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Old 06-10-2008, 04:53 PM   #108 (permalink)
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Originally Posted by SuperTrooper View Post
I was using a conclusion contained in the Finnish study of RR on roads:

"2.1.7 Size of Tire
Both the radius of the tyre and the aspect ratio H/W influence the rolling resistance. Rolling resistance can be reduced by increasing the radius and decreasing the aspect ratio of the tyre. Lower aspect ratio means increased stiffness of the sidewalls and thus lower hysteresis losses."

By taller I meant a larger radius. The trick is finding a larger radius tire with a lower aspect ratio (on a larger wheel) that doesn't widen the tire.
Unfortunately, our Finnish friend is wrong.

First, it wasn't a conclusion. It was in the introduction and he was referencing another paper - one on vehicle modeling - and I'm sure he misinterpreted what was written. I'm in the process of locating a copy of the reference paper in order to understand what where he went wrong.

Second, if you are stuck with a certain vehicle - as opposed to designing one from scratch - you are going to have trouble with the size / aspect ratio thing. The fenderwells limit your ability to go with larger diameter tires. Out another way, you can't go with a larger diameter tire and go lower in aspect ration without also affecting the load carrying capacity - and as we can see in the graphic that Tasdrouille posted - load carrying capacity (as evidenced by the tire size) greatly affects RRC.

Originally Posted by tasdrouille View Post
RR is mostly about hysteresis. Reduce hysteresis and you reduce RR. Since roughly 80% of the hysteresis in tires happens in the sidewalls, make the sidewalls stiffer and RR will go down. The easiest way to do so is to increase the pressure. Btw, if you're adding material to make the sidewalls stiffer (same material, just ticker sidewalls), that won't work nearly a well as added pressure. Even though it'll flex less, more material will be flexing so overall hysteresis will pretty much stay unchanged.

Re tread width. It's not about skinny or wide, it's about finding the right contact patch size. Long contact patches from skinny tires increase RR due to increased hysteresis. Make the tread wider and RR goes down, up to a point where RR goes up again.

You can validate that in the graph posted earlier.
I'm afraid you are mistaken in this. I think the source of the misunderstanding is that you are trying to look at too big a picture.

If you imagine a tire with a given load on it - and you can change the tire, but you can't change the load carrying capacity - then narrow treaded tires work better - less volume of rubber moving = less hysteresis.

BTW, the graph posted earlier is RRC (Rolling Resistance Coefficient) and not RR (Rolling Resistance) - and that might be part of the confusion.

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