on our trucks. You will get more hop at higher pressures. Sorry Trippin, bad advice on my part.;)
The less tire pressure you have, the slower you go. So you do not want to go lower than necessary. But do you want more pressure in the front or rear? Ideally we want identical traction at all 4 tires, because we are a "locked" transfer case. We must spin both ends of the truck or nothing. I believe (with no proof) that a poor balance of front/rear traction increases hop, and a balanced truck it much harder to make hop.
When you launch, the torque from the front wheels pushes the rear of the truck down, and the torque of the rear wheels pulls the front end up. This is important because your "contact patch" is a function of tire pressure and weight. A tire at 20PSI has twice the contact patch as a tire at 40PSI, and bigger the contact patch, the greater the traction.
I want the contact patch on launch to be the same for all 4 tires. Well if there was no launch torque, it would be an easy calculation. Say you have a 7000lb truck, with 4000lb on the front, and 3000lb on the rear. You would run 40PSI Front, and 30PSI Rear. That would yield 100sqin of contact at each end. But torque screws that up, especially with a 4WD. First we need to know what the torque is at the axles. Assuming we are launching at 1000ftlb of engine torque, first we divide the torque in 2 (two ends of the truck), so that's 500ftlb. Then multiply it by the first gear, say 3:1, is 1500ftlb, then multiply by the axle gears, say 4:1 (yes, I'm making up numbers), so we end up with 6000ftlb at the axle. Now we need to take into account tire radius. The bigger the tire, the less the torque. For simplicity, say tire radius of 1.5 feet = 4000 ftlb at each end.
So 4000 ftlb at each contact patch is trying to rotate the truck. Now the wheelbase comes in. Say it's 12 ft. You take the 4000/12 and get 333lb of "lift" on the front of the truck from the rear axle, and the same amount of "push" downward on the rear from the front axle. So now at launch, your front weighs 3666lb, and the rear weighs 3333lb. As axle torque goes up, weight bias shifts to the rear. At low power levels, you will need more air in the front than the rear, and moderate power levels it will be equal, and at high power it you would want less air in the front than the rear.
Or am I way off base? I need to get actual numbers to figure this out right. If you figure a 1.70 60' time and assume constant acceleration, and know the weight of the truck, you can calculate effective axle torque, and from there with the wheelbase and weight bias, you can figure the ratio of tire pressures.
Any math wizards out there want to take up the challenge? Come up with a formula that takes:
Weight front
Weight rear
60' time
Wheelbase
And come up with a F/R tire pressure bias number?