Competition Tire sizes and Dyno numbers?

[JaMan]

Does tire sizes affect dyno numbers or are they the same and the truck just slower?



Jared

 

[JaMan]

No one has any info on this?

 

[RKohn]

"shouldn't" make any difference

Since HP is basically a calulation of weight, distance, and time, changing the effective gear ratio might let you turn more weight, but you won't move it as far in the same amout of time, so no net change. But... . (knew that was coming, didn't ya) peak acceleration may be a little higher and depending on the program, you may see a little more with smaller rubber.



rick

 

[mattymac]

Some say that on the really big HP trucks that the dyno doesnt load enough and the bigger tires help load the motor making more boost and actually make more HP.

 

[DavidTD]

My experience so far has been it makes little difference. There is a limit though. When I dyno a truck with 44's, it will rob power. Tire weight is really what seems to makes the difference. 44" boggers are heavy and it will result in less HP but not 50 hp difference.

 

[jetenginedoctor]

David is 100% correct. The size of the tire/wheel isn't the determining factor. Rather, it's what the end rolling resistance of the wheel/tire/road interface. On an eddy current dyno such as ours, we can hold the rollers/tires/engine at a steady speed and directly measure the HP at the rollers. When we do so, there is no inertial losses from the drivetrain or tire/wheel weight, because there's no change in speed. On sweep tests (ShootOut style tests like everyone's used to seeing,) we can essentially tune out the effect of the drivetrain and wheel inertia, so the effect of a heavier wheel/tire combo can be compensated for.



On our dyno, your best HP numbers will come from using a very hard tire with stiff sidewalls (highway towing tire) rather than the big gumball Super-Swamper style off-road tires. A stiffer tire will deform less under a load, and will consume less power than a tire that produces a big wrinkle or bulge in the sidewall under load. We actually make much better traction with a narrower tire than with the wide off-road tires, because the off-road tires tend to deform not only the sidewall under load, but also the tread surface as well, causing the tire to slip-grab-slip-grab rapidly, manifesting itself as a howling chatter sound. Adding more air pressure to the tire usually helps this problem, but it could be avoided altogether by using a highway tire for testing.



The point of all this? If you come to one of our dyno events and want to make the best numbers you can, leave the Swampers at home in the garage and test with your highway tires.