Higher tire pressure -> lower MPG??

[Hersbird]

What about centrifugal force throwing the tires out at higher speeds? You can definitely see that effect on Top Fuel dragsters. Then again 60 mph is past in a fraction of a second with those things.

[user removed]

On dragsters you don't have steel belts and the increased diameter is a part of the overall gearing.

regards
mech

[Donahue]

I would think that even if diameter expansion was to blame, I would still expect it to be more linear. It's that flat before the curve that has me curious. Its like comparing it to metallurgy. That curve is the beginning of the yield curve. I know they are completely different things, I'm just trying to get an idea of a similar physical process a material goes through that has such a drastic change.

[cyclopathic]

Originally Posted by CapriRacer

This says otherwise.



https://www.tut.fi/ms/muo/vert/7_raw...p_image008.jpg

And there is other data to support that RR is relatively the same until you hit about 60 mph, then it increases dramatically - which is probably where the video you looked at was coming from.

I am still not sure the graph above compares oranges to oranges. It shows that RR force increases with speed, but OP never argued this. The argument is that in the range of speeds tire used the increase is liniar, so driving one mile at 25mph and 50mph will require to overcome XYZ kJ of RR

[cyclopathic]

Originally Posted by Donahue

I'm curious of the physics involved in this. On the surface, you would think tire resistance would be linear. What is causing the curve, is some sort of internal dynamics on the tire causing the change? Is is something like heat, or a harmonic vibration or something? Do speed ratings have any effect on when that curve occurs?

I think what is going on that at higher speeds there more slippage as tire doesn't get enough time to bend into optimal shape (is it also a reason why too much pressure could be bad?) Or higher hysteresis at higher speed

Here is more on physics: https://en.m.wikipedia.org/wiki/Rolling_resistance

[jray3]

Originally Posted by cyclopathic

I think what is going on that at higher speeds there more slippage as tire doesn't get enough time to bend into optimal shape (is it also a reason why too much pressure could be bad?) ]

Amen on slippage. The iMiEV has skinny LRR tires that I run a bit tight (44 psi). High speed sweeper turns with the ASC turned off feel just fine with no howling tires and don't feel loose to me, but switch the safeties back on, and the nanny systems intervene! I didn't think the car was loose enough on a 60 mph turn to kick in the calipers, but the wheelspin sensors don't lie! Tire wear has also indicated slippage, as though the 145 mm fronts scrub hard in turns, it is the 175 mm rears that wear faster on this RWD car! Weight distribution is 45/55 and the rear has 20% more rubber, so that's proportional, but the relationship between load and traction is non-linear. Perhaps the relationships between load and wear and contact area and wear are also non-linear.

[CapriRacer]

Slippage?

I think there are 2 different concepts being discussed.

1) Slippage in the direction of travel. As tires are required to generate more forward force to overcome wind resistance, the tire has to slip relative to the direction of travel to generate that force. This MIGHT have implications on RR - BUT - when RR is measured in a lab, there is no wind resistance. The only thing being measured is pure RR. That's why I think the standing wave is the reason for the increased RR.

2) Lateral slippage - as in a turn. Another way to look at this is grip. And, yes, slippage due to loss of grip is going to affect the energy loss a vehicle experiences, but I don't think that is impacting what that graph is showing.

[cyclopathic]

Originally Posted by CapriRacer

Slippage?

I think there are 2 different concepts being discussed.

1) Slippage in the direction of travel. As tires are required to generate more forward force to overcome wind resistance, the tire has to slip relative to the direction of travel to generate that force. This MIGHT have implications on RR - BUT - when RR is measured in a lab, there is no wind resistance. The only thing being measured is pure RR. That's why I think the standing wave is the reason for the increased RR.

2) Lateral slippage - as in a turn. Another way to look at this is grip. And, yes, slippage due to loss of grip is going to affect the energy loss a vehicle experiences, but I don't think that is impacting what that graph is showing.

The cause of the splippage could be different but the process is the same: you have a segments of rubber in contact patch moving in relation to road surface and causing abrasion. Standing wave, lateral turn load, ?too much?/insufficient pressure could be causing it.

If an issue with wave was an issue, than there would have been a drop/increase at resonance frequency.

Wheels are rotation at relatively slow speeds to have the wave make all the way around wheel. At 60MPH wheels rotate at 700-900RPM.