Cycling UK Forum

I have just replaced the battery in one of my cycle computers. Unlike my others (which require the wheel circumference) this one is obviously idiot proof as it allows you to scroll through various wheel sizes. My wheels on this bike are Shimano WH-RS10, which I understand are 700c with a 24mm rim. The computer will let me select 700x 23 or 700x 25. I wonder if some person brainier than me can tell me which I should use (assuming it is one of these two of course). Many thanks.

700x23/25 are tyre sizes rather than wheel sizes so this is what it's looking for. Best bet is that the size will likely be written on the sidewall of your tyre.

That is what I thought originally! The figures on the side wall of the tyre are 25-622..........which is not one of the options on the computer ( a Bontrager trip 5w). Any further thoughts would be appreciated.

That's 700 x 25. 622mm is the precise diameter of the tyre bead (not the tread). 700 is a fairly meaningless number these days; the 700c tyre was originally a fairly fat tyre for a French farmer's bike (a bit like the modern 29er) and was nominally 700mm in diameter.

burroc wrote:That is what I thought originally! The figures on the side wall of the tyre are 25-622..........which is not one of the options on the computer ( a Bontrager trip 5w). Any further thoughts would be appreciated.

25-622 I believe is another way of writing 700x25c

Many thanks for all the contributions. 700-25 it is then.

Bet you that won't be accurate though! After all tyres blow up to differnt sizes according to the manufacturers version of 25mm, the rim they are on and how much puff is in the tyre. Doubt it will be much but whne bored one day I compared a preset setting like above to a measured tyre. they were different. Doubt it really mattters though.

mattsccm wrote:Bet you that won't be accurate though! After all tyres blow up to differnt sizes according to the manufacturers version of 25mm, the rim they are on and how much puff is in the tyre. Doubt it will be much but whne bored one day I compared a preset setting like above to a measured tyre. they were different. Doubt it really mattters though.

That's been my experience as well. I prefer to manually put in the circumference of the wheel after measuring it, rather than use the 'factory' settings.

mattsccm wrote:Bet you that won't be accurate though! After all tyres blow up to differnt sizes according to the manufacturers version of 25mm, the rim they are on and how much puff is in the tyre. Doubt it will be much but whne bored one day I compared a preset setting like above to a measured tyre. they were different. Doubt it really mattters though.

Pumping a 25-622 to 100 psi instead of 120 psi reduces the effective radius by 1 mm.

1mm x 2 = 2mm. 2 x pi = 6.3mm

Of a tyre circ of 2.1 m, 6.3mm is 0.3% error.

That's 3 metres in 1 km. Or 300m in a 100 km Audax. Can you live with that?

If you cannot, pump the tyre up to the pressure indicated on the manufacturer's specification.

Sum wrote:

mattsccm wrote:Bet you that won't be accurate though! After all tyres blow up to differnt sizes according to the manufacturers version of 25mm, the rim they are on and how much puff is in the tyre. Doubt it will be much but whne bored one day I compared a preset setting like above to a measured tyre. they were different. Doubt it really mattters though.

That's been my experience as well. I prefer to manually put in the circumference of the wheel after measuring it, rather than use the 'factory' settings.

+1

Draw a chalk mark on the tyre at the valve position. On a flat surface, rotate the wheel until the mark is central at the bottom and mark this as a start point on the floor. Now roll the wheel until the mark rotates to the same position as before and mark this as the finish. Measure between these two points and this is the circumference to be entered into the computer.

Hope this helps as it is more accurate than the given settings.

Ref measuring the actual circumference by marking the tyre and rolling the bike along the ground.
Make sure:
1. That you are sitting on the bike when rolling the bike (your weigh compresses the tyre and changes the effective diameter and thus the rolled circumference).
2. That you roll the tyre in a dead straight line (this is difficult at slow speed when siting on the bike...most computers are fitted to the front wheel, which moves from side to side when you ride slowly).
3. That you are not on a compressible surface (like carpet or grass).
This all sounds daft, but is the result of experience trying to obtain an accurate, consistent and repeatable circumference on my old mountain bike.
It took me ages, some help from wife, and lots of runs to get a consistent result.
Good luck.