Author Topic: What's your Rohloff combo (chainring, cog)?  (Read 115586 times)

buffet

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #315 on: August 26, 2021, 08:28:17 AM »
I haven't seen anywhere where Rohloff have stated the maximum torque the hub is designed to withstand,

https://www.rohloff.de/en/experience/technology-in-detail/specifications

130nm. Which is max input torque at the hub itself. Say you have a 2:1 gear ratio, that translates to 260nm torque at your cranks. This may sound like a crazy figure, impossible to reach when just pedalling, but I'm fairly sure that you can achieve it in short term by mashing and jumping on your pedals when pedalling standing. Actually it's one of the things I'm trying to re-learn after I switched from singlespeed to Rohloff: keep your arse on your seat, do not mash standing.
« Last Edit: August 26, 2021, 08:32:41 AM by buffet »

PH

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #316 on: August 26, 2021, 10:44:01 AM »
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 haven't seen anywhere where Rohloff have stated the maximum torque the hub is designed to withstand,
https://www.rohloff.de/en/experience/technology-in-detail/specifications

Thanks both, interesting stuff.  I don't mind admitting I'm out of my depth, I struggled with the difference between power and force at school and I was probably a bit sharper then!
Still, I remain convinced that the torque at the hub with a powerful E-bike is greater than I'm ever likely to produce on my own.  The way the bike I tested worked was partly by using a multiple of the torque inputted by the rider, up to 360%.  I'm not sure that's significantly different to the way a rider inputs, I was watching a demo of an E-MTB where stomping on the pedals could be used to instantly lift the front wheel over an obstacle  :o I know I could accelerate up a hill in a way I could only normally dream of, this on a 35kg bike with a 95kg rider and a 20kg payload... The Bosch trained rep put this down to an increased torque rather than power, indeed the motor has recently had an increase via software and the power remains the same, confused?  I was...

Andre Jute

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #317 on: August 26, 2021, 04:20:44 PM »
The Bosch trained rep put this down to an increased torque rather than power, indeed the motor has recently had an increase via software and the power remains the same, confused?  I was...

No, I'm not confused. But I'm not surprised that you are, given your scientism.

Quote
From the discussion of brake mean effective pressure from the section "Relating Parameters" in DESIGNING AND BUILDING SPECIAL CARS by Andre Jute (Batsford, London; Bentley, Boston; proprietary editions by motor companies), pp 151-152 of the Batsford, London edition, given in Imperial measures:

Torque is the amount of work done by an engine while its power relates to the rate at which it works... All these things are related to each other:
HP = (Torque x RPM)/5250
Torque = (HP x 5250)/RPM
at any particular engine speed...

Just in case you're tempted to your refrain of "that's only your opinion", I wrote the book to educate hotrodders but automobile companies bought licenses to make reprints to give to their annual intakes of design engineers, so it is the opinion of thousands of hotrodders and engineers, Arthur Mallock ("the Major", the leading designer of club racers) who taught a course for racing mechanics out of it, all the leading motoring magazines, a text at innumerable technical colleges, etc.

With a little algebra, you can do the rest yourself. The 5250 is a constant to relate imperial measures to each other and can be ignored for merely dealing in the principle of the matter.

Those relationships, with others a bit too technical to go into here, explain why I advise those who shop for an aftermarket electric motor to buy by torque because the lack of torque will be the greatest source of dissatisfaction (next to inadequate batteries and therefore range) in any installation, which generally means Bafang (in Europe called 8FUN) because Bafang has all along been the masters of the torque game. The Bafang BBS on my bike has 80Nm of torque, as much as you claim Bosch considers enough for a cargo bike; not in in my town ("the Rome of West Cork") it won't be.
« Last Edit: August 26, 2021, 04:37:00 PM by Andre Jute »

Moronic

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #318 on: August 27, 2021, 03:24:31 AM »
Fun topic and I'm still in lockdown here so I'll join in.  ;D

Check my logic but I reckon you can get an "at least" figure for the torque a rider of a given weight can exert at the bottom bracket just by considering his applying his weight fully to one pedal with the crank horizontal.

If we take AndrewJ's stated weight, just over 100kg, then if the crank were a metre long that would produce a little more than 100kg-m of torque, which conveniently is 1000Nm.

If in fact the crank is 0.17m long, then we have torque of 1000 x 0.17 which is 170Nm.

