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Old 30th October 2006, 21:56   #1
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Here you have to be clear what u want from your car after the upsize.
The dynamics of a tyre upsize
As a whole increase in terms of diameter and wider contact patch would lead to lower accerlation as compared to say the orignal low sized dia tyre but conversely the top speed would increase as the wheel is covering a larger distance for each revolution of the crank but it would be doing the same slowly.One has to use wider tyres because there is no use of power if u cannot put it down on the road, it handles better both due to larger contact area and mostly enthusiasts prefer large dia tyres with low aspect ratio;ie;thin sidewalls that minimize tyre flex as there is less air which causes better handling and a poor ride because there is now a very thin column of air which can absorb shocks or flex .So one has to fix a compromise between accerlation ,handling and ride.A wider tyre would also decrease fuel efficiency because of the larger contact area or greater rolling resistance.
Purpose for using wider tyre=grip only on high powered cars
Purpose for using larger dia tyres =To ensure stability not to
make tyre rotate too fast;ie;beyond a certain rpm ask yourrself why do scooters with lower dia tyres are more unstable at high speeds as compared to large dia tyre motorbikes.
However CAUTiON a bigger tyre may also lead to an increase in weight of the tyre assembly as a whole leading to an increase in unsprung weight;i.e;weight not supported by the springs and hence u have to apply more effort to turn the steering and it would become more slow because engine has to do more work to rotate a tyre of greater mass;ie;why in formula 1 they use magnesium wheels to reduce unsprung weight.
Choosing the right size
Mostly unless we require the upgrade for special purpose for road use we try to keep the rolling diameter of the existing and replaced tyre the same.The following ilustrates how to calculate this
1.A number like this xxx/yy/Rzz .The xxx part represents the contact patch in mm .yy represents the represents the sidewall height in terms of the % of the contact patch .R represents radial and the zz corespods to the rim diameter in inches. get the rolling diameter covert xxx into cm to this add yy as the % of xxx in cm +zz in cm
For example if your tyre reads like this 175/70/R13
then 17.5cm+70% of 17.5 cm+13x2.54cm will give u the rollig diameter.
I hope now you will also consider the dynamic advantages of tyre upgrade and not go in for one only for the looks factor alone.
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Old 31st October 2006, 10:01   #2
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Old 29th September 2009, 12:18   #3
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Default Effect of tyre on performance….

I would like to know the effect on performance with changes in diameter and width. Let us consider any stock vehicle and change only 1 parameter – Tyre for my understanding.

Effects of using a wider track tyre maintaining the recommended diameter.
Effects of upsizing by 1 or 2 inches maintaining the standard width.

In the above cases, there are no changes in the profile of the tyre in the above scenarios (the ratio of tyre width % remains unchanged) or the thread pattern. Radials are considered and lets use these mainly for dry conditions – city roads and highways.

By effects, I mean:
  • Strain on the engine
  • Changes in top speed
  • Changes in pick-up
  • Mileage
Even if the “effects” mentioned above is not very significant, I would like to know the theoretical outcome of the changes.

I will jot down my comments:
A wider tyre would produce better grip resulting in better pick-up, cornering capabilities but lower top speed, and has a higher strain on the engine. By reducing the dia by an inch or two, the strain on the engine can come down. But, 1 rotation of the hub would result in .9 rotation of the tyre which in total brings the mileage down. Also the extra width would reduce the mileage by .5 Km to a liter.

Not sure if am right; please comment.

Can some veteran in this group throw some light on other probabilities also like increasing the dia and reducing the width and so on?

Last edited by Jaggu : 29th September 2009 at 12:20. Reason: Removing [Font] tags, please preview before submitting and avoid using external font editors. thanks
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Old 29th September 2009, 12:52   #4
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This is a myth that wider tyre = more contact patch = more grip. The reasons are completely different.

Please see this link -

Browser Warning

A picture to show that contact patch doesn't change with tyre width -

And please don't forget the tyre compound - this is the most important factor for tyre performance compared to tyre width, profile or diameter.
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