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Old 08-29-2012, 10:55 AM   #21 (permalink)
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Originally Posted by Lbar View Post
Also, my accord is a death trap in the rain with those skinny tires and no ABS.
If your tread patterns were similar, narrower will typically work better in the rain and snow as there is more weight per unit area, displacing the water more effectively.

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Old 08-29-2012, 05:41 PM   #22 (permalink)
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Originally Posted by CapriRacer View Post
When tires are tested for RR, they are tested at a particular load and inflation pressure. The result is a Force.
Perhaps it's because I'm unreasonably dense, but isn't the Force the thing that directly affects coasting?

Are you saying that we can never assume that the load is the same from tire to tire, even on the same car and if we set the same pressures?

I think, that if I am looking for tires to put on one specific car (e.g., my own car) that if I am allowed to set whatever tire pressure I want, the load would be the same no matter what the specific tire was. So in that case, the tire with the lowest measured force resisting movement would be the best one for efficiency.

Can you please show me where I am getting this wrong?

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Old 08-29-2012, 05:58 PM   #23 (permalink)
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^^^ That sounds correct. The business about a particular load and inflation pressure is to gain the ability to compare tires directly, independent of the application.
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Old 08-30-2012, 06:41 AM   #24 (permalink)
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Quote:
Originally Posted by some_other_dave View Post
Perhaps it's because I'm unreasonably dense, but isn't the Force the thing that directly affects coasting?
You're right - it's the RRforce that actually causes the drag and needs to be overcome to move the vehicle.
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Old 08-30-2012, 09:35 AM   #25 (permalink)
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Quote:
Originally Posted by some_other_dave View Post
Perhaps it's because I'm unreasonably dense, but isn't the Force the thing that directly affects coasting?

Are you saying that we can never assume that the load is the same from tire to tire, even on the same car and if we set the same pressures?

I think, that if I am looking for tires to put on one specific car (e.g., my own car) that if I am allowed to set whatever tire pressure I want, the load would be the same no matter what the specific tire was. So in that case, the tire with the lowest measured force resisting movement would be the best one for efficiency.

Can you please show me where I am getting this wrong?

-soD
Let me put it like this:

- and for the purposes of discussion and simplicity, I am going to confine this to passenger car tires - standard load only.

When tires are tested for RR, the test method will describe the testing conditions. Typically the pressure is specified (remember, we are talking about SL PC tires!) - say 30 psi.

What is also specified is the load. Since the rated load on a tire varies by tire size, this is usually expressed as a percent of some value in the load table. Larger tires will have more load than smaller tires, but the way it is calculated would be the same.

So when you try to compare tires of difference size, since they were tested at different loads, the RRF values are NOT comparable. The only way to compare tires of different size is by RRC - which takes the load out of the picture.

You should also be aware that RRF varies by inflation pressure - and to my knowledge, no one has developed a formula that covers every tire to account for changes in inflation pressure. Put a different way, there isn't a standard way of comparing tires tested with different inflation pressures.
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Old 08-30-2012, 09:44 AM   #26 (permalink)
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Quote:
Originally Posted by rmay635703 View Post
I think what is being lost here are the effects of rolling mass and aero.

So when driving down the road which trumps which
Rolling Mass + Aero or Rolling resistance by itself.

Obviously crap tires made of bias crap compounds won't score well.
Compared to the weight of the vehicle, the mass of tires is a tiny fraction - it's impact would be minimal.

The difference in inertia between tires is also fairly small - especially when you consider the inertia of other rotating components.

Aero? The difference in aero drag due to tire width is also pretty small - and I've been able to demonstrate that the difference in RR due to width more than offsets the change in aero.

Bottomline: You can neglect everything else if you want to talk about tire size and its affect on RR. But you also have to consider that tire size has a small affect compared to the difference between tires (meaning make and model). Spend your time worried about tire make and model and not so much about tire size.
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Old 08-30-2012, 07:43 PM   #27 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
... Since the rated load on a tire varies by tire size, this is usually expressed as a percent of some value in the load table. Larger tires will have more load than smaller tires, but the way it is calculated would be the same.
So what you're saying is that tires of different sizes are being loaded with different loads (as expressed in lbs of force), because those are the same percentage of the load value in a table somewhere.

OK, that makes sense of the results--one of the assumptions I was making was invalid, so my reasoning fails at that point.



Quote:
You should also be aware that RRF varies by inflation pressure - and to my knowledge, no one has developed a formula that covers every tire to account for changes in inflation pressure.
True, but a limited number of examples tested have shown us that RRF drops as pressures rise. The relationship is non-linear to be sure, but it does seem to be monotonic in the range from typical placard pressures to the sidewall pressure. It's definitely not a complete model by any means though, and it is quite possible that there are exceptions.


Anyway, thanks for clearing up my understanding of the data!

-soD
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Old 09-04-2012, 08:13 AM   #28 (permalink)
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Originally Posted by CapriRacer View Post
The best information I have been able to dig up is that bigger is better - no matter how you define "Bigger".
Its true the real tests we have all show slightly bigger is almost always better.
I went to a slightly wider and 2 inch taller tire on the suburban I was fully expecting fuel milage to go down, but it stayed the same and may have even gone up slightly.

Wasn't there a test were some one on here with a metro put bigger tires on the front (where all the weight is) and doughnut tires on the back and picked up like 4mpg?
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Old 09-04-2012, 10:14 AM   #29 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
Wasn't there a test were some one on here with a metro put bigger tires on the front (where all the weight is) and doughnut tires on the back and picked up like 4mpg?
I run donuts on all 4's of my 1980 commuta 125r13's and my rolling distance is nearly double that of a legitimate 155r13 (but I am only using a very small percentage of the donut tires load rating)

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Old 09-04-2012, 11:27 PM   #30 (permalink)
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If you can pull those from the junk yard new, all day for $10 or $20 each, its unbeatable.

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