Various informed sounding discussions around the net claim that in practice most riders can exert a force greater than their body weight on the crank arms at this point, in part because they can hold their body down through gripping the handlebars, and in part because their opposite leg if clipped in can do the same and add torque.

A third additional factor is the inertia of the rider. Even without being clipped in and with no pressure on the handlebars, the rider can exert more than 100kg downforce on the pedal by attempting to accelerate his body more than gravity does. To see this, imagine that someone holds your bike upright on a steep hill in a gear tall enough that when you stand on one pedal with that crank horizontal the bike rolls neither backward nor forward. Are you stuck like that or can you move your bike up the hill? Well, you could move it a little if you bent your leg and then straightened it sharply. You're accelerating your body upwards, and the force generating that acceleration would add to the force at the pedal. The bike would move forward briefly.

Maintain that mental image, hold on the the handlebar, clip in your other leg, and now see what you can do. It's pretty clear that you'd be capable of advancing the bicycle.

So when you look at the maximum instantaneous torque a 103kg rider could exert on the crank spindle, we can be confident it is greater than 170Nm. What's less clear is how much greater. Still, the 247Nm at a gear ratio of 1.9:1 that would meet Rohloff's specified 130Nm maximum at the hub for light riders (thanks buffet) is not a huge leap away.

What about with electric assist? From what I can see the rated peak torque output of the strongest bicycle motors is about 80Nm. So if a 103kg rider balancing at a standstill on one pedal were to add maximum specified torque from a pedelec motor that applied it all at the spindle, total torque at the spindle would be 170 + 80 = 250Nm.

Note that even with the light-rider 1.9 input ratio this barely exceeds Rohloff's specified maximum at the hub of 130Nm.

As I think we've seen, the rider if clipped in could increase that figure. By how much is not clear. From would I could find in a quickish scan of the webs, researchers haven't paid a lot of attention to the peak torque a fit rider can exert through a crank system.

If that's so it might be because it's not that important. In practice it would never be delivered, because in practice there is no other good reason to conduct the exercise I've just described. The bike will always be moving forward at least fast enough to stay upright, and the possibility that the rider can time his peak output to match precisely the horizontal crank position with each revolution is slim. What matters much more is the maximum average torque he can exert as he pedals a circle. That multiplied by his cadence and a factor appropriate to the power unit chosen (thanks Andre) will give his power output at his maximum torque, and that will determine the tallest gear he can hold briefly for a stated rate of climb - road speed at slope steepness. As the slope steepens, the gear he needs falls, and with it his road speed at cadence. Given infinitely variable gearing, at a certain slope he'll be too slow to balance the bike. It is not likely that at this point he'll attempt to generate peak torque (as distinguished from max average torque). It's not likely either that if he tried he could manage it, and even if he did manage it he could not sustain it for more than a moment.

What's the implication for hub life?

Glad you asked. I think that without knowing how much peak torque a rider might add to the torque he generates when standing on a pedal with the bike stationary, we can draw some conclusions about risks to a hub.

Lets observe at first that Rohloff's stated maximum input torque of 130Nm is not likely to be the torque at which the hub goes crunch and stops. Rohloff doesn't say this, but to my eye that's common sense. There would be no point in Rohloff specifying that figure, because it's of no help to anyone. What we need is the maximum torque it can accept reliably and sustainably. It's quite likely that's 130Nm. Which is what a 103kg rider exerts momentarily through a 1.9:1 input ratio if he stands on a horizontal pedal while assisted by an 80Nm motor that's operating at maximum torque.

Obviously using the 2.5:1 ratio adds a margin of more than 25 per cent.

Now let's look at the rider - we've theorised about the peak load he might impose momentarily, but what might we expect him to reach even briefly in practice?

I've rambled on for long enough, so I'll leave you all to speculate about that. My guess is it's under what he might exert when standing on a horizontal pedal. Yes he could exert more when climbing or sprinting out of the saddle, but I reckon he exerts less because he's much more interested in exerting a high average torque - i.e. in sacrificing some peak torque in order to carry that sub-peak output output further through the pedal circle.

Finally, what of the DC electric motor? Well, they tend to make peak torque at zero rpm. At typical cadences they'll be doing less than that. But I've assumed above that the advertised torque is what reaches the crank spindle - if what's specified is the torque at motor output shaft and it multiplies that through a gear train to the cranks then my theorising is out by the factor of the gear ratio.

