Are 92’s a bit extreme because it’s a 26 or would that be the case regardless of wheel size?
If it depends on wheel size, then it would be cool to modify the chart. For each column in your chart you put a line showing the longest “recommended” crank length and a line showing the shortest?
I guess even if it doesn’t depend on wheel size, it would might be nice to have those lines… just straight across.
That would make it more obvious that although I might have the same TGR (26 with 90’s and 36 with 135’s both equal 3.39) I might be going too extreme with one.
I am a big fan of the concept and I took a good while to read this since I often find myself doing calculations on my unicycle. I was just yesterday thinking about the advantages of different sizes of wheels andthis sums it up pretty well!
When I first made the chart I highlighted the crank lengths from 165 to 110 since that is what I like to use but decided to undue the highlighting because some people like to crank away with 185mm bike cranks for big wheel MUni and some people are really good at spinning little wheels with ridiculously short cranks.
With the 92s you will probably be able to reach top speeds similar to a 36 with 130s but won’t have the same stability or control. That is exactly the reason I have pieces of my leg holding my shoulder together, but then I was pushing the limits a bit to far. You should be fine as long as you aren’t trying to squeeze every last ounce of speed out of your setup while wearing a large backpack. :o
It seems that for a non-geared uni it should be TGR = axle height / crank length
Axle height should be measured with the uni mounted so that it equals the length that it will be when riding. So the result depends on rider weight and tire pressure.
Firstly, I personally think that 92’s are not really extreme on a 26". Sask must be with me on that one, or his colour coding in the table is wrong. 92 on a 26" show between “allround road riding” and “hilly road rides”.
Secondly, it DOES depend on wheel size. That’s the whole idea of Sask’s table and the colour coding in it. E.g., 165 mm cranks make good sense on a geared 36" but not on a 20". Those lines you’re talking about are already in the table, as the borders between differently coloured areas. But it’s not a black and white picture, literally.
Yup, that is what it is. If people want a really accurate number they should probably use this method.
Most people just want a general idea of what their ratio is and I think the chart does a pretty good approximation.
The formula I used in the chart is
Gain ratio=(diameter in inches)*12.7/(crank size in mm)
more accurate ratio would be found with the formula
Gain ratio=(loaded axle height)/(crank length)
For schlumpf gearing I just multiplied the wheel diameter by 1.5 but for a more accurate number you should use 11/7. Mostly I used 1.5 so i could use the 36" column twice.
The chart is simplified for simplicity
Really I think people just want to use this to see what would be similar and don’t need to calculate to 5 decimal places.
You are right that the result depends on rider weight and tyre pressure. It also depends on tyre wear, uni weight (in addition to the rider) and some less significant factors.
However, the rollout of a wheel is not the axle height times 2 pi. That would only be true if the tyre deforms to its loaded shape everywhere it is in contact with the road. The contact patch however is flat (as opposed to constant radius), and so outside the middle of the contact patch the tyre has deformed less. The tyre undergoes deformation from hitting the road to leaving the road: it goes from unloaded to maximum deformation and back to unloaded (and this is only 2D, in reality the deformations are still different outside the centre plane of the wheel). Therefore the rollout is 2 times pi times an “effective radius” that is between the axle height and the unloaded radius. I’ve published some thoughts on this in 2003 on my otherwise somewhat outdated Coker rollout page.
The more I think about it the more I think Doug would be fine with the 92s. I was using 102s on a 29 equivalent (27x1-1/4) and my crash was due to many factors including a weighted wheel with road tire at 120psi, poor roads, large backpack filled with text books and binders but not properly strapped on, and excessive speed due to the fact I was going to be almost late for my Native Law final. If he takes it easy and just has fun he should be fine. I guess I am just a bigger wheel instead of smaller cranks kind of guy.
Thanks, I pulled 11/7 out of my head and even as I was posting it it felt a bit course. I was close, all 1s and 7s and all Would have looked it up but last minute post before going to work.
It does seem to make sense like that, if you want to see the results of swapping cranks from one uni to another. And your point is especially clear, if those cranks are fairly extreme (165) for a particular uni (20).
But if I am looking to get different cranks for my uni to match the TGR of another ride, I might find there isn’t anything prohibiting me like pedal strikes. For example, if I want a 26 to have a TGR of a G26 on flat roads (green), I look it up and see I’d want a TGR between 4.13 and 4.50. Looking back to the 26, I see I can get 4.40 with 75’s. But wouldn’t that be significantly less comfortable to ride?
(I really don’t have the experience to say, I just gather from these forums that that would be a bit extreme.)
Not exactly clear to me which two setups you are comparing. One is a 26" with 75 mm cranks, I think. The other a geared-up 26"? With what cranks?
Anyway, for most people a 26" wheel with 75 mm cranks will be less comfortable to ride, especially when one wants to ride fast, than a bigger wheel (say, 36" - or a geared-up 26" for that matter) with the same TGR. OTOH, ride comfort is also a matter of getting used to a particular setup.
TGR is a mathematical number referring to the total gear ratio resulting from crank size, wheel size and gearing. It can be construed as an approximate indicator for speed potential, but “at your own risk”. In any case, it is not an exact predictor for maximum or cruising speed. Even less so (I think), it is a predictor for ride comfort.
I love this whole concept and the chart is amazing. I think it would be really interesting to disect the 29" format where these numbers would end up being vastly different from set-up to set-up. I have 2 29" tires that measure almost 2.5" inches in diameter different. By these calculations my Kenda Kwest with 125mm cranks only comes out to 2.69(measures marginally better than 26.5"), which only makes it slightly better than a 26" with the same lenght cranks, whereas my Schwalbe Big Apple 29 (actually 29") gives me a much better TGR for cruising at 2.95 with 125mm cranks.
Comparing my limited experience of several different crank lenghs on my 24" & 29" uni’s I agree completely with what the chart says for my effort/comfort etc.
It also supports my thinking that 150’s on my yet-to-be-delivered 26" Guni may be just a bit too long after I’ve got the hang of it.
This chart should be a ‘sticky’ somewhere so newbie’s can find it easily.
I’ve got a set on my 29" and change the position depending on what I’m riding (3 times yesterday for Muni, then fire road then back to Muni in a 12 mile ride) - hoping to get away from this messing about with theb geared hub as I’m going to use the 150’s I’ve got spare…although I can feel an ‘upgrade’ coming on!!
Ha, I like it. You guys did a great job explaining the effect of wheel size. I think though that you should have included more about crank length, and probably also about gearing (hub). At least if your goal is to explain Total Gear Ratio, which really depends on all three.
Would have looked it up but last minute post before going to work.
