94AWDcoupe

iTrader: (125)

that sag you see is from weight of the car. the weight of the car does not change at speed. so the sag you see when car is not moving is same sag you will see at 100mph.

actually you havent read the this thread. that .5 is showing 20-30whp change. pretty significant.

Fireescape

Hey
Yeah I understand what causes the sag in the tyre but I'm suggesting that this sag will be pulled out of the tyre, to some degree, by the centrifugal force of the rotating tyre at speed. To say that a tyre rotating at xxxx rpm as the car travels at 100mph is not effected by this is incorrect. The question is by how much and by how much this effect has on the accuracy of VD. Even the difference in air pressure in your tyre will have an effect. Lower your tyre pressure to 20psi and do the rotation measurement I mentioned early then pump it up to 35psi and repeat. I'd suggest there will be a difference in circumference.

94AWDcoupe

iTrader: (125)

Tires that dont have steel other suitable reinforcements do expand and can become bigger like you suggest. drag tires, RC tires etc. street tires simply dont. When you see the RPM data from tire manufactures do you see a * ? (* tested at xx MPH and xx air pressure) no you dont. because MPH doesnt change the rolling distance, and air pressure is assumed to be inflated correctly.

the sag in a radial tire does not get centrifugally straightened to any degree. and even if it did it still would not change the rolling diameter of the tread. tread rolls same distance inflated 20psi or 40psi. something you can prove in your own driveway. something I have already done.

mrm95

iTrader: (1)

Well some other factor is playing into that then. up to a 30wp loss is ridiculous. I mainly hear 5-10whp

94AWDcoupe

iTrader: (125)

doesnt seem you read the posts in here then. the VD does not use an accelerometer like g-tech. with g-tech you only need to enter weight being accelerated. with VD you need to enter correct gear ratios so it can calculate an accurate speed over time. if you dont enter correct tire size the mph will be off. 1/2 tire diameter throws the calculation off pretty dang much. thats why I took the time to make this thread. the VD numbers are only as good as the numbers being entered into it. with some people entering correct tire sizes and others entering incorrect sizes it makes comparing hp numbers inaccurate. kinda like comparing dynojet numbers to mustang numbers. useless if you ask me. they never read the same. now we have VD numbers that are useless to compare as well.

batty200

iTrader: (3)

I am going to do this tomorrow and see how it affects my power readings.

Fireescape

Centrifugal growth
A tire rotating at higher speeds tends to develop a larger diameter, due to centrifugal forces that force the tread rubber away from the axis of rotation. This may cause speedometer error. As the tire diameter grows, the tire width decreases. This centrifugal growth can cause rubbing of the tire against the vehicle at high speeds. Motorcycle tires are often designed with reinforcements aimed at minimizing centrifugal growth.

Straight from Wikipedia.

mrfred

iTrader: (50)

jerry understands what you are talking about, and i'm pretty sure that quoting wikipedia isn't going to change his opinion. the centripetal force can actually be quite large at 85 mph (top of third gear) - a 1/16th pie section of a 25" diameter tire weighing 25 lbs generates somewhere around 800 lbs of force, but this is a very simplistic calculation, and i think that tire construction limits the sidewall shape change as jerry said. if i had to bet, i'd say that the sidewall above the contact patch on a 235/45 tire increases in height by ~1/8" at 85 mph.

mrm95

iTrader: (1)

Apparantly you didn't read your own thread. If you didn't realize, both of those were in response to something I posted. Read your thread man