Assuming I haven't made some silly error with the weight-torque conversion.  ::)


« Last Edit: August 27, 2021, 04:11:32 AM by Moronic »

Moronic

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #319 on: August 27, 2021, 08:32:00 AM »
Someone with more comfort reading scientific language than I have might get some interesting conclusions from this paper, even if again it does not attempt to measure peak pedalling forces:

https://www.researchgate.net/publication/7528120_Torque_and_Power-Velocity_Relationships_in_Cycling_Relevance_to_Track_Sprint_Performance_in_World-Class_Cyclists

A couple of suggestions from what I can make out.

First, world class sprinters can generate an average torque in the 250Nm range at low cadences, for body masses over a 70-90kg spread. Obviously that's way more than any of them would generate from standing on a pedal with the crank horizontal, so the ability of a human to generate torque while locating himself on a bicycle crankset doesn't have much to do with how gravity acts on his body. Since this is average torque (and calculated from other observations rather than measured directly), and since torque production oscillates across a pedal cycle, we can know that the peak torque will be higher (once again we are in the dark about how much higher).

So that makes my theorising from rider weight on a pedal look a bit silly.

It also suggests an elite sprint cyclist can generate an average torque that would reach the max input torque for a Rohloff at a 1:.9:1 input ratio, and would exceed it at peak points in the cycle.

OTOH the decoupling of torque output from body weight works the other way too. If you're not a world class sprinter, simply being heavy won't guarantee you'll be generating average torques in this range. Even though a 150kg rider will be able to generate a 250Nm peak through a 170mm crankset just by standing on the pedal.

The other interesting bit there though is that these riders generated maximum power at average torques roughly half their max average torques (so far as I can make out). Their average max torques fell as their cadences rose, but by halving the torque they could more than double the cadence and win a net gain in power.

Again I'll leave people to make more of this if they wish to. I find it broadly supportive of my argument in my previous post, if we're talking about non-elite riders. Even the elite guys sustained those high torques over a distance of only 200m.

Andre Jute

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #320 on: August 27, 2021, 09:29:02 AM »
Re your second post above, Moronic. I tend to agree with you, though I take a much darker view than your 25% allowance of Herr Rohloff's safety margins. I don't think we've seen the last of his "final" relaxations of the transmission ratio permission. (I once asked Frank Sinatra, who was sitting in a chair in my kitchen in Melbourne having my crimper cut his hair before one of his farewell concerts, how many comebacks he had made. "I never retired!" he said so explosively that the crimper told me, "STFU, Andre, before I cut off his ear." That describes my view of Herr Rohloff's relaxations very well.)

I had a Peugeot bike once built of some fancy-smancy metal which wouldn't let my bike mechanics do up the crank bolts tightly enough to stop their infernal creaking. Eventually I ran out of patience and invited them all around for a drink, at which I demonstrated the right way to do the job. I blocked one pedal and its crank up against a sturdy tree in my orchard, put my biggest torque wrench preset to the right torque on the crank bolt on the other side, than put one foot on the lever of the torque torque wrench and, holding on to the tree, raised my other foot off the ground, and when the torque wrench clicked in the appalled silence at my abuse of a tool none of them had in their workshops, put my foot down again. When the crank bolt had to come out again a couple of years later, it was found to overtorqued by 60%, about 25Nm for a total of 65Nm but it had been dead silent, which was what I cared about. My normal weight, which has held since I was a young athlete, is 95kg, and was probably within a couple of kilograms at that time. So I reckon your guesstimates for the torque a cyclist who means to mash the pedals can put into the rear axle are pretty close.

I'm off to bed, so I don't have time to look up whether it is you I'm kibitzing, or someone else, but unless a single application of torque almost instantly twists something off, I would say that any damage done by torque is better represented by the mode, the torque most often achieved in a period of riding, than by one or two exceptional peaks. Cycling torque peaks, outside of road racing, just aren't that long and sustained, because they aren't sustainable for cyclists, nor for batteries in E-bikes.
« Last Edit: August 29, 2021, 02:13:59 AM by Andre Jute »

Moronic

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #321 on: August 28, 2021, 03:12:04 AM »
Funny stories Andre, and some excellent points. In fact Rohloff doesn't specify whether its 130Nm input limit is a limit on momentary peaks or sustained or modal drive. And yes, given the documented longevity of these hubs, it is easy to imagine Rohloff has built in a big safety margin.

