Slave Posted February 23, 2009 Posted February 23, 2009 I have a Bont 25c tyre on the front wheel of my bike (Race Lite). (Yeah TNT,serious overkill for my 900 year old Beast.) So, over the weekend I managed to get a micro wireless cateye (two things now on the bike that exceed it's monetary value). So I measured the rolling circumference of the tyr and came up with 2085. Seems to be out of synch with the generally accepted rolling circumference of a 700c wheel and tyre. What have I done wrong? I was on the bike and tied fishing line around the wheel so I had a nice "thin line"to work with. Made nice thin pencil lines on paper (Stuck to the garage floor with celotape) Measured the distance between the lines. I think the genarally accpeted measurement for 700c wheels is closer to 2096 mm. Eish, talk about foncused.
Johan Bornman Posted February 23, 2009 Posted February 23, 2009 Mine are all 2085. You're in the ballpark.
carlo_sg Posted February 23, 2009 Posted February 23, 2009 Clearly you are a lot heavier than me...Mine was 2120mm Much of a muchness, PPA never get the distance of the race correct anyway... LOL
Slave Posted February 23, 2009 Author Posted February 23, 2009 Thanks JB. I was worried there for a bit. I think the generally accepted circ for a 700 c is in the region of 2096. Maybe I misread something somewhere. Not at all unlikely for me.Phew. Worries over, I will leave the setting as it is then. Glad to find I got it about right.Thanks again.
Minion Posted February 23, 2009 Posted February 23, 2009 My Garmin reckons mine are around 2085 too. 2096 might be the unloaded circumference.
Slave Posted February 23, 2009 Author Posted February 23, 2009 Clearly you are a lot heavier than me... Mine was 2120mm Much of a muchness' date=' PPA never get the distance of the race correct anyway... LOL[/quote'] What, on the same tires? I am more intersted in having the correct setup so that my training logs are accurate. I tested the thing against my vello 8 and the vello 8 reads 1 km more after about 38 k's. So the vello 8 is reasonably accurate then. A little over 97% which is not bad at all considering price. Your diffs in tyre size is big compared to mine though. Tyres are pretty hard and I weigh 91 now. Lost some weight in the last few months.
Slave Posted February 23, 2009 Author Posted February 23, 2009 Edman, that Garmin of yours is clever. They make em smart these days.
cat-i Posted February 23, 2009 Posted February 23, 2009 My Garmin reckons mine are around 2085 too. 2096 might be the unloaded circumference. how do you use the garmin to determine the circumference? go out and ride, then compare distance? does the garmin include the vertical ascent in the distance calculation?
Minion Posted February 23, 2009 Posted February 23, 2009 how do you use the garmin to determine the circumference? go out and ride' date=' then compare distance? does the garmin include the vertical ascent in the distance calculation?[/quote']The Edge 705 has an auto-calibration function if you use the additional magnet/cadence sensor. It usually determines a value within the first few hundred meters of the ride. I'm not actually sure whether it accounts for vertical elevation when calculating distances use the GPS data points.
mountain_lion Posted February 23, 2009 Posted February 23, 2009 My Garmin reckons mine are around 2085 too. 2096 might be the unloaded circumference. how do you use the garmin to determine the circumference? go out and ride' date=' then compare distance? does the garmin include the vertical ascent in the distance calculation?[/quote'] You can use a speed & cadence sensor with the Edge 305 (and 705) which uses a magnet on the wheel and crank respectively to count wheel and crank revolutions..If you go for a ride, The Edge gets the distance traveled from the GPS satellites and will calculate the wheel size using this distance and number of wheel revolutions. GPS devices do not take vertical ascent into account when calculating distance. However if you do the math, you will find that the difference between the 2D & 3D distance is negligible for gradients which you can cycle up.
