Author Topic: What's your Rohloff combo (chainring, cog)?  (Read 120165 times)

PH

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #300 on: August 22, 2021, 11:59:16 PM »
I feel that being 5% over the weight tier I am probably within an acceptable safety factor to go with a ratio that is 3.2% under the tiered ratio, as long as I treat the unit with due respect while climbing.
Your call of course, but I'm inclined to agree with you. 

Moronic

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #301 on: August 23, 2021, 12:01:58 AM »
If I recall correctly, the 2.5:1 minimum input ratio is also specified for tandem crews, with no weight limit stated.

If I'm right there may be food for thought in that.

Andre Jute

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #302 on: August 23, 2021, 12:29:41 AM »
In that case, Andrew, it sounds like a plan.

If you haven't yet, change the oil when you change the gearing, and again between 1500 and 2000km, and thereafter at the normal interval of 5000km/3000m. That removes little pieces of metal knocked off the gubbins as the box is run in, avoiding stress-raisers. Better safe than sorry.

Aleman

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #303 on: August 23, 2021, 10:37:01 AM »
If I recall correctly, the 2.5:1 minimum input ratio is also specified for tandem crews, with no weight limit stated.
That's correct. We have a Rohloff Equipped Raven Twin running 47-17. Bike weight 23Kg, Crew Weight 150ish Kg and can probably add another 30Kg for touring gear

buffet

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #304 on: August 23, 2021, 01:12:12 PM »
I don't think this limitation has much to do with bike weight, gear weight, uphill inclination, those are irrelevant. The only thing that matters is how much force the rider can apply to the pedals. And Rohloff went with a logical assumption that a heavier rider can put more power on the cranks than a lighter one.

I'm 93-95kg myself, running 36/16 now. Every now and then I have this fear that I'm on the edge of the 100kg threshold and I need to increase the ratio.
« Last Edit: August 23, 2021, 01:14:18 PM by buffet »

martinf

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #305 on: August 23, 2021, 03:46:23 PM »
I don't think this limitation has much to do with bike weight, gear weight, uphill inclination, those are irrelevant. The only thing that matters is how much force the rider can apply to the pedals. And Rohloff went with a logical assumption that a heavier rider can put more power on the cranks than a lighter one.

I reckon pedalling style and crank length are also important.

I am a spinner (turning a low gear relatively fast) rather than a masher (turning a high gear relatively slowly). I believe that spinning puts less strain on the gears, as well as on my knees.

I also use significantly shorter cranks than standard - either 155 mm or 150 mm. For the same force applied at the pedal, the leverage is less with the shorter crank, so the torque applied at the hub is also less. 

So even if I put on enough weight to break the 100 Kg limit set by Rohloff I wouldn't worry about not respecting the limitation. Taking into account my own circumstances of course, a heavy rider with a different pedalling style or extra-long cranks might be more likely to damage the hub.

mickeg

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #306 on: August 23, 2021, 05:53:37 PM »
...
Ah, the Speedhub itself is probably five or six years old
...
It came to me with a threaded 16T cog, ...

Some people have had difficulty getting the threaded sprocket off the hub.  If you installed it, did you grease it first?  Did someone else put it on, in which case you do not know if it was greased?

I have had no trouble getting mine off, but I grease the threads and I use long tools with leverage to remove the sprocket.



...
I appreciate your comments, and with an Engineering background, I understand well the factors or torque and materials fatigue on a complex & precise system like this IGH. I feel that being 5% over the weight tier I am probably within an acceptable safety factor to go with a ratio that is 3.2% under the tiered ratio, as long as I treat the unit with due respect while climbing. But as I'm new to it all, appreciate comments from those who know it the best.

Then you have the appropriate education to make an informed guess on whether or not it would be appropriate in your case.  Do what you think is best, knowing the risk.


If I recall correctly, the 2.5:1 minimum input ratio is also specified for tandem crews, with no weight limit stated.

If I'm right there may be food for thought in that.

Yeah, sum the weights of the pair of riders.  I suspect that this is not printed anywhere, but if you put the weight of two people on two cranksets that are linked together, what other option would make sense?

in4

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #307 on: August 23, 2021, 10:42:47 PM »
I went with 45 x 19, at SJS’s suggestion. I’ve  found that combo fine for most of my rides. That said, I recall an Andymantra saying ‘Gear Low’ so if I were to ride somewhere with plenty of inclines, perhaps over broken surfaces I’d be inclined to change to a lower gear combo and forgo the speedy top end delights.

JohnR

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #308 on: August 23, 2021, 10:45:37 PM »
The jump from lowest recommended ratio of 1.9 for a 99 Kg rider to 2.5 ratio for a 101 Kg rider tells me there's got to be some middle ground in there.
I wonder if it would be worthwhile contacting Rohloff support and explain your dilemma, pointing out the discontinuity in their recommendation. When I contacted Rohloff about the poor chainline on my Birdy Rohloff they said that it wouldn't damage the hub (I think I've got something less efficient and noisier than it could be which could be why they recommend getting the chainline correct).

Also, in the very unlikely event that something does break, how does Rohloff know what gearing ratio you have been using?

