Rdriver

I have been thinking. Should I take the time to swap out wheels to get a baseline dyno run, or would that be a waste of time. If I have posted in the wrong forum sorry...

Thanks

Steve

Wesmaster

If it's a true baseline, and you're not looking for bragging rights, you should dyno on whatever you think you will race with or dyno with in the future. If you're trying to get a good number to impress your friends, then throw on the 16s.

Rdriver

How much will 18's hurt? I wonder if anyone has tried dynoing with 16's v. 17's v.18's??????

Uncle Fester

My guess is that as long as the overall diameter was maintained when going to 18 in wheels, there shouldn't be much of a difference. Of course, if the 18" wheels / tires weigh 100 lbs each, that might make a difference with dyno results... I'm exaggerating, but you get the point.

Wesmaster

[QUOTE]Originally posted by Uncle Fester
[B]Of course, if the 18" wheels / tires weigh 100 lbs each, that might make a difference with dyno results...

ejis

As far as dyno numbers/acceleration are concerned, weight of the wheels doesn't matter. Moment of inertia does. Moment of inertia can be thought of as an object's resistance to change of rotational motion - like mass is for linear acceleration. The further away from the axis of rotation a concentration of mass is, the slower it will accelerate for a given moment. Since a lot of a wheel's mass is concentrated at the rim, a bigger wheel will have a higher moment of inertia, even if they weigh the same (as long as the construction is similar). Even if your 18"s are lighter than the 16"s, they may have a higher moment of inertia, and would take away power to the ground.

If you want to know for certain, you can either do a spin test, applying a known moment to the wheels and measuring how quickly they spin up (a reverse dyno, or sorts) or calculate the moment of inertia.

The moment of the rim is:
m*R^2
- m = the mass of the rim portion of the wheel (you can estimate this by taking the thickness of the metal, multiplying it by the width of the wheel and by the circumference)
- R = the radius of the wheel

The moment of the spokes portion may be a little more difficult, depending on the design. One decent estimate could be:
0.5*m*R^2
- m = the mass of the wheel minus the mass of the rim
- R = the radius of the wheel, or to be more accurate, the radius of the dish portion.

Add these two together for the total. These are both estimates, of course, since the shapes are much more complex than this represents, but it may give you an idea if what effect the changes you make will have.

Austblue

I think that the primary objectives that are achieved from using a dyno are to tune, compare and/or brag.

As Wes said if you want some numbers to put against your car then throw on the sixteens. If you want to tune your car and as you say get a baseline chart that can be used to measure gains from future mods, then go with whatever you will be using in the future.

This will reduce the unknown variables that would have you making excuses later. Its grade 10 chemistry theory