[Scubadriver]

My question builds on the very interesting tire size discussion in the "M235i Modified" thread. Specifically, this post:

http://www.2addicts.com/forums/showp...2&postcount=53

I didn't want to hijack that thread so I thought I would start a new one. I'm very much hoping that Apex Eddy will chime in on this. I would have PM'ed him, but I thought this would be interesting and informative for others.

In that thread Eddy recommends 18x8.5" ET45 Apex EC-7 with 235/40/18s on the front and 18x9.5" ET58 Apex EC-7 with 265/35/18 on the rear. I think these look great and may look even better with lowered springs. I would get anthracite...though I'm already getting a bit tired of people telling me how much brake dust my car generates when they see the gray stock wheels.

So my question is, if you go to non-spec, wider tire sizes like those above, how does one determine appropriate inflation pressure?

Thanks.

Bob

[Scubadriver]

And by the way, you gotta love Apex's photostream:

https://www.flickr.com/photos/apexra...h/14906107741/

[Eddy@ApexWheels]

Originally Posted by foglesre

My question builds on the very interesting tire size discussion in the "M235i Modified" thread. Specifically, this post:

http://www.2addicts.com/forums/showp...2&postcount=53

I didn't want to hijack that thread so I thought I would start a new one. I'm very much hoping that Apex Eddy will chime in on this. I would have PM'ed him, but I thought this would be interesting and informative for others.

In that thread Eddy recommends 18x8.5" ET45 Apex EC-7 with 235/40/18s on the front and 18x9.5" ET58 Apex EC-7 with 265/35/18 on the rear. I think these look great and may look even better with lowered springs. I would get anthracite...though I'm already getting a bit tired of people telling me how much brake dust my car generates when they see the gray stock wheels.

So my question is, if you go to non-spec, wider tire sizes like those above, how does one determine appropriate inflation pressure?

Thanks.

Bob

Bob,

Tire pressures and how precise you are with them will depend on what you're using the tires for. On the street you'll experience a range of pressures due to the wild temp changes throughout the seasons and even the day, so no matter where you start in the day there will be a change. At the track you monitor them more closely.

On the street you may run higher pressures to get better gas mileage, or a bit lower to soften the ride. On the track tire pressures are tire brand and application specific as you're optimizing for what feels right to you, and from what you're visually seeing on the sidewall for tire wear.

If i had to throw a single catch-all number out there, I'd set street tires to 38~ psi

I'm sure you'll get many other opinions.

[trkshoe]

Way to chime in Eddy... that's what I call 'support'

[Scubadriver]

I was hoping there might be a bit more science to it... It's all about the size and shape of the contact patch, right?

In my mind, as you widen the tire while keeping diameter the same, your goal would be to increase the width of the contact patch. To do so, you would need to decrease the pressure (as the pressure in the tire times the area of the contact patch is producing the force to support the car). I must admit though...it's been a long time since I drew a free body diagram...

[Scubadriver]

Here's a pretty good discussion on the topic:

https://www.physicsforums.com/thread...debate.330790/

[Eddy@ApexWheels]

Originally Posted by foglesre

I was hoping there might be a bit more science to it... It's all about the size and shape of the contact patch, right?

In my mind, as you widen the tire while keeping diameter the same, your goal would be to increase the width of the contact patch. To do so, you would need to decrease the pressure (as the pressure in the tire times the area of the contact patch is producing the force to support the car). I must admit though...it's been a long time since I drew a free body diagram...

There is a lot of science to it, but it still starts with a simple question: What is the desired outcome? Will you be using these on the street or the track?

Without a benchmark there is no correct PSI. Once you know what your goal is, then you measure the results and fine tune your PSI to your specific application as your application, specific car, tire brand/model are all just as much of a factor in what you'll end up running if you want a very specific number.