AndrewJ

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #309 on: August 24, 2021, 05:22:46 PM »
The jump from lowest recommended ratio of 1.9 for a 99 Kg rider to 2.5 ratio for a 101 Kg rider tells me there's got to be some middle ground in there.
I wonder if it would be worthwhile contacting Rohloff support and explain your dilemma, pointing out the discontinuity in their recommendation. When I contacted Rohloff about the poor chainline on my Birdy Rohloff they said that it wouldn't damage the hub (I think I've got something less efficient and noisier than it could be which could be why they recommend getting the chainline correct).

Also, in the very unlikely event that something does break, how does Rohloff know what gearing ratio you have been using?
I think it's going to be lost in the rounding error... I tried fitting my 42T chainring, which gives a 2.47 ratio with the 17T cog, and I'm content to call it "close enough for practical purposes" and go about enjoying the bike. With my big 700x45 tires, I have a gearinch range of 19.3 to 101.5, which is broader than I was used to on the other touring bike (22 to 97).

And with the 42T ring, I think I like the chainline even better, here's a picture of the Surly tensioner now... lots of chain wrap on that cog!

Steel & brass, with lugs. Berkeley California US

Andre Jute

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #310 on: August 24, 2021, 07:16:24 PM »
Not so long ago the ratio for a sub-100kg rider was 2.35. It was reduced once, twice, the last time to 1.9, and at that time I wrote (again) that it seemed as if there was some German CYA padding in there, and nobody disagreed. Looks to me, Andrew, like you made a safe decision at 2.47, near enough 2.5 to argue cases for your 105kg being a transitional mass deserving some flexibility. Objectively you're well on the right side of not just the last but the penultimate permission limit for the fractionally lower weight, at neither of which ratio to weight limits there has been a problem as far as I know.

George Hetrick

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #311 on: August 25, 2021, 03:29:00 AM »
Physics not being my strong point, is the torque independent of the wheel size? You'd need much larger gear ratios to get a 90" gear with a 20" wheel, than with a 26" wheel. Does that mean using a 63T/19T and a 20" wheel is kinder to the hub than 50T/20T and a 26" wheel?

Andre Jute

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #312 on: August 25, 2021, 07:58:53 PM »
Physics not being my strong point, is the torque independent of the wheel size?

I replied to this point in this thread in 2012 (the thread has grown whiskers!) at
http://thorncyclesforum.co.uk/index.php?topic=4412.msg20957#msg20957
I'll copy the relevant paragraph here to save hunting back years:

"The lowest ratio Rohloff permits has nothing to do with wheel size. It is specifically and exclusively the ratio of teeth between drive sprocket and chainring, as in 38/16 = 2.375 or 42/17 = 2.471 whereas 36/16 = 2.25. If you think about it carefully, the power that goes into the box is localised at that point by this ratio, regardless of the rolling radius of the wheel. The wheel size selected is thus a 'free multiplier'."

Don't be embarrassed by not knowing it; it is one of the most counter-intuitive concepts in cycling!

PH

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #313 on: August 25, 2021, 10:38:54 PM »
I haven't seen anywhere where Rohloff have stated the maximum torque the hub is designed to withstand, or where they calculate the applied force. Using rider weight seems a crude way to estimate it, though I don't have the expertise to suggest another. It seems clear that any calculation has many permutations and because it's possible for rider X to apply Y it doesn't follow that they do.
I had a test ride on a Rohloff cargo E-bike last week, 55/19 belt so will within the higher limit, even so with Bosch's excellent cargo motor adding 85Nm to whatever mine is, it must be seeing a larger force than any solo rider regardless of the ratio. 

martinf

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #314 on: August 26, 2021, 07:21:23 AM »
I had a test ride on a Rohloff cargo E-bike last week, 55/19 belt so will within the higher limit, even so with Bosch's excellent cargo motor adding 85Nm to whatever mine is, it must be seeing a larger force than any solo rider regardless of the ratio.

When I last did an effort test as part of a cardio check up a few years ago at age 60 +, according to the meter on the hospital exercise bicycle I put out a maximum sustained power output of 400W at about 100 rpm. I could have gone a bit higher, but the medical personnel told me to ease up and reduce pedalling cadence.

That translates to about 38Nm torque at the cranks if the formula I found on Internet is correct. I reckon a younger and fitter rider could apply a much higher sustained power output and hence a higher torque.

I also reckon that the way the motor applies torque to the transmission is different. A motor is pretty much constant, whereas for the same power output I think pedalling must have peaks of higher torque and troughs of lower torque, depending on the point in the pedal stroke. This will also vary according to pedalling technique.
As for as this peak torque is concerned, looking on Internet I found a figure of 112.9Nm for the maximum amount of torque momentarily produced by a professional racing cyclist.

So the motor might not be applying more maximum peak torque than a young, fit, well-trained cyclist.

Of course, the combined torque applied at the same time by the motor and a young, fit, well-trained cyclist will be much greater than the torque applied by most Rohloff equipped cyclists.

I think that even though I use a low input ratio (small chainring and largish sprocket) my own pedalling style (short cranks, relatively high pedalling cadence, low gears) probably puts less strain on the transmission than someone who uses long cranks, relatively high pedalling cadence and high gears. And I don't put out 400W in normal cycling, to keep going for any meaningful distance I will probably be in the 100W to 200W range quoted for touring cyclists.

So I'm not at all worried about damaging the Rohloff hub. Even if I did damage it, it would cost a lot less to repair than my knees.