700x45c tyre = ??inches (installing computer)

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pgmbmc

Regular
Hi, I have a Specialized Crosstrail Comp Disc, I am fitting a new wireless computer, but struggling to work out the what the wheel factor setting is, my tyres fitted are the Specialized Borough XC Sport, 700x45C, could some-one please tell me what my wheel diameter sizing is, my options on the set up instructions are 26"(650A), 26.5"(TUBULAR), 26.6"(700x25C), 26.8"(700x28C), 27"(700x32C), 28"(700B), the 700x45C is not listed, need advice on which is the closest, your help is much appreciated.

P.s. is there a specific place to place the sensor and the magnet, some sites show near hub, others centred between hub and rim.
 

Andy_R

Hard of hearing..I said Herd of Herring..oh FFS..
Location
County Durham
The formula to use to get your diameter is 622 + (2*45)
so your diameter is going to be 712mm
your roll out(how far you move when the wheel rolls once) is going to be the circumference of the tyre, so 712mm * 3.1415 = 2236.814mm
 

Rickshaw Phil

Overconfidentii Vulgaris
Moderator
Personally I never set up a cycle computer by the sizes stated in the instructions as they are often quite inaccurate. For example the stated circumference in the instructions for mine is 50mm bigger than the actual rolling circumference of my wheels & tyres.

Best thing is to get out your tape measure and measure one complete revolution of the wheel. It's easy to do and doing it that you shouldn't be more than 5mm out.
 
OP
OP
pgmbmc

pgmbmc

Regular
Thanks for that, if I'm guessing right, the 2236.814mm, rounded up to 2237 is my wheel factor, and according to my installation sheet this is the 28"(700B). Thank you.
 

gbb

Legendary Member
Location
Peterborough
Personally I never set up a cycle computer by the sizes stated in the instructions as they are often quite inaccurate. For example the stated circumference in the instructions for mine is 50mm bigger than the actual rolling circumference of my wheels & tyres.

Best thing is to get out your tape measure and measure one complete revolution of the wheel. It's easy to do and doing it that you shouldn't be more than 5mm out.
Exactly what i always do...valve at the bottom, mark the floor, one revolution, measure. It nearly always comes out different to the manufacturers specs.
 

Andy_R

Hard of hearing..I said Herd of Herring..oh FFS..
Location
County Durham
Can it make a difference if you sit on the bike when you use the floor/one revolution measurement
If your tyres noticably "squash up" when you sit on the bike, then they need :pump:. If they're properly :pump:, then it should have no bearing on the matter. But I could be wrong:whistle:
 

Rickshaw Phil

Overconfidentii Vulgaris
Moderator
Can it make a difference if you sit on the bike when you use the floor/one revolution measurement
It does make a difference (about 3mm on my tyres) even when the tyres are up to full pressure. For this reason I do tend to sit on the bike when measuring it. It's a small difference but it adds up over a few miles.
 

Pauluk

Senior Member
Location
Leicester
Mind you Phil that's less than 0.5% on a 700mm wheel. 150 yards in 20 miles or 0.08 mph error at 20mph. Not sure if its worth it. Wonder if you get similar errors in a normal ride anyway, or error in getting off your bike to mark and measure the floor :smile:

Not too off topic, what do people think is more accurate for distance, cycle computer setup correctly or a sat nav. Any views?
 

Rickshaw Phil

Overconfidentii Vulgaris
Moderator
Mind you Phil that's less than 0.5% on a 700mm wheel. 150 yards in 20 miles or 0.08 mph error at 20mph. Not sure if its worth it. Wonder if you get similar errors in a normal ride anyway, or error in getting off your bike to mark and measure the floor :smile:

Not too off topic, what do people think is more accurate for distance, cycle computer setup correctly or a sat nav. Any views?
Yeah, I'm fussy though.:addict:
 
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