Wheel size/weight effecting dyno numbers?

[JamieP]

Ive read a few things on the net over the years saying this effects numbers greatly.

 

Opinions? anyone ever tested the theory?

Edited September 4, 2008 by JamieP (see edit history)

[n boost]

Just stick to a dynapack/hub dyno and u wont have a problem lol!

[dandan]

...

Edited September 5, 2008 by dandan (see edit history)

[bolarbag]

I've read something similar in one of my books, and not to dyno with slicks as they warp,

 

An accurate hub dyno in 5th gear would be as accurate as you would get in terms of chassis dyno

[dandan]

Ive read a few things on the net over the years saying this effects numbers greatly.

 

Opinions?

 

Running on a "regular" rolling road is subject to all sorts of external influences for the power @wheels figure. Wheel size will have an effect (think of it as gearing). Whether you'd ever notice/record the effect with all the other inconsistencies is very arguable especially given the max difference in rolling radius you can actually achieve in practice on your Supra with those big brakes. Tyre pressure will have an effect, tyre grip will have an effect etc etc. Wheel weight...I guess the greater the wheel weight, the greater the load you are "putting on the engine" which (especially on turbo cars) could have a positive effect on making the bottom end torque a little better.

...anyone ever tested the theory?

 

Almost. This was many many years ago on small capacity race prepped NA engines.....but rather than wheel size I was experimenting with final drive ratios. I've never really given the wheel weight question any thought to be honest.

 

Just stick to a dynapack/hub dyno and u wont have a problem lol!

 

Everybody knocks Thor as their numbers are high but if you want back to back consistency between visits then an engine dyno would be first choice, a hub dyno (like Thor) would be second, then after that you're into "rolling roads" and my first choice there might well be a Mustang Dyno and then a Dyno Dynamics setup like SRR.

[SimonB]

There's a lot of rubbish talked about rolling roads. It's true that the power at the wheels figure is pretty meaningless. That's because the friction between the tyre and roller depends on lots of things, largely how many rollers there are (one or two) and how hard the car is strapped down and at what position on the roller. That is why rolling roads do a coast down at the end of a run - this is used to work out the losses so you get an accurate flywheel figure. People still seem to think that the "at the wheels" figure is the accurate one though, despite this being basically wrong!

 

The thing that can affect how accurate they are is the fact that the tyre can squat down and distort when it's under power compared to when the coast down is done.

 

Hub dynos may be repeatable, so great for comparing things, but not much good for absolute power figures as you can't do a coast down on them so you have no idea what the losses are and have to guess to produce a flywheel figure.

 

Of course the other thing that affects the final figure is the fudge factors that are applied to compensate for air temperature and pressure in the dyno cell. Unscrupulous operators can stick in a stupidly high ambient temp for example - the software will then increase the power reading to compensate as there's a standard temperature that power readings are supposed to be measured at.