Sheldon Brown
- simonineaston
- Posts: 8063
- Joined: 9 May 2007, 1:06pm
- Location: ...at a cricket ground
Re: Sheldon Brown
Hoorah - I can plonk it on my newly-aquired server-a-like (as Synology DS 220j)
EDIT: I was waaay overestimating the complexity of the page, which looks to be quite lightweight
EDIT: I was waaay overestimating the complexity of the page, which looks to be quite lightweight
Last edited by simonineaston on 14 Jun 2021, 6:56pm, edited 1 time in total.
S
(on the look out for Armageddon, on board a Brompton nano & ever-changing Moultons)
(on the look out for Armageddon, on board a Brompton nano & ever-changing Moultons)
- patricktaylor
- Posts: 2303
- Joined: 11 Jun 2008, 11:20am
- Location: Winter Hill
- Contact:
Re: Sheldon Brown
Mine worksthere are others
I just haven't been able to update some of the sizes (which I got originally from a table on the old CTC website).
Re: Sheldon Brown
SB's gear calculations don't use rolling diameter, so they are a tad high.
No mention of tyres.
No mention of tyres.
Mick F. Cornwall
Re: Sheldon Brown
What a shame that such a long established cycling business, with an excellent reputation, hasn't found a way to continue. A reminder of how tough retail is Worldwide, for anyone who thought they were all UK problems.
I hope someone takes over the Sheldon Brown resource, and maintains it, it's no small undertaking to keep such things up to date rather than just a historical record. There's loads of useful stuff on there, though it isn't always the easiest to find.
It's also my go to gear calculator, in part because it contains all the IGH ratios, I don't know if any others do, I haven't needed to look.
I hope someone takes over the Sheldon Brown resource, and maintains it, it's no small undertaking to keep such things up to date rather than just a historical record. There's loads of useful stuff on there, though it isn't always the easiest to find.
It's also my go to gear calculator, in part because it contains all the IGH ratios, I don't know if any others do, I haven't needed to look.
- simonineaston
- Posts: 8063
- Joined: 9 May 2007, 1:06pm
- Location: ...at a cricket ground
Re: Sheldon Brown
that and it keeps pace with product availability, too. It's been so useful over the yearsalso my go to gear calculator, in part because it contains all the IGH ratios
S
(on the look out for Armageddon, on board a Brompton nano & ever-changing Moultons)
(on the look out for Armageddon, on board a Brompton nano & ever-changing Moultons)
Re: Sheldon Brown
Sit on your bike with the rear tyre pressure you normally have whilst marking the floor/ground with chalk as your rear wheel has the valve at the bottom. Mark that point.
Employ an assistant who can keep you upright and gently propel you forwards.
Count the revolutions of the rear wheel. Do as many revs as you can manage and that you have a tape measure to reach, and stop with valve at the bottom.
Divide the distance travelled by the revolutions done.
That is your rolling circumference.
Divide by Pi and you get the rolling diameter so you can work out your gear ratios.
I have 20" 406 wheels on Moulton, and the rolling diameter is 18.3inches.
Mercian 700c wheels and has 26.2" rolling diameter.
Mick F. Cornwall
-
- Posts: 36780
- Joined: 9 Jan 2007, 2:44pm
Re: Sheldon Brown
Gear tables are only a ready-reckoner to help with the calculation and comparison of gear ratios. For a rider who sticks with the same wheel size, then there will be slight differences with tyre section but not much. There may be bigger differences with different wheel sizes because nominal inch sizes are not all calculated the same way. I can't see that these slight differences have any significant effect on the comparison.
Counting wheel revolutions to measure distance travelled is a bit different.
Counting wheel revolutions to measure distance travelled is a bit different.
Re: Sheldon Brown
Exactly.thirdcrank wrote: ↑15 Jun 2021, 4:12pmCounting wheel revolutions to measure distance travelled is a bit different.
You know nothing about gear ratios unless you measure the rolling diameter.
You can be near perhaps, and more than probably high, but not correct at all.
If you're going to promote and publish gear tables, you need to be correct.
Measuring rolling diameter is the only way.
Mick F. Cornwall
-
- Posts: 11039
- Joined: 7 Jul 2014, 9:45pm
- Location: Near Bicester Oxon
Re: Sheldon Brown
Makes sense. That's what I had to do when I set up a cycle computer (they're called that, I think? I only bought the one, and it was a long time ago)Mick F wrote: ↑15 Jun 2021, 4:43pmExactly.thirdcrank wrote: ↑15 Jun 2021, 4:12pmCounting wheel revolutions to measure distance travelled is a bit different.
You know nothing about gear ratios unless you measure the rolling diameter.
