Dyno's and Wheels

[l__i__l]

i'm hoping someone can clear this up for me. How do wheel size affect dyno readings? I understand the weight of the wheel has less effect compared to the diamater. why is that? Is it becasue the final gear ratio is changed?

if i was to dyno with 18's how much roughly will the dyno vary when compared to 17's? I know it's impossible to estimate but i wanna know how much of a factor wheels play in dyno readings.

assuming they both weigh the same am i to guess right that the 17's will dyno higher then the 18's? Or will 18's give a higher reading due to the less work required for 1 revolution?

[noodleman]

i put a stock jazz on the same dyno before

15inch = 64kw
16inch = 47kw

[l__i__l]

wow that's a pretty big difference! 17kw between an inch! were the weights similar??

[air23box]

weight dun affect dyno figure at all....the major factor is the size of the rims.....on dyno they try to run the gear as similar to 1:1........so different car with different gear box may run different gear.......by changing ur rims is equal to changing the gear ratio so the reading is different.

[tinkerbell]

weight dun affect dyno figure at all....the major factor is the size of the rims.....on dyno they try to run the gear as similar to 1:1........so different car with different gear box may run different gear.......by changing ur rims is equal to changing the gear ratio so the reading is different.

but weight of rims do affect the engine power? heavy rims = less acceleration... thus affecting dyno reading?

[air23box]

but weight of rims do affect the engine power? heavy rims = less acceleration... thus affecting dyno reading?

Depends on how much is the difference......it does affect in some way but not as much as the size I reckon......

[RichyR]

stickiness of the tyres and tyre pressures affect the dyno readings as well

[ChargeR]

If a dyno was a perfect device then changing the rolling diameter of the wheels/tyres wouldn't change the power output as long as the rotational moment of intertia of the wheel/tyre package remained the same.

Changing to a larger wheel invariably brings with it an increase in the moment of inertia (the resistiance of the wheel to being accelerated rotationally sorta) because even if the wheel is lighter the mass is further from the axis of rotation. So a car with 18" wheels generally should dyno lower than a car with 17" wheels if the mass of the wheel and tyre remain constant.

[benson]

There are alot of variables that can affect dyno readings. If you want dyno numbers, pump up the tyres, run street tyres, and have little to no camber.

I know for our car we dyno the car as it runs on the circuit more than -ve 3 camber, 225/50 semi slicks, 8inch wide rims. Imagine in street trim

[l__i__l]

If a dyno was a perfect device then changing the rolling diameter of the wheels/tyres wouldn't change the power output as long as the rotational moment of intertia of the wheel/tyre package remained the same.

Changing to a larger wheel invariably brings with it an increase in the moment of inertia (the resistiance of the wheel to being accelerated rotationally sorta) because even if the wheel is lighter the mass is further from the axis of rotation. So a car with 18" wheels generally should dyno lower than a car with 17" wheels if the mass of the wheel and tyre remain constant.

that's interesting shouldn't dyno's be able to correct for veichle inertia?

[rvong]

correct me if i'm wrong.

i was told that smaller rims would read a higher kw output on a roller dyno. also having stiff settings on your suspension plays a part aswell. but then again i know someone who has more mods than i do yet had a lower kw reading with smaller rims.

[DC2RKID]

just do hub dyno ...

[fatboyz39]

correct me if i'm wrong.

i was told that smaller rims would read a higher kw output on a roller dyno. also having stiff settings on your suspension plays a part aswell. but then again i know someone who has more mods than i do yet had a lower kw reading with smaller rims.

Somewhat true. Rim size, camber, tyre pressure plays apart in dyno readings. Also methods of strapping it down plays a role.

[rvong]

just do hub dyno ...

hub dyno always reads higher. i read higher by 5kw on the hub dyno.

[DC2RKID]

at least u know there correct....no loss through traction