For Those Who Are Better At MAth Than I: Tire Size/Speedo Question
O-K, Poindexters, this 'uns for you! :D
I recently put a 700c X 28 Gatorskin on my rear....but I still have a 700 x 25 in the front, where my bike-computer sensor is. Do I need to change the measurement input on the computer to still remain accurate? [It seemed "pretty accurate" over the course of the 21 mile ride I did T'other day...maybe off a few tenths- but I'm wondering. I keep thinking that the front wheel still spins the same number of revs per mile, etc.....but I dunno- am I missing something?) A few years ago I would have known this... Points are dropping off the old IQ....seriously! |
use the actual circumfrence of the wheel to which the sensor is attached
for you it sounds like the front is the answer did you do a roll-out test or just estimate or use a chart to find the wheel size? |
the comp is still measuring the distance traveled by the front tire so you are still good to go.
|
Thanks, guys! That's what I thought- just wanted to be sure.
Wilfred, I used both a chart and actually measured the circ umfrence with a string- if memory serves. It seems to be very acurate (For some reason, I thought I got a slightly different mileage total on my first and only ride since I changed the rear tire....but it's been a while since I'd done that route, so maybe I'm just remembering wrong). |
Oh, so this is not about the speedo swimming suits? :lol:
|
Originally Posted by fietsbob
(Post 15273914)
Oh, so this is not about the speedo swimming suits? :lol:
|
Given that you now have a smaller tire in front as compared to the rear (which implies that the two wheels will be rotating at slightly different RPM's), you need to be careful that the difference in rotational speed doesn't cause your frame to stretch...
|
An actual roll-out is the best method of measurement. Mark the pavement, line up the valve stem with the mark, roll out at least once and mark again when the valve stem comes around again. Measure, divide by number of revolutions if necessary, and convert to metric if necessary. 1 inch = 25.4 mm
Just as a ballpark number, though, a 28mm tire on a 622 rim would be 622 + (2x28) = 678mm diameter. Circumference is pi(d) so 678 x 3.14 = 2129mm. Or, if your computer wants cm, then it would be 213cm. That would assume that the tire was a true 28mm tall when you were on the bike and the tire was squished down. Various factors could affect that number by 5 or 10. edit: Yes, the tire should have a load on it when you're rolling it out. If you want it to be accurate. |
Originally Posted by dcf
(Post 15274176)
Given that you now have a smaller tire in front as compared to the rear (which implies that the two wheels will be rotating at slightly different RPM's), you need to be careful that the difference in rotational speed doesn't cause your frame to stretch...
larger rear wheel will provide less force when braking relatively smaller front wheel provides more force while braking frame will be compressed not stretched sheesh! |
Originally Posted by digger531
(Post 15273838)
the comp is still measuring the distance traveled by the front tire so you are still good to go.
|
I heard of this one guy who went back in time by having a smaller front wheel on a motorcicle....
No...wait...he just dressed like he was back in time- the 60's. (And I think that was the last time he bathed...) |
The front tire gets there first, so it's obviously going faster than the back wheel. To get an accurate measurement, you want the average of the two, since you'll be sitting between the wheels.
|
Originally Posted by BlazingPedals
(Post 15274918)
The front tire gets there first, so it's obviously going faster than the back wheel..
[Waits for Steven Wright to show up in this forum...or maybe Stephen Hawking] |
Because the wheel first goes half way around, and then goes half of the remaining half, and then half of the remaining arc, etc., etc.,
It never does go all of the way around! |
When I get a flat, I just move the tire so the flat part is not on the bottom,and continue on my merry way.
|
In a similar vein......one of my bikes has 20"/26" front/rear wheels. And rolls forward on a level road without pedaling. Honest.
|
Don't bother doing a roll out or any other measurement, just use the chart.
