What determines tire width on a rim?
 

Question - given the same diameter clincher or tubeless tire (on a hooked rim), what impact does the internal rim width have on the inflated width of the tire?
For instance, if installing a 700 x 23c tire on a rim with an internal width of say 17mm vs 19mm, would one expect the inflated tire to be wider on the wider internal width rim, or the narrower?

I have a rim with an 18.3mm internal width, and a 23 tires measures out to 26.5mm when inflated with a tube. Would a narrower rim cause the tire to "bulb" more, less, or are there other factors beside internal rim width that impact this?

Thanks!

the only important rule, or guideline is,
narrow tires to not work well on wide rims, wide tires however do work fine on narrow rims
but to answer your question
a 2mm difference, 17mm vs 19mm rim, with a 23mm tire is not going to make any noticeable difference at all

Thanks. I'm running a set of 23mm Conti GP4000 clinchers on a Boyd Altamont rim right now with an 18.3mm internal width. I run 23s (26.5 give or take inflated) because on my older rim brake frame and a 25c tire actually rubs the chainstays.

I'm about to pull the trigger on a set of carbon rims, with a 25mm external and a 17.9 internal rim width. They have this chart of inflated tire sizes, and it lists a Conti GP 5000 700 x 25 as 25.87 inflated.

I'm scratching my head a bit. Unless the tire spec is really different between a Conti GP5000 and a GP4000, not sure why the inflated width of 25c tire would actually be less than of a 23c??? I'm buying these wheel direct from mfgr, so have no option to try before I buy, which means I've spent a lot of time with digital calipers trying to make sure they will fit (I have no doubt the rims will fit, but if I'm going to by new wheels I don't want to stay limited to 23c clinchers if I can help it. Not because I hate 23s, just because they are getting harder and harder to find.

Thanks!

as a guide, my Rubino Pro clinchers were ~24mm on 15mm internal width rims and ~26mm on 17mm rims. Stands to reason that, in general, a greater separation of the beads (ie increased rim width) will result in an increase in overall tire width, although it might not be that noticable

not all tires of the same quoted size are equal
I believe the 5000 tires are closer to true
while the 4000 tires run a tiny bit wider

You have hit the nail on the head; GP 4000 and other Continental tires of that generation, e.g., Grand Sport Race are noticeably wider than GP 5000. This is allegedly because GP 5000 are designed for wheels with wider internal width.

When I first installed a GP 5000 in my 17 mm internal width wheel, the tire measured just < 23 mm wide. After a few cycles of pre-ride inflation and riding, the tire has stretched to just about 25 mm wide.

Yes. The internal width of a rim will impact the tire width.

In addition to that, some tire mfg’s are changing their tire design to accommodate the current trend of wider rims.

GP4000’s were designed to run on narrower rims, so when mounted on a wider rim they measure wider.

GP5000’s are designed for wider rims so they will run truer to the correct wide tire on current wider rims.

A 23mm GP4000 measuring to 26.5mm is a bit extreme. I could see a 25mm GP4000 measuring 26.5mm on an 18mm internal width rim.

I would also expect a 25mm GP5000 to run closer to 25mm on the same rim.

John

A simple formula to remember is adding 1mm to rim internal width will add .5mm to the mounted tire width.

I just happen to have the same tire on two different rims of 21 and 25mm width (I borked my rear 21mm rim and put on the 25mm one as a substitute for now). I just measured my tires and the 21mm was 30.5mm wide and the 25mm was 32.5mm wide .. so it exactly matches this simple approximation .. not bad! The tires are labeled 700x30c so they must have been put on 20mm or some such rims for the 30c number.

Thanks! New question- involved so moving to a fresh thread!