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Tyre Help Please


Hybrid_From_Hell
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Nige

There are quite big differences between different brands of tyre

Sit a 7.50R16 Michelin XS against a 7.50R16 Avon Rangemaster to see what I mean....

....probably about an inch difference in the height! maybe a bit more (don't have them handy to check but side by side you would never believe they were the "same size")

so a lot of the measurement depends on what sort of tyre you are using - and I think 7.50R16 is a "particularly variable size" - the metric sizes are more accurately specified - 205R16 is actually 205/80R16 and they shouldn't vary quite so much

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John,

The program I have works out speed at given 500 rpm jumps up to a preset max.

It does it for all gears and low and high box, and needs 1/2/3/4/5th Ratios, Diff Ratio, Circumference of tyre, and TBox Ratio...

I have worked out that I am approx 20% higher geared than a factory V8 on 205s, and slightly less if on 7.50s / 235s.

With the V8 it dioes sort of overcome the extra gearing, but it is a step maybe too far, and I think 1.222 will stay and I'll if I do change much will do P&Ps,

On a run 5th is a struggle up a moderate hill !, its since the simexs went on , it was fine on 7.50s / 235 grizzlys...

Nige

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On a run 5th is a struggle up a moderate hill !, its since the simexs went on , it was fine on 7.50s / 235 grizzlys...

Nige

That's 34" Simex, 300BHP and it struggles on a MODERATE hill :o

No wonder I need LT77 (0.830), LT230 (1.192), 4.7 R&P (4.75 wanted) and 35" Simex to match my bog std TDi.

Did go to measure some tyres (got 205s, 7.50 SAGs and 7.50 XS stacked in the garage) but its cold, dark and I'm going to have to pull them all out to get a tape round them.

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Yes, but the inflated circumference of the tyre will not give the correct values. You need the rolling radius or rolling circumference which is quite bit smaller.

Yep, but WTF am I going to get those !

I have looked at my Simex 34s, they are (off the 90) 107.5 inches circumference, versus the 7.50s 235s, and 205 etc, would you think the percentage difference of circumference versusus rolling circumference would be reasonable similar say 5-7% ?..........

Nige

Edited by Hybrid_From_Hell
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If you measure the loaded radius carefully, you can estimate the rolling circumference. Measure from the ground to the center of the drive member (on a flat surface). Multiply this by 2 and then by pi to get the rolling circumference.

A better way is put a mark on the side of the tyre where is touches the road and on the road. Have someone drive slowly forward and count out say 5 revolutions. Measure the road distance travelled. Make sure the road is fairly smooth and flat, say a parking lot.

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If you measure the loaded radius carefully, you can estimate the rolling circumference. Measure from the ground to the center of the drive member (on a flat surface). Multiply this by 2 and then by pi to get the rolling circumference.

What about the "caterpillar track" effect? :P

A better way is put a mark on the side of the tyre where is touches the road and on the road. Have someone drive slowly forward and count out say 5 revolutions. Measure the road distance travelled. Make sure the road is fairly smooth and flat, say a parking lot.

You'll actually get less than this on the road though because of rotational slip under power... You could work it out with GPS and an accurate tachometer... but for the purposes here the %age change ought to be the important bit - the actual ratio is academic.

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Hi Nige,

I have used the calculator on this site to very good effect:

http://www.4lo.com/calc/gearratio.htm

Just put the tyre diameter in inches.

Whilst it does not take into account differences between different brands of the same tyre, it is very useful for comparing between sizes and if comparing effects of different transfer ratios with the same size tyres.

For the purposes of the discussion my 235/85 X 16 Hankook MT tyres are 32.5 inches in diameter (102.05 inches circumference) compared to a dunlop 7.50x16 which measures 32 inches diameter(100.48 inches circumference).

Hope this helps,

Regards,

Diff

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