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Old 08-08-05, 05:44 PM   #1
FarHorizon
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How large a tire can I use?

My wheels are currently fitted with Vittoria 700c x 23mm tires. I have about 8mm (0.36") clearance between the top of the existing tire and the bottom of the brake caliper. I have about 12mm (0.46") clearance between the sides of the existing tire and the sides of the brake caliper when in the functional (closed and ready for use) position. What is the largest tire that I can hope to fit on this rim without rubbing the brake?

I know this question is complicated by the fact that different manufacturer's tires of the same listed size (22mm, 26mm, etc.) have different dimensions. I'm hoping that someone with LOTS of experience in this area can give me a recommendation without me having to "try it on and see if it fits."

I also desire to run the highest pressure tire possible (125psig +) because I'm riding on rough pavement and don't want to damage my expensive rims.

Suggestions appreciated.

Thanks!
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Old 08-08-05, 05:56 PM   #2
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My first suggestion is to lower your pressure to 100 - 110. Even lower if you get bigger tires. You can get a 25c in there. After that it
just depends... A Continental 28c might fit,some of their tires run a bit small. But you would just have to try it. Hutchinson used make the Profile U, it was perfect for these situations. I have no idea why they stopped making it. Only tire they made i liked.
Rivendell has a great 27c tire, but it is exactly 27c, meaning it's prob too big.

The thing to do is find a good bike shop. They can look at it and should be able to tell you what would fit.
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Old 08-08-05, 06:02 PM   #3
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My first suggestion is to lower your pressure to 100 - 110. Even lower if you get bigger tires. You can get a 25c in there...
Why would lower pressure make a difference? The reasons I'm wanting larger tires (and high pressure) are to avoid pinch flats and protect my rims. I'd think that higher pressure would be a step in the right direction, or am I missing something? Thanks for the prompt reply!
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Old 08-08-05, 06:09 PM   #4
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Originally Posted by FarHorizon
My wheels are currently fitted with Vittoria 700c x 23mm tires. I have about 8mm (0.36") clearance between the top of the existing tire and the bottom of the brake caliper. I have about 12mm (0.46") clearance between the sides of the existing tire and the sides of the brake caliper when in the functional (closed and ready for use) position. What is the largest tire that I can hope to fit on this rim without rubbing the brake?
Based on those measurements, you could go up to a 32 mm tire, probably. I say "probably" because not all tires labeled 32 mm are actually 32 mm, the manufacturers play fairly loose with the numbers.

But there are a couple of other things you haven't considered:
1. How much clearance is there where the tire crosses the chainstays? Many modern racing bikes have VERY little clearance at the chainstays, some can't take anything larger than a 25 mm tire...
2. How wide are your rims? If you have very narrow rims, wide tires may feel squirmy on them. (see the chart at the bottom of this page: http://sheldonbrown.com/tire-sizing.html)

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I also desire to run the highest pressure tire possible (125psig +) because I'm riding on rough pavement and don't want to damage my expensive rims.
First of all, high tire pressure will prevent you from bottoming out your rims and denting them, but when you go over bumps the higher pressure will cause more of the force to be transmitted to the rim, spokes, hub, and frame. This puts unnecessary stress on the bike and makes for a very rough ride, and it's part of the reason why mountain bikers on rough terrain use a LOW tire pressure.

Expensive road wheels are usually very light and meant for racing. They're often the weakest part of a bike. I don't know what particular wheels you're using, or what you're using them for, but if you're planning on riding on rough roads, do yourself a favor and get a pair of sturdy 32 or 36 spoke wheels and save the good wheels for racing.
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Old 08-08-05, 06:15 PM   #5
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Originally Posted by FarHorizon
Why would lower pressure make a difference?
Because you're gonna be in for a crazy rough ride at 125 psi on bad roads

Quote:
The reasons I'm wanting larger tires (and high pressure) are to avoid pinch flats and protect my rims. I'd think that higher pressure would be a step in the right direction, or am I missing something? Thanks for the prompt reply!
The wider your tires, the lower the recommended pressure in general. This is because a wider tire has a wider contact patch, and therefore doesn't need as high a pressure to support the same weight (Pressure = Force or weight / Area).

Pinch flats are often caused by low pressure; this does not mean that raising your pressure really high will protect you more and more from them. Usually if your tire pressure is within the manufacturer's recommended range, you won't get pinch flats. Going too high won't provide much additional protection, and will invite other problems as I mentioned in my above post.
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Old 08-08-05, 06:23 PM   #6
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Originally Posted by moxfyre
...1. How much clearance is there where the tire crosses the chainstays?...
2. How wide are your rims?...
The bike has 5mm (0.2") of clearance between each chainstay and the current tire. Thanks for suggesting that I check - I hadn't thought of that as being a potential limitation.. My rims are Campy Ventos:



These rims are not only narrow but also have few spokes. They've been holding up fine, though with no out-of-round or out-of-true problems so far (even with the current tires). The previous owner raced on them and had them professionally trued and tightened.

I'm a clydesdale, and currently tip the scales at 255 (down from 265 in January!). I'm riding to lose another 25# by the end of the year. My weight gave me pause on buying this bike at all, but after my first 50 miles over fairly rough pavement, the wheels are still round & true.

The "conventional knowledge" is that more spokes and more crosses are stronger. I'm sure this is true, but with deep-V rim sections, how many spokes are really needed? Are these wheels doomed to destruction if I keep riding them, or will they hold up if I pad them with larger tires?
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Old 08-08-05, 06:35 PM   #7
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Yeah, get a larger tire. Unfortunately, from the pic it looks like there is no way you are going to get anything larger than a 25c in there.
When you do, try 100 psi.
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Old 08-08-05, 06:45 PM   #8
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Originally Posted by FarHorizon
The "conventional knowledge" is that more spokes and more crosses are stronger. I'm sure this is true, but with deep-V rim sections, how many spokes are really needed? Are these wheels doomed to destruction if I keep riding them, or will they hold up if I pad them with larger tires?
Good question... some very knowledgeable wheelbuilders think that the low-spoke count wheels are basically a silly marketing gimmick with insufficient advantages to warrant their use by anyone who isn't paid to ride them

Deep-V rims probably are a bit stronger, however I think that practically any mechanic would recommend that a 250 lb rider stick with 32/36 spoke wheels. (Search the forums for a sampling of this advice.) Particularly if you're on rough roads.

I won't say much else, because I'm only 165 lbs and use fairly beefy wheels, so not much experience in this area. I'd recommend posting on rec.bicycles.tech, where you'll probably hear from Jobst Brandt, who literally wrote the book on wheelbuilding
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Old 08-08-05, 06:55 PM   #9
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..you'll probably hear from Jobst Brandt, who literally wrote the book on wheelbuilding
I've read Jobst's book - he recommends more spokes and more crosses than what I've got. My LBS, though, says they've sold wheels like this to 300+ pound customers who've had no problems whatsoever (!!). I think that for the time being, I'll try some wider tires and hope for the best. If I see any signs of deformation, I'll retire these until I've lost my next 25# and buy some 36-spoke, 3-cross everyday wheels to use in the mean time.

Thanks much for your feedback!
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