195/60r15 tires on 7.5" wheels

[JHZR2]

Hello,

In response to the 196/65r-14 thread and my own desire to look into "upgrading" my wheels to an OE lookalike that may enhance performance a bit, Im looking into getting a set of 15" wheels to replace my 14" ones. Because of the nature of my other questions, Ill post another thread, as this one is purely a rim width/tire size question.

My issue is as follows: the correct, direct fit 15" wheels for my 91 BMW 318i have a 7.5" width. However, the tires that I would prefer, p195/60r-15, generally are specced for 5.5-7" rim. The other size, which is more commonly chosen, is p205/55r-15, which Id rather not pick because I want a compromise between sidewall height because of the roads I drive on, and narrow width for hydroplaning resistance. However, most p205 tires are specced for 5.5-7.5"

so, Ive seen mixed answers using the search to see about using tires at wider rim widths than recomended. It seems that small differences won't make such a difference, and may even "crisp" a tires handling characteristics, however, go too wide and one might negatively effect the tire's ability to provide a smooth ride.

So, question is, what do you think the effect of running a p195 tire (7.6" or so in width) on a 7.5" rim? How significant do you think the difference would be, compared to an exact same tire, wheel, etc. in a p205 size (8.07" or so).

For a pretty light car, what do you think the handling, safety and performance implications would be from using the p195 size?

Thanks in advance for your insight!

JMH

 

[JHZR2]

anyone??? I think it is a simple question, just want clarification. Ive gotten some answers from searching, but the similar questions were pertaining to much larger differentials of wheel width and tire size.

Simply, a tire specced for 5.5-7 inch rims - will it be OK on a 7.5 inch rim? What are my risks/issues?

Thanks,

JMH

 

[CapriRacer]

There is a reason the rim width range is 5 1/2" to 7". The ideal rim width is 6".

Narrower than this tends to arch the tread across the face making the tire's footprint long and narrow. This will tend to make the car a bit unstable and susceptible to groove wander and side winds.

Using a rim that is wide tends to do the opposite - it makes the footprint a bit wide and short, and this tends to make the car unstable in turning. It will be susceptible to bumps in the turn, not to mention that rain puddles to increase the likelihood of hydroplaning in midturn.

If you stay within the rim range, the chances of this happening are minimized.

One last issue of using a rim wider than the range is that the tire may be difficult - and sometimes impossible - to seal up during the mounting process.

 

[TylerL]

JMH, I wanted to do the same thing as you. I have 7.5 rims and wanted 195 width.

The Japanese tire maker Sumitomo shows that 7.5 is ok on the HTR H4 tire:
http://www.sumitomotire.com/theline/chart.asp?id=41 http://www.sumitomotire.com/theline/chart.asp?id=41

That 205 has a sidewall that is a 1/5 of an inch shorter than the 195, that's not very much.

And don't listen to capriracer, he gives bad advice.

Just kidding; CapriRacer is the best tire expert BITOG has.

 

[glennc]

I would not only avoid tires outside the recommended range, I would try to stick in the middle half of the range or so. I believe much of the improvement you might be looking for in a different-sized tire, whatever it might be, will be quickly eliminated if the profile of the rubber on the road, and the profile of the sidewall itself, is not close to ideal. If possible I would stick to BMW approved sizes.

I think wheel/tire compatibility is an under-appreciated part of all-around performance, however you define it: ride, handling, wear or any combination of the three.

My humble opinion only, of course.

 

[JHZR2]

Thanks all, VERY much appreciated

Best regards,

JMH

 

[Shannow]

I remember an old magazine article that recommended narrower tyres on wider wheels, showing how the "trapezoid" cross section stayed "more flat" as the car rolled and with side forces.

capriracer's post certainly put points out the error in purely considering a tyre as a one dimensional shape.

Good thread guys.

 

[CapriRacer]

Shannow,

There was an old racer's trick if the rules had a limitation on the tire. That was to use as wide a rim as possible. For practical purposes, it takes the sidewall out of the picture and the tire becomes incredibly stiff. If you think about it, the sidewall is a column that has buckled outwards. If you use a wider rim, the sidewall approaches the configuarion of a beam - a completely different set of problems.

 

[Shannow]

Yep, that'd be it.