Aspect Ratio Question

[Desertnate]

I have been following JST's search for the proper tire side for eductional purposes here
(JST's Tire Search (http://www.bimmerfest.com/forums/showthread.php?t=56563)), and it got me thinking.

EXAMPLE: 225/40/18 front 245/35/18 rear (strictly for purposes of this discussion)

When going to a staggered set up, why is the rear tire a lower profile? I realize that when increasing the size of the diameter you change profile to keep the overall diameter roughly equal to what it would be on a smaller wheel. I also am familiar with the philosophy of a staggering.

What I have not caught on to after reading the thread above and others like it, is why you would go to a smaller diameter set up on the rear since you have the same diameter wheel, yet less sidewall?

My uneducated brain would tell me that you would buy a set up with equal sidewall profile and stictly adjust the width to match your handling tastes/objectives.

I am missing a piece to the aspect ratio puzzle? :dunno:

[doeboy]

BMW chose to put slightly smaller diameter tires on the rears... I don't know why... :dunno:

When I got my wheels, I went with 225/40/18 in front and 255/35/18 in the back.

As to why the aspect ratio number is lower in the rear though is becasue aspect ratio is a percentage of the tire's width... so.... the amount of sidewall height doesn't change much... it's the ratio of it to the tire's width... so to keep the same sidewall height as a tire gets wider, the aspect ratio goes down.... Is that what you were trying to figure out?

[Mr. The Edge]

I have been following JST's search for the proper tire side for eductional purposes here
(JST's Tire Search (http://www.bimmerfest.com/forums/showthread.php?t=56563)), and it got me thinking.

EXAMPLE: 225/40/18 front 245/35/18 rear (strictly for purposes of this discussion)

When going to a staggered set up, why is the rear tire a lower profile? I realize that when increasing the size of the diameter you change profile to keep the overall diameter roughly equal to what it would be on a smaller wheel. I also am familiar with the philosophy of a staggering.

What I have not caught on to after reading the thread above and others like it, is why you would go to a smaller diameter set up on the rear since you have the same diameter wheel, yet less sidewall?

My uneducated brain would tell me that you would buy a set up with equal sidewall profile and stictly adjust the width to match your handling tastes/objectives.

I am missing a piece to the aspect ratio puzzle? :dunno:

you're not using a smaller diameter on the rear tire. The ratio number represents the height of the tire's profile in relation to the width of the tire.

So, the rear tire is usually a lower ratio simply because it is also wider. (staggered setup)

[Kaz]

the Aspect Ratio is a percentage. The height of the sidewall is determined by the width of the tread (225mm, 255mm, etc.) multiplied by the aspect ration in % (0.40, 0.35, etc.). THis means, that to keep the overall sidewall height the same, the wider tire must have a lower percentage aspect ratio.

225x0.40=90mm
245x0.35=85.75mm

Double these numbers (there is a tire sidewall on bottom and top of the wheel, remember), and add these to the wheel diameter (with the proper metric/english units) and you get the overall wheel/tire height.

In the above example, the overall height difference is 8.5mm, which, IMO, is on the high side.

[Mr. The Edge]

also, play around with this tire calculator to see how changing each # affects the overall diameter of the tire

http://www.miata.net/garage/tirecalc.html

[doeboy]

the Aspect Ratio is a percentage. The height of the sidewall is determined by the width of the tread (225mm, 255mm, etc.) multiplied by the aspect ration in % (0.40, 0.35, etc.). THis means, that to keep the overall sidewall height the same, the wider tire must have a lower percentage aspect ratio.

225x0.40=90mm
245x0.35=85.75mm

Double these numbers (there is a tire sidewall on bottom and top of the wheel, remember), and add these to the wheel diameter (with the proper metric/english units) and you get the overall wheel/tire height.

In the above example, the overall height difference is 8.5mm, which, IMO, is on the high side.

Yep... 255x0.35 = 89.25mm which is much closer. :)

[·clyde·]

The height of the sidewall is determined by the width of the tread (225mm, 255mm, etc.)

Actually, that width measurement is the section width (widest point of the carcass when mounted on the reference size wheel), not the tread width.

[Desertnate]

Wow, thanks guys...

I had used the tire calculator to work on changing the entire size of the package (i.e. moving from my 16s to 17s), but never used it to compare in a front/rear set up. Very interesting.

I was more ignorant of tire measurements than I thought. I had always been under the impression that the section width was based on the width of the rubber on the road (not too far off it seems) and that the profile was measured as a width of the sidewall (why I thought the diameter would change), not as a ratio. Now it makes more sense, and running the scenarios on the tire calculator demo helped out a great deal.

[Pinecone]

A couple of things, 1 is to check the info on the Tire Rack site.

For most tires they have overall diameter, and more improtantly revs per mile.

You will find that not all 225 tires are really 225, it is just they in between 215 and 235. Also the aspect ratio of 40 doesn't always mean exactly 40%, but not far enough to either side to be 45 or 35.

This varies from brand to brand and even model to model. For real fun compare the size of R-comps like Hoosiers to normal street tires. The sizes are not comparable. :)

Check these out:

Michelin Pilot Sport

225/45-18 25.8" diamater
245/40-18 25.8" diameter
255/40-18 25.9" diameter
255/40-18 26.0" diameter (second line for slightly different tire)

Revs per mile for the tires in the same order.

804
805
770
803

So the first two and the last one are basically identical in rolling radius, the third one is strange, but the two do have different load ratings.

[Desertnate]

Could that 770 be a misprint? I couldn't imagine a load rating having that much difference from a tire of the exact same size. Even if it does bulge more due to the weight. Even then I would think they would adjust the recommended pressure to make it ride with a similar profile to a tire with a lesser rating :dunno:

The points your brought up regarding section widths combined with the comments Gary had yesterday about wear ratings makes me think that the tire world is more voodo magic then science. The tolerances are pretty wide.

[Pinecone]

Not sure, but if you look at a bunch of tires, you will find similar things.

Well in a lot of ways it makes sense to use standard markings, otherwise you would have hte unwashed masses going nuts because their car came with 228 tires and now someone put 226 tires on it. :)

[BradS]

And one other variable to really drive you nuts, because there is no way to measure it until you mount the tires on the vehicle, is static loaded diameter--the diameter of the tire on the vehicle. Differring inflation pressures, sidewall heights, and carcass stiffness all play into how tall it will actually be on your car with its weight.

Unless you are trying to jam the absolute biggest tire you can under your ride, this measurement may be of only academic interest.

[Pinecone]

Actually several tire brands do give you the loaded radius. And you can back calculate from the revs per mile also.

[BradS]

Actually several tire brands do give you the loaded radius. And you can back calculate from the revs per mile also.


Cool. Is that with a set vehicle weight? Or doesn't it vary too much depending on if your vehicle weighs 3000 or 5000 lbs?

[Pinecone]

Well one would assume that it is based on a weight within the load rating for the tire. :)

I have no idea which part of the range, but I would suspect the upper range. But figure that the number would only be vlaid for comparisons within a brand of tire.