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Old 08-28-2012, 06:09 AM   #11 (permalink)
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Quote:
Originally Posted by mcrews View Post
You need to define 'smaller'

What you mean is narrower. Smaller means smaller diameter.

Smaller would prove for worse mileage.
Narrower would provide such a small gain that I would not concider the lose of handling and comfort.
Have you aired up the tires to 40-45psi?
Sorry, my fault. I meant narrower tires, with same diameter.

Both tires are stock recommended sizes. 175/80R14 is the narrowest regular size for this car, 195/65R15 is the most sold tire in germany, and also stock recommended. In size 175 there are only an few different tires to buy, but in 195/65R15 there are about 300 different profiles and a lot of LRR-tires to buy.

In the moment im driving very bad all-weather tires from the last owner of my car. So i need to buy some new tires and rims for summer.

Ill drive the tires at maximum sidewall pressure, but actually the mounted tires are so bad, i wont try it with them...

Kind regards

Benny

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Old 08-28-2012, 09:45 AM   #12 (permalink)
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Bigger is better!

The best information I have been able to dig up is that bigger is better - no matter how you define "Bigger".

That said, the effect is small - especially compared to the differences between tires.

In other words, spend your time researching tires and very little researching tire size - but if you have the opportunity, go larger.
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Old 08-28-2012, 11:47 AM   #13 (permalink)
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Not a direct comparison. However my 95 accord with 185/70/14s and lowed CoD does not roll with EOC nearly as well as my 98 Cavalier with a high CoD and 195/65/15s. Both are around 50psi in the tires. Also, my accord is a death trap in the rain with those skinny tires and no ABS.
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Old 08-28-2012, 03:37 PM   #14 (permalink)
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wasn't there some one that tested the idea of using 4 space saver tires on a auto and lost some mpg.
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Old 08-28-2012, 03:47 PM   #15 (permalink)
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Quote:
Originally Posted by trooper Tdiesel View Post
wasn't there some one that tested the idea of using 4 space saver tires on a auto and lost some mpg.
Oh please dont bring that back.....It didnt work/wont work.
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Old 08-28-2012, 04:05 PM   #16 (permalink)
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I compared the rolling resistance of 4 space saver donut spares on the car against various other tires in my fleet of various sizes. The donuts were the worst of the lot: http://ecomodder.com/forum/showthrea...uts-19094.html



from: http://ecomodder.com/forum/showthrea...e92-19126.html
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Old 08-28-2012, 05:43 PM   #17 (permalink)
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Barry/Capri, can you please help me understand the coefficent of RR versus the RR force?

To me it seems that we have a fixed weight (to a close approximation) of vehicle, so we would be putting a fixed load on a tire. I would also assume (and it may be a bad assumption) the same inflation pressures in each case. And for comparison we also need to assume the same construction, rubber compounds, etc. In that case, the lower RR force for narrower tires would seem to mean that those tires would coast farther.

Why is it the coefficient, which you have to multiply by the load, which actually shows you which tires would coast further?

-soD
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Old 08-29-2012, 09:46 AM   #18 (permalink)
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Quote:
Originally Posted by some_other_dave View Post
Barry/Capri, can you please help me understand the coefficent of RR versus the RR force?

To me it seems that we have a fixed weight (to a close approximation) of vehicle, so we would be putting a fixed load on a tire. I would also assume (and it may be a bad assumption) the same inflation pressures in each case. And for comparison we also need to assume the same construction, rubber compounds, etc. In that case, the lower RR force for narrower tires would seem to mean that those tires would coast farther.

Why is it the coefficient, which you have to multiply by the load, which actually shows you which tires would coast further?

-soD
Rolling Resistance Force (RRF) = Rolling Resistance Coefficient (RRC) X load on tire.

When tires are tested for RR, they are tested at a particular load and inflation pressure. The result is a Force. If you divide by the test load, you get a dimensionless number (a coefficient) that applies to that particular test fixture and that particular test procedure at that particular inflation pressure.

If you want to compare tires, you need to run them all at the same time, at the same place, and under the same conditions.

Questions:

1) Do the values vary according to the test fixture? Yes, but a correlation between test fixtures (and therefore test facilities) could be done so they could be compared. I'm not going to go into what it would take for that to happen but it is considerable!

2) Do RR values vary according to the test method? Yes, but a correlation between tests can also be done. Again a considerable coordinated effort would need to take place.

3) Is RRC constant for a given tire? No. Not only does it vary according to inflation pressure, but it varies according to load. The best information I have is the variation due to load is small enough to ignore (but it is something to keep in mind)

Coming back to the same tire in different sizes, I think the reason the data shows that "Bigger is Better" is that an increase in tire width of (say) 10mm increases the width of the tread a fraction of that (say 7 mm), but the increase in load carrying capacity is pretty close to the increase in width. The net effect is that there is slightly less material in comparison to the load.

And did I mention that while the sidewalls only play a small role in RR, every tire only has 2. Changing the tire size doesn't change how many of them there are.
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Old 08-29-2012, 11:15 AM   #19 (permalink)
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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.
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Old 08-29-2012, 11:19 AM   #20 (permalink)
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Short answer: it depends.

You can find a good discussion of aero losses vs tire width in this thread: http://ecomodder.com/forum/showthrea...g-cd-7475.html

You can also use the aero/rolling resistance calculator to estimate effects of changes to Cd/A & RRC from tire changes: http://ecomodder.com/forum/tool-aero...resistance.php

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