I was wondering how deep the sidewalls are on 275/35/18 and 275/40/17 tires.

I measured the sidewall of my stock 245/50/16 tires and estimate 4 inches. From pictures I have seen the 18"s look to be about 2 inches but I would like confirmation.

I need to get an idea of how harsh the ride will be in order to help me pick a wheel size. The roads around here are a bit uneven!

 

The sidewall height is defined as the section width X aspect ratio.

In a 275/40-17, the section width is 275mm = 10.83". The aspect ratio is 40/100 = 0.40

10.83 x 0.40 = 4.33" sidewall height.

If you ran a 275/35-18 to get a similar (within 1/10") outside diameter:

10.83 x 0.35 = 3.79"

In effect, the sidewall is about 1/2" shorter on the 18" tire. This can vary slightly from manufacturer to manufacturer, and even among different models from the same manufacturer.

 

How much sidewall is exposed when this tire is mounted on a typical 18" rim?

I've just been out and measured my 245/50/16, the measurements confirm the math that 245mm (9.75") x 0.50 = 123mm (4.88") Thats from the bead to the outside of the tread. The exposed sidewall of this tire from the rim to the start of the tread is 3" (4" including the tread).

The 3.79" you quoted, minus an inch for tread, and another inch for the sidewall thats hidden behind the rim lip leaves less than 2 inches of useable sidewall, is this correct?

 

Your use of the terms "sidewall" and "tread" is confusing. The sidewall dimension is the distance from the very inside of the bead to the rolling surface of the tread. How much of the "tread" wraps onto the sidewall is going to vary with the design of the tire and tread.

I gave you the dimension from the bead to the rolling surface of the tread. I would suspect, from memory, that the rim bead retaining flange covers up about 3/4" to 1" of the sidewall. So the visible sidewall is going to reflect that "subtraction", which is going to be the same amount subtracted no matter what the diameter of the wheel.

Point is, if you use a rim that is 1" larger, and you keep the outside diameter of the tire the same, you are going to reduce what can be seen by 1/2".... that's simple math. How much is plain sidewall and how much is wrap-around tread is something you have to measure yourself. Just because the tread pattern wraps around the sidewall, I don't think that renders that part of the sidewall as "not useable". It is still there to absorb the shocks.

 

Thanks for your input, I was confusing myself a bit there!

You have answered my question, thank you, and now I have to decide wether to go 17" or risk 18" on these roads, the sidewalls on the 18"s are pushing the limits a bit and I dont want to regret spending the money afterwards if I don't like the ride quality!

 

Hard to believe that the roads in London and vicinity are any worse than the roads in New Jersey.... they seemed a lot better than our roads, the few times that I have been there.

 

Inner London maybe, but in the outskirts the local authorities are not too quick off the mark at repairing potholes, the roads around here are a mess, especially the minor country roads