Tyre sizing hassles!

[Pedaldog.]

I just finished putting together a ss using a BSA 531 frame.It looks excellent but the tyre clearances are only 1 or 2mm.
Took it for a test last night and it felt slow and draggy! I think that the rider weight is deforming the rear tyre just enough to make it catch the chain stays.
They're 27 X 1 1/4, 32 X 630,  and I was told that a 700C is actually a slightly smaller wheel. This is confusing me so can somebody please let me know the score and what I need to do?

[rafletcher]

Looks like 700C wheels are somewhat smaller diameter (by 8mm) than old-stylee 27", despite being called "28""

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

[Polar Bear]

I'd be surprised if the 'deformed' tyres are catching the chainstays.   Once inflated I guess that it's the bulk of a 1 1/4 tyre which is the real issue.   A narrower tyre would help. 

I note that Conti do a 27x1 1/8 which is I guess 3 to 4 mm narrower.

[Ningishzidda]

As you've said, the 27" rims are 630mm across the bead diameter.
700C are 622mm.

You will have to remove a tyre and put a rule across the rim to measure the internal width.

Sheldon's chart of tyre widths vs rim width will tell you the max width tyre to put on the rim.


What's puzzling me is the tyre is bulging to rub the chain stay with a rider on the bike?? Never heard of this before. Mind you, always done measurements of the frame, rim and tyre before.

http://www.sjscycles.co.uk/tyres-27-(630)-dept203/

As you can see, there is a Conti Supersport 28-630. Bloody pricey tho'.

[Biggsy]

Also ensure ample brake block clearance.  Sometimes flex from rider weight makes the brakes rub.

I'd expect all of the tyre bulge from the rider's weight to be below the chainstays.  You could try a dab of wet paint to test.  But anyway I'd want more than 2mm of tyre clearance, to allow a slightly buckled wheel.

By the way, you could use a 700C wheel if you can lower the brake blocks by 4mm (by filing the bottom of the slots if necessary).

[rafletcher]

As others have said, any deformation will be around the contact point on the ground, not near the chainstays, so look for other reasons.

[Ningishzidda]

Are you sure this BSA 531 frame wasn't built for sprints? ( Tubulars )

In the old days, you could specify a Sprints frame or a 27" HP frame.

Top end lightweights would be more likely to be for sprints than HPs.

[Bledlow]

By the way, you could use a 700C wheel if you can lower the brake blocks by 4mm (by filing the bottom of the slots if necessary).

If there's that little clearance, I wouldn't be surprised if the calipers would accommodate rather lower blocks. It should certainly be easy to find suitable calipers which wouldn't need any filing.

[Pedaldog.]

We are thinking it was designed as a track bike for tubs. Going to the LBS and offer up a 700c wheel and tyre to see what the score is, thanks.
27 X 1 1/8 is the other option but the front is only  2mm from the fork crown  and they wouldn't make any difference there would they?

[Biggsy]

The bike being designed for tubs means it's probably designed for 700C, I think.  Tubbies were early adopters of 700C.

There might be some difference from 27 X 1 1/8, but I'd forget about 27" altogether now.

[Ningishzidda]

Sprints and tubs; and 700C should be as close as damnit interchangeable. Only a small tweak of the brake blocks will be necessary.

Haven't seen 18mm 700C 'track clinchers' for years. These went on a 13mm inside rim width.

My Brother-in-law rode a Bianchi in the eighties with this size. Too low profile for country lanes.

[Pedaldog.]

LBS guy, friend of mine, called at my flat yesterday and had a nosey. He says it is a definite for 700C so I'll be looking out for a (cheap) pair soon. No rush now that it's not bugging me.