Rim and tyre widths

[robinlh]

Because I'm very old and sad,and because I don't like cycling in the dark,I've been messing about in the workshop.
I thought that the relationship between the inner widths of rims and actual tyre sizes,measured across the tyre when inflated, might be an interesting bit of minor research.
As everyone knows the actual width of a tyre is not,usually, the same as the marked width.
What I seem to have come up with is that the actual tyre width should be about twice the distance between the flanges,measured internally.
Thus my rather nice old Alesa 913s,which measure 13mm between flanges,seem ideally suited to 28mm gatorskins which actually measure 26mm.I've other similar results,Mavic mod 3s,are 15mm,between flanges and seem just right with 32 mm Paselas which are about 31 mm when inflated.
I also discovered that the Paselas weigh 15 ozs and the Gators 13ozs.I had expected less of the Gators.
I've got some Schwalbes on order and when the trike re-appears with its Snypers I'll do some more measuring.In the meantime if anyone has anything to add to this,or comment generally,I'd be interested to hear.
Robin

[Erudin]

http://www.sheldonbrown.com/tire-sizing.html

Thanks for the post, very helpful for me. Looked up Sheldon and he found the same dishonesty in sizing and has a chart showing "Which tire fits safely on which rim?"

I've bought some 28c tyres (the icy winter has damaged a lot of the roads around here) to replace the 23c ones I've got but am not sure if they will fit my current rims. Ordered some wider rimmed wheels that recommend 28c but they are taking forever to be built.

I will give your method a go and get the calipers out and see if they fit my current wheels. Looked at the Mavic website and it says current Open Pro's can take 19 to 28 mm so hopefully it will work.

Update: Found this thread about Open Pro's and 28mm looks like they will cope fine until my new wheels arrive: http://tiny.cc/v879f

[niggle]

Of course the width of the rim will affect the width of the tyre when inflated

[reohn2]

Tyre sizes aren't exact and I suspect it depends which rim a tyre is fitted on can alter the inflated width eg,I have recentlly fitted some 25mm Gatorskins to the same rims (DRC STtouring 22mm overall width) I have on another bike which is fitted with 28mm G/skins, the 28mm measure 27mm and the 25mm measure 25mm so not a lot of difference there.
Someone will tell me that 22mm rims are too wide for 25mm tyres and at the lower end for 28mm,there is a 2mm difference!
I suspect that if the 25mm G/skins were fitted to say a DRC ST17(20mm overall width,Mavic open Pro are similar) they may be narrower still,the profile maybe better(?)

As an aside,I had occasion to weight both wheelsets (I too am sad) both are DRC touring rims built with the same spokes both 36hole but one set is built on Shimano XT(M752) hubs and the other set are built onto Shimano Ultegra (6600) hubs,bare wheels no cassettes or tyres fitted, they both weighed the same,which was a bit of a shock as I would have thought the MTB hubs would have been much heavier.

[andrew_s]

As far as the tyre is concerned, what counts is the internal width of the rim. The external width is typically about 6mm larger than the internal width. The rim should say what internal width it is with a "13-622" or whatever sticker, but I wouldn't rely on it. The manufacturer's websites often state the external width only.
Tyre width should be in the range 1.5x to 2x the internal rim width, so a typical 13mm racing width rim (DRC ST17, Rigida Chrina) would take tyres 18 to 25mm. A wider 19mm "trekking" rim would take tyres 28mm to 38 mm. It's OK to push the width a size, but if you push too far the handling goes to pot. Narrow gets harsh, and wide starts to get wallowy on sharper corners, especially if on the soft side.
Deciding what is OK is complicated by tyres not being the size they say they are. I've measured 28mm Gatorskins and GP4S as 26mm, and 25mm Pro2Race as 27mm (both on 15mm Open Pro rims).

[robinlh]

I think you can oversize the tyre and get away with it,like mountain bikers do.My 13mm internal Alesas had 32 (31) mm paselas on for a time,and I did'nt notice any handling problems,but I don't exactly bung the tourer into corners banked hard over.....
robin

[CREPELLO]

I think you can oversize the tyre and get away with it,like mountain bikers do.

True. I did read somewhere recently though, that if the tyre is too oversized (apart from wallowy handling) there could be a risk (with certain combinations of rim and tyre) of the tyre wall chaffing away on the rim bead.