andsaw
Peloton Rider
Posts: 268
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Post by andsaw on Feb 11, 2014 16:39:52 GMT
I had to replace my battery to my computer, and i when i looked at instructions, the recommended size for a 23mm tyre was 2112, so i thought i would actually measure mine im using a Michelin lithium 2 on the front, and it came up 2118, rechecked still same, so a 6mm difference, it would be interesting if other makes differ much, don't know the difference in speed and distance (sure someone will work it out) so whats yours.
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Post by Rocket on Feb 11, 2014 17:27:48 GMT
I'm using a measured 2,100mm on a 23mm Btwin FRC.
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Deleted
Deleted Member
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Post by Deleted on Feb 11, 2014 18:00:05 GMT
Conti GP 4 seasons 23mm said 2122mm on the box, haven't bothered to check how accurate that is.
6mm is roughly a 0.28% difference, which means 20mph would become 20.06mph
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Post by fatmanona5a on Feb 11, 2014 18:34:22 GMT
Conti GP 4 seasons 23mm said 2122mm on the box, haven't bothered to check how accurate that is. 6mm is roughly a 0.28% difference, which means 20mph would become 20.06mph As far as I know, car speedos are only required to be within +\- 10% accuracy, so the odd % here or there is negligible!
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Post by Rocket on Feb 11, 2014 18:36:55 GMT
Speedos can be up to +10% but cannot read under as you could be speeding without knowing it.
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Post by fatmanona5a on Feb 11, 2014 19:08:23 GMT
Conti GP 4 seasons 23mm said 2122mm on the box, haven't bothered to check how accurate that is. 6mm is roughly a 0.28% difference, which means 20mph would become 20.06mph As far as I know, car speedos are only required to be within +\- 10% accuracy, so the odd % here or there is negligible! As far as I know was wrong! Vehicle speedos cannot read slow, but they can be fast by 10% + 6.25mph! So a vehicle doing 40mph, could have a speedo reading 50.25mph! But I still think the odd % error on a bike is negligible!
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Post by cbrdeano on Feb 11, 2014 19:17:00 GMT
I had to replace my battery to my computer, and i when i looked at instructions, the recommended size for a 23mm tyre was 2112, so i thought i would actually measure mine im using a Michelin lithium 2 on the front, and it came up 2118, rechecked still same, so a 6mm difference, it would be interesting if other makes differ much, don't know the difference in speed and distance (sure someone will work it out) so whats yours. You measured a fully round tyre. When you are riding the bike the tyre distorts a little at the bottom and changes the effective circumferance. The ideal way to measure the circumference is to be on the bike and roll forwards 1 full revolution. The most accurate way would be to use a satnav vs your speedo and see what the % error is when riding. Then change the figure in the speedo by the same % Go for another ride and the satnav should now agree with your speedo.
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Deleted
Deleted Member
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Post by Deleted on Feb 12, 2014 1:50:46 GMT
Tbh the 6mm difference is probably more accurate than a gps. GPS at it's best will only be accurate to something like 10m, and if you're on any sort of incline then the speed will have even more error.
Not really worth worrying about, you're already within less than half a percent of the true value, you're not going to get much more accurate than that with anything.
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Post by captslog on Feb 12, 2014 11:10:16 GMT
You measured a fully round tyre. When you are riding the bike the tyre distorts a little at the bottom and changes the effective circumferance. The ideal way to measure the circumference is to be on the bike and roll forwards 1 full revolution. [/quote] That's what I do. However, because I work with things where we have really want to know, I measure it over two or three revs and repeat that several times. Bloody pointless I know, but I can't help it.
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Post by chas on Feb 12, 2014 11:24:50 GMT
There are so many variables the best way is to calibrate over a known distance, possibly a local time trial course, divide the known distance by the measured distance and multiply by your circumferance setting (I may have got that the wrong way round) or trial and error altering the setting and riding the route again - If you can use all or part of your commute it's easy. (I always start with 2100mm, and use the same figure when calculating gear restrictions for my kids when racing)
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andsaw
Peloton Rider
Posts: 268
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Post by andsaw on Feb 12, 2014 12:22:08 GMT
Did a recalculation today and pumped up to 100 psi which i normally have i think i did a miscalculation on the circumference the first time around, i did the without me on and measured 2114, and then with me on, i lost 14mm so ended up 2100, i did it all 3 times and each time the same, couldn't get it at first as you would think if i have squashed the bottom the squashed bit would spring back on the rest of the tyre so you wouldn't lose any but obviously it doesn't work that way, so i set my computer to 2100, don't think i feel like im cheating now even though i will go slower now, another weird fact, i was more curious about if different makes have anomalies in their sizes, my width is 25mm and is listed as 23mm so not correct in width either.
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