E88 Tyre pressure if changing width question

[Fraserb64]

Hope you all had a great xmas

Hopefully nor repeating the same old questions again

I have gone through the various site and forums regarding increased widths etc.
so changing from the standard Front=215/40/18 and Rears=245/35/18
to Fronts=225/40/18 and Rears 255/35/18 seems like the largest non impacting change without affecting "speedos",offsets and clearances etc...

One thing i could find is what pressures the increased width Tyre pressure should be?
I am running 34-psi Front and 37-Psi rear,

Thanks

Brian

[nachob]

Brian, good morning. You posted this in the 1M forum and our cars are wholly different. We start out with 245 in front and 265 in rear. Our fenders are different as well as offsets, etc. I think you will have better success in the regular 1 series forum:
http://www.1addicts.com/forums/forumdisplay.php?f=228

With that said, I am a car person myself with several BMWs so I will throw out my 2 cents worth. Hope you don't mind.

You are dramatically altering the front and not so dramatically altering the rear width. You will get significantly more bite from the front so I would keep these pressures and adjust after. You will want a little more air pressure in the rear at first and 37 sounds right to me. One thing that worries me is that you are increasing the diameter of the rear slightly and adding a little more weight. While a car like the 1M with 320-369 feet of torque has more than enough to compensate, it might make your car slower. Tire/wheel weight affects smaller motors more and in this case a slight increase in diameter too.

You didn't mention what you wanted to accomplish. Is this for track, autocross and want fastest lap time? Or just for street where you want less understeer? You also didn't mention if you were using the same stock wheel widths or buying new wheels. If you will be using the stock wheels, the wider tire will be pinched and you will have a rounded contact patch. A thinner tire with a wider wheel sits flatter and can get more rubber on the road than a wider tire with too narrow a rim.

I just personally did this experiment with my 1M. I put the stock front tires on an 8 1/2" wide rim. While 8 1/2 is the recommended width, I could still feel the difference in handling and grip when going back to the 9" rim with the exact same tire. The wider rim pushes the edges of the sidewall down for a very even, flat contact patch.

In the end, it's only tires and you can always go back later so long as it doesn't damage your front fender but it seems like a very expensive experiment.

Finally, I had an E46 325i that had stock Touring tires on it. It was a blast to drive. I could launch it by spinning the rear tires slightly and kick the rear out. I loved it. Then I decided I wanted the best possible tires for it so I switched to Michelin Supersports. So now I could tell it would grip so much better on a high speed sweeper but it had so much grip on a low power car that I could no longer launch the car without it bogging down. It actually made the car less fun to drive.

Best wishes!

[Fraserb64]

Originally Posted by nachob

Brian, good morning. You posted this in the 1M forum and our cars are wholly different. We start out with 245 in front and 265 in rear. Our fenders are different as well as offsets, etc. I think you will have better success in the regular 1 series forum:
http://www.1addicts.com/forums/forumdisplay.php?f=228

With that said, I am a car person myself with several BMWs so I will throw out my 2 cents worth. Hope you don't mind.

You are dramatically altering the front and not so dramatically altering the rear width. You will get significantly more bite from the front so I would keep these pressures and adjust after. You will want a little more air pressure in the rear at first and 37 sounds right to me. One thing that worries me is that you are increasing the diameter of the rear slightly and adding a little more weight. While a car like the 1M with 320-369 feet of torque has more than enough to compensate, it might make your car slower. Tire/wheel weight affects smaller motors more and in this case a slight increase in diameter too.

You didn't mention what you wanted to accomplish. Is this for track, autocross and want fastest lap time? Or just for street where you want less understeer? You also didn't mention if you were using the same stock wheel widths or buying new wheels. If you will be using the stock wheels, the wider tire will be pinched and you will have a rounded contact patch. A thinner tire with a wider wheel sits flatter and can get more rubber on the road than a wider tire with too narrow a rim.

I just personally did this experiment with my 1M. I put the stock front tires on an 8 1/2" wide rim. While 8 1/2 is the recommended width, I could still feel the difference in handling and grip when going back to the 9" rim with the exact same tire. The wider rim pushes the edges of the sidewall down for a very even, flat contact patch.

In the end, it's only tires and you can always go back later so long as it doesn't damage your front fender but it seems like a very expensive experiment.

Finally, I had an E46 325i that had stock Touring tires on it. It was a blast to drive. I could launch it by spinning the rear tires slightly and kick the rear out. I loved it. Then I decided I wanted the best possible tires for it so I switched to Michelin Supersports. So now I could tell it would grip so much better on a high speed sweeper but it had so much grip on a low power car that I could no longer launch the car without it bogging down. It actually made the car less fun to drive.

Best wishes!

Hey Nacbob, thanks for reply (apologies for wrong forum) didn't notice to be honest.

I thought the changes were fairly even front and rear based on what i have read across on standard 1 series sites/pages?

I am after more grip at both ends the back is pretty good anyway as I have a Quaife ATB LSD fitted excellent in the dry but the "lights" still keep flashing in anything changeable even at sensible speeds. The standard tyres are very sensitive to pressure and surface changes (especially the fronts).

I have an old ford Galaxy that I can drive more confidently in changeable conditions than my 125 (without having to constantly tweak and change pressures). I think based on feedback across the various forums going from 215 to 225 front and from 245 to 255 rears will help enhance grip especially at the front.

So given the change in width but maintaining the same staggered rims does this require a different calculation for pressure given there is more volume to inflate.

I wont be doing this untill i need new tyres so wont be experimenting just yet


thanks for taking the time to reply

Have a great new year

Brian

[nachob]

You too, Happy New year and report back to the forum what you find out. You would be surprised how many others have the same questions and concerns.

Tire pressures are definitely the cheapest tuning anyone can do. When I used to run hot hatches like my old front wheel drive Scirocco Mk 1 and Mk 1 Jetta, you could do wonders by lowering rear pressure and raising front to make them very neutral front drivers.

Best wishes!