How accurate do you really need to be? It might be fun for a highly technical type, but in my experience with a bunch of bikes, different tires and speedometers, just using the chart if very accurate.... and if you're off by a couple of 1/10ths every 10 miles who cares? Frankly, I use the same computer on my 700X23 road bike and my 700X32 commuter and I doubt there's more than a 1/4 mile difference in my normal 13 mile commute.... that's more than accurate enough for me. Probably it's a little short for one and a little long for the other, I have no idea because even my GPS doesn't give me the exact same reading every ride. Now mind you, I'm not an engineer or computer scientist and I guess that these forums are biased in that direction. I work in a world where darn close is good enough and you move on! |
Originally Posted by Camilo
(Post 15279836)
Don't bother doing a roll out or any other measurement, just use the chart.
|
Originally Posted by BlazingPedals
(Post 15274200)
Just as a ballpark number, though, a 28mm tire on a 622 rim would be 622 + (2x28) = 678mm diameter. Circumference is pi(d) so 678 x 3.14 = 2129mm. Or, if your computer wants cm, then it would be 213cm. That would assume that the tire was a true 28mm tall when you were on the bike and the tire was squished down. Various factors could affect that number by 5 or 10.
edit: Yes, the tire should have a load on it when you're rolling it out. If you want it to be accurate. |
Originally Posted by thomasbrent
(Post 15280623)
I think you are wrong on this. Even though squishing the tire deforms it, it shouldn't affect the overall circumference and thus shouldn't affect the overall distance traveled per revolution.
I would guess the difference is pretty minor, though. Like 1 or two mm in total. |
Originally Posted by Camilo
(Post 15279836)
Don't bother doing a roll out or any other measurement, just use the chart.
How accurate do you really need to be? It might be fun for a highly technical type, but in my experience with a bunch of bikes, different tires and speedometers, just using the chart if very accurate.... and if you're off by a couple of 1/10ths every 10 miles who cares? Frankly, I use the same computer on my 700X23 road bike and my 700X32 commuter and I doubt there's more than a 1/4 mile difference in my normal 13 mile commute.... that's more than accurate enough for me. Probably it's a little short for one and a little long for the other, I have no idea because even my GPS doesn't give me the exact same reading every ride. Now mind you, I'm not an engineer or computer scientist and I guess that these forums are biased in that direction. I work in a world where darn close is good enough and you move on! I did measure when I installed a bicycle computer on my motor scooter....'cause the size tire on the scooter wasn't listed in the chart...so it was the only way- and I guess the degree of precision would matter a little more on a much smaller diameter tire, like on the scooter; and one that goes at a higher speed....than a millimeter or two would matter on a c. 28" tire. I notice, sometimes on the bike, I'll get slightly different mileage readings over the exact same route on different days- Maybe one day I didn't ride as straight; or cut the corners wider; or had to swerve to avoid things, etc. so what would be the point of worrying about a few millimeters? |
Originally Posted by BlazingPedals
(Post 15274918)
The front tire gets there first, so it's obviously going faster than the back wheel.
Ride through a puddle and then make a turn. Now go back and examine your tire tracks. The back tire cuts the corner and travels a little less distance at every turn. |
Originally Posted by DayGloDago
(Post 15280971)
I notice, sometimes on the bike, I'll get slightly different mileage readings over the exact same route on different days- Maybe one day I didn't ride as straight; or cut the corners wider; or had to swerve to avoid things, etc. so what would be the point of worrying about a few millimeters?
Just a quick math check says that it makes about 1% difference if you don't load the tire while doing the roll-out. Some may see that as close enough, I'd rather take a little extra time to do it right. |
OK, I had set the computer by doing a roll-out. Recently, my buddy explained it to me this way: "Imagine that you had a larger and less inflated tire. The flat area at the contact patch will be bigger when loaded, and the axle will be lower- say 1/2". That, then is the radius used to calculate the circumference- shorter radius= shorter circumference."
I tried it just now. 700/23, pressure a little low- 85 lbs, I weigh about 165. Unweighted: 212cm, weighted:211cm A little less than .5%. About a 1/4 mile in 50? |
I'll save the mental math gymnastics for driving.... I have to remember that my truck, in reality, is going 12% faster than what the speed-O-meter says, 'cause I gots bigger than stock tires on it..... (I just figure 10% and add 1 or 2 MPH). Maybe I should do a roll-out test with the truck? :)
|
All times are GMT -6. The time now is 02:27 AM. |
Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.