Slave Posted February 24, 2009 Author Posted February 24, 2009 I realise that 11 mm is not a big difference, but it's obviously significant enough to make a difference.Clearly, 2085 seems to be a norm then. The question now arises as to why cateye manufacturers put the default setting for 700c wheels at 2096? According to the owner manual for 23c tyres, they have the circ at 2096 and for 25c tyres, 2105. Are some 700c tyres genuinely 2096 mm in circumferance? JB, would the loaded and unloaded weight make that much of a difference?Slave2Love2009-02-24 01:03:56
Johan Bornman Posted February 24, 2009 Posted February 24, 2009 I realise that 11 mm is not a big difference' date=' but it's obviously significant enough to make a difference.Clearly, 2085 seems to be a norm then. The question now arises as to why cateye manufacturers put the default setting for 700c wheels at 2096? According to the owner manual for 23c tyres, they have the circ at 2096 and for 25c tyres, 2105. Are some 700c tyres genuinely 2096 mm in circumferance? JB, would the loaded and unloaded weight make that much of a difference?[/quote'] Cateye has no way of knowing what size tyre you're going to fit. They're all 622mm dia at the bead but since a clincher is roughly tubular, a small difference in its width makes a big difference in its diameter. For instance, the difference between a 21mm and 28mm tubular or clincher would give you a difference of 2 x pi x 7mm difference in circumference. Some 30 millimeters. Also, weight makes a difference but again, that difference is a function of weight and tyre pressure. I just keep all my bikes' computers at 2085 (I measured that back in 19voertsek) and ride. Who cares if I've done 45,7 or 46 kms today? Johan Bornman2009-02-24 01:16:39
Slave Posted February 24, 2009 Author Posted February 24, 2009 Fair comment. What made me wonder was my teenager has a catey velo 8 (new model) and I have an older velo 8 (purchased in 2006). Same universal fitting so they are interchangable. I tried them both and on average, the older vello 8 reads 1 km/h more than the micor cordless, while the newer velo 8 reads on average 0.7 to 0.8 more. Small stuff I know,but one would have to say that 23c is probably the most poluar size of tyre, while I have a preference for the 25c. I just think that the tables they use for default settings is slightly out of true. I'll let you know the result of my experiment tomorrow. I will measure all my tyres, loaded and then unloaded.
Johan Bornman Posted February 24, 2009 Posted February 24, 2009 All is not what it seems. A 23mm tyre doesn't meaure 23mm - more like 21.5mm and a 28 is over rated by a good 3mm. Most 28s are about 25mm. I have a 28 continental that's 25mm and a 28 Maxxis that's barely 26.5. The best still is Specialized's 33's. They're only 28mm. It's like American trouser sizes. I drown in a Yankee 32 but bloat out a ZA one. This stupidity is a throw-back from weight weenie days where bike magazines used to rate tyres with a scale. The lightest 23mm won the Editor's Choice. Stupid editor never bothered to measure the tyre across its inflated girth. The tyre companies lied about their sizes in order to come in below the others in weight. Poor old Cateye cannot be sure if you're fitting a cheater's 23mm or an honest company's 23mm. Bottom line is you should forget about the tables and just measure the distance on your kitchen floor. PS - the only honest tyre company I've come across is Avocet. They made a conscious decision not to play the weight game and fitted true measurement. Unfortunately rim width also plays a role in a tyre's measured width but generally the rims are within a narrow range of sameness. Johan Bornman2009-02-24 01:43:39
Shebeen Posted February 24, 2009 Posted February 24, 2009 Fair comment. What made me wonder was my teenager has a catey velo 8 (new model) and I have an older velo 8 (purchased in 2006). Same universal fitting so they are interchangable. I tried them both and on average' date=' the older vello 8 reads 1 km/h more than the micor cordless, while the newer velo 8 reads on average 0.7 to 0.8 more. [/quote'] silly question, but are they both calibrated at the same measurement? picking up the signal at 2Hz is pretty easy, so it's not likely that the machine 'skips' any of the revolutions.
Slave Posted February 24, 2009 Author Posted February 24, 2009 shebeen, the velo 8's only have tyre size as a coice. There is no specialist calibration that you can use to set them up differently. The micro cordless can take the exact measurement of the tyre. Both velo 8's are set for 700c wheels. That is as close as it gets. 700c not 700 X 23 or 25 c. Just plain old 700 c. So one would expect them to have the same setting. And it's not as though we used different tyres,wheels or bike. I just intechanged the unts on my bike with the same wheel, tyre and tyre pressure. Go figure. JB, I have noticed that the 25 c seems to have more tyre (If I can put it that way.) between the road and the rim. 23 c's by comaprison seem to be lower profile than 25 c's. At my weight of now 91 kg, the 25 c is a little more comfortable and I prefer the overall feel of the ride on 25's. I was checking out the "table" and the closest measurements in the 700 range is the 700 X 20C. Made a mistook a while ago and bought a 20c tube. Dam thing was so skinny it popped inside my 23c tyre when I was inflating it. Fortuantely no damage to anything other than tube. We live and learn. Now I check the boxes and the contents. Some jokers like to switch stuff around in those boxes on the shelves.
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now