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Old 02-20-09, 01:30 PM   #1
meb
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Sizing tire chainstay clearance for a 700C tire with 27s or 700C fronts

I'm 235 lbs. I want to put wider more robust tires on a GT road bike that has 700x23s. I was intending 700x32 or 700x28. I pulled a rear off on of one of my Raleighs that has 700x35 and tested it in the GT for chainstay clearance and found it is so snug I realize even 700x32s won't fit and I am questioning whether 700x28 will fit.

I still have mounted the original 700x28 front that used to be on that Raleigh-could I get meaningful chainstay clearance info using the front in rear?

I have other road bikes mounted with 27x1F/R, 27x 1 1/8 F/R and 27x1 1/4F/27x1 3/8R. Could I get reliable sizing info from the 27s and plug in a compensating factor?
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Old 02-20-09, 05:45 PM   #2
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Well, a 28 should nominally be 28mm wide. You'd have to guess on where that point is but 14mm above the rim would be a good guess. The trick of course is that not all 28mm tires are actually 28mm wide. Now that I think of it, I'm not sure ANY 28mm tire is exactly 28mm wide! Kinda like shoes.....

edit: and oh yeah - that front would make a great test case. No difference usually between from and rear road tires (they usually aren't specific) and the bead seat diameter is always the same. If it's really close you'd want to make sure you get a rear rim that is as narrow or narrower than your test rim. Maybe put a rear skewer through it and put it in the dropout like that. It's be way to narrow, so don't try to get it all the way tight, but you should be able to generate enough friction to keep it in place for measuring purposes in the shop. You'd want to be very careful if it's a carbon frame, a bit careful with aluminum and only kinda careful with steel.

edit 2: it would help if I actually read your post! Mount that 28 on your rear wheel and give it a try.

Last edited by GV27; 02-20-09 at 05:51 PM.
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Old 02-21-09, 10:41 AM   #3
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Here's what I do:

1) Measure the width of the tire that's currently on the rim. Don't trust the labeled size.
2) Measure the tire clearance with the frame/fork
a) Front clearance at the crown (tire height) and at the fork blades (tire width)
b) Rear clearance at the brake bridge and seat tube (tire height) and chain stays (tire width)
3) Calculate how much wider the tires can be and still clear by 1-2mm.
a) For tire height clearance measurements, you can increase tire size by the full amount, i.e. if the fork crown clearance measurement is 7mm, you can go 5mm larger.
b) For tire width clearance measurements, you can increase by twice the minimum clearance, i.e. if the chain stay clearance is 4mm on the left and 5mm on the right, you can increase by (4-2)*2 or 4mm.
c) Take the smallest of the calculations to get your tire size increase. Add this to your measured tire width.

You could try to estimate using the Raleigh front wheel but since 700x28 tires are not always 28mm and the Raleigh tire is not what you would be putting on there, I'd recommend the clearance measurement procedure to make sure you have room for any 700x28 tire (tires are rarely larger than their labeled size).
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Old 02-23-09, 04:39 AM   #4
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Quote:
Originally Posted by Gonzo Bob View Post
Here's what I do:

1) Measure the width of the tire that's currently on the rim. Don't trust the labeled size.
2) Measure the tire clearance with the frame/fork
a) Front clearance at the crown (tire height) and at the fork blades (tire width)
b) Rear clearance at the brake bridge and seat tube (tire height) and chain stays (tire width)
3) Calculate how much wider the tires can be and still clear by 1-2mm.
a) For tire height clearance measurements, you can increase tire size by the full amount, i.e. if the fork crown clearance measurement is 7mm, you can go 5mm larger.
b) For tire width clearance measurements, you can increase by twice the minimum clearance, i.e. if the chain stay clearance is 4mm on the left and 5mm on the right, you can increase by (4-2)*2 or 4mm.
c) Take the smallest of the calculations to get your tire size increase. Add this to your measured tire width.

You could try to estimate using the Raleigh front wheel but since 700x28 tires are not always 28mm and the Raleigh tire is not what you would be putting on there, I'd recommend the clearance measurement procedure to make sure you have room for any 700x28 tire (tires are rarely larger than their labeled size).
The 700x35 is a true 35 mm.

I mounted the 27 x 1 1/4 front and it cleared and the 27x 1 1/8 rear and it likewise clears. The 27 x 1/4 is a fraction of a 1/8 inch narrower than the 27x1 1/8-both measure 27x1 1/8 to the nearest 1/8 with the 27x 1 1/8 having a true width. Seems 700x28 should work on this wheel. The chainstay clearance appears insufficient to fit a true 700x32 in but a narrowly made 32 may fit. Since I don't have any 32s, any ideas on which 32s run narrow?

The 700x28 front I have is at work and the bike is at home, so it wasn't readily available to swap-but in leu of the trails with the 27s, it appears moot.
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