You can be near perhaps, and more than probably high, but not correct at all.
If you're going to promote and publish gear tables, you need to be correct.
Measuring rolling diameter is the only way.
-
- Posts: 36780
- Joined: 9 Jan 2007, 2:44pm
Re: Sheldon Brown
The point you seem to be missing is that for the purpose people use gear tables - a comparison of ratios - the accuracy to which you think they should aspire is irrelevant. Years ago, time trial riders had a stop watch mounted in a Terry clip (from the same stable as the Primus clip and the hockey stick holder) on their handlebars. By counting pedal revs AKA cadence against their stopwatch they had an idea of their speed (and perhaps the mental arithmetic helped them forget the pain in their legs) but the world has moved on.Mick F wrote: ↑15 Jun 2021, 4:43pmExactly.thirdcrank wrote: ↑15 Jun 2021, 4:12pmCounting wheel revolutions to measure distance travelled is a bit different.
You know nothing about gear ratios unless you measure the rolling diameter.
You can be near perhaps, and more than probably high, but not correct at all.
If you're going to promote and publish gear tables, you need to be correct.
Measuring rolling diameter is the only way.
It seems to me you are advocating spurious accuracy, which is a polite way of saying nitpicking.
=======================================================================
You do the need accurate info about wheel circumference to set up a computer but in that context the gear ratio is irrelevant.
Re: Sheldon Brown
Back in the day when I had a Garmin 705 with a speed/cadence sensor on the rear chainstay with a magnet on the rear wheel and a magnet on the crank, the device would tell you (obviously) your cadence, but also your rear wheel rolling circumference.
It knew the position and speed from GPS and it continually altered your real speed over the ground by comparing rear wheel revs to GPS.
It was interesting to go to the correct screen and see how the rear wheel affected the speed and the gear ratio.
Go back to very old threads on here about how a few of us measured wheel revs going up hills varied with going down them.
The steeper the hill, the lower the gear ratio due to tyre deformation. Can't remember the figures, but a few gear inches difference.
There's more to all this than nominal wheel diameter.
It knew the position and speed from GPS and it continually altered your real speed over the ground by comparing rear wheel revs to GPS.
It was interesting to go to the correct screen and see how the rear wheel affected the speed and the gear ratio.
Go back to very old threads on here about how a few of us measured wheel revs going up hills varied with going down them.
The steeper the hill, the lower the gear ratio due to tyre deformation. Can't remember the figures, but a few gear inches difference.
There's more to all this than nominal wheel diameter.
Mick F. Cornwall
Re: Sheldon Brown
Gear inches are most useful in comparing between setups, the couple of percent difference in tyre diameter is insignificant in reality.Mick F wrote: ↑15 Jun 2021, 4:43pmExactly.thirdcrank wrote: ↑15 Jun 2021, 4:12pmCounting wheel revolutions to measure distance travelled is a bit different.
You know nothing about gear ratios unless you measure the rolling diameter.
You can be near perhaps, and more than probably high, but not correct at all.
If you're going to promote and publish gear tables, you need to be correct.
Measuring rolling diameter is the only way.
Do I get down to or under 10”? Do I get up to or over 150”?
A couple of percent matters not.
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
There are two kinds of people in this world: those can extrapolate from incomplete data.
Re: Sheldon Brown
+1
It's just that, a comparison. Can anyone ride in a gear and tell exactly what it is? Say within the same sort of accuracy as using the generic tyre size? I like the bikes likely to be ridden loaded to have a 20" bottom gear, I doubt I'd notice if it was 19" or 21".
Re: Sheldon Brown
To be really accurate of course your effective diameter will vary with temperature, exact tyre pressure, torque applied, probably even the texture of the surface, how much water you carried and how far you have ridden.
It's much like the question regarding the length of a coastline - It all depends on how long your ruler is.
It's much like the question regarding the length of a coastline - It all depends on how long your ruler is.
Last edited by [XAP]Bob on 16 Jun 2021, 8:53am, edited 1 time in total.
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
There are two kinds of people in this world: those can extrapolate from incomplete data.
Re: Sheldon Brown
For me, the theoretical gear (say 27") is just for comparison with another theoretical gear that I would get if I found a bigger sprocket. What it actually works out as when the tyre has squished a bit is academic.
Where the effective radius of the tyre (with squishing taken into account) matters is with setting up a cycle computer where accurately measuring how far forward one turn of the wheel takes you means less inaccurate speed and distance figures.
Where the effective radius of the tyre (with squishing taken into account) matters is with setting up a cycle computer where accurately measuring how far forward one turn of the wheel takes you means less inaccurate speed and distance figures.