The 25 per cent I referred to was just about the difference from the 1.9:1 ratio to the 2.5.

The bit that interested me most from the paper was that even for elite sprinters, average torque had fallen to half its max level by the time they reached the cadence that gave them peak power. Applying that to the rest of us suggests that it will be rare that we get anywhere near the max torque we could apply to the cranks. Even climbing out of the saddle, we'll choose a gear and cadence that privileges power development over torque development.

That is very likely true even for so-called mashers. No one climbs in a gear where they can barely move the cranks at max effort. We go down gears so that we can spin with some comfort. If a slope is so steep that bottom gear doesn't allow that, we'lll be walking in very short order.

What of the electric assist that multiplies your effort by three? Well if the max 80Nm from a motor really is what's applied at the crank spindle, that is not going to happen when a strong rider is trying to grind up a hill at max effort. In other words, it's mainly marketing. Except for the reality that it really can multiply your effort by three if you're not applying much effort.

Even taking the example from PH of a climbing rider using electric assist to lift the front wheel over logs, wr know that in a low gear a strong rider could do that easily enough with max effort even on a heavy bike. If he were using max effort with 3 x assist, he'd be over backwards on his head. The point of the motor is that it lets him float the front wheel seemingly without effort, so that he can continue the climb at a comfortable human output rate.

Andre Jute

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #322 on: August 28, 2021, 09:06:31 AM »
In any event, Andrew isn't likely to do expensive damage to his Rohloff HGB by excessive torque. Herr Rohloff doesn't just cover all possibilities with high safety margins, he does the job right from the inside out, as a correspondent has just reminded me privately. Rohloff built in a failsafe from the beginning, nylon sacrificial shear pins  which, in the context of not having to pay for a civil war between the gears inside an HGB, cost peanuts and are conveniently placed to reduce labour costs for disassembly and reassembly. But it serves anyone who shears them off right if his wheel has to go to Germany for a rebuild at the height of the riding season. In my opinion, anyone who manages to commit serious (rather than cosmetic) damage to a Rohloff is either an evil genius or a malicious criminal.

But I must tell you, I am uncomfortable telling of such thoughtful design as those shear pins in public where HGB wreckers and other cowboys can read of it, and perhaps be inspired to greater destructive efforts.

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I've probably lifted the front wheel once a week, with and without a motor, all my cycling life, in the beginning just from glee at riding, and sometimes to hop onto pavements, and more recently because blending into the much increased traffic going uphill from my front door is a fraught business one wants to attempt quickly rather than slowly. Lifting the front wheel doesn't take all that many "torques" (to quote Jeremy Clarkson), certainly not 80Nm. You can probably work out something mathematically close to a general rule, especially if you can persuade JimK to help you, but on most bikes you just need to eyeball the proportions to discover that the rider is in fact precariously stable front to rear, with most of his weight only a couple of degrees on the right side of terminal rearward rotation around the rear axle.

JohnR

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #323 on: August 28, 2021, 01:09:36 PM »
Herr Rohloff doesn't just cover all possibilities with high safety margins, he does the job right from the inside out, as a correspondent has just reminded me privately. Rohloff built in a failsafe from the beginning, nylon sacrificial shear pins  which, in the context of not having to pay for a civil war between the gears inside an HGB, cost peanuts and are conveniently placed to reduce labour costs for disassembly and reassembly. But it serves anyone who shears them off right if his wheel has to go to Germany for a rebuild at the height of the riding season.
Which reminds me of old Land Rovers which had a deliberately weak point on the half shaft to protect the resut of the transmission from damage.

Anyway, getting back on topic it's also possible that Herr Rohloff didn't want the torque from the hub damaging the bike although the Rohloff torque management provisions are robust compared to Shimano who rely on a bent washer.

buffet

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #324 on: August 29, 2021, 09:53:44 AM »
Am I correct that should one succeed in  putting excessive torque to the hub, the only damage will be the nylon shear pins, but not the internals?

Andre Jute

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #325 on: August 29, 2021, 07:26:47 PM »
I'm not so sure about the postwar Land Rovers, JohnR. Britain was in a desperate state, and building cars with scrap metal for export. I rebuilt several Bentleys of that period into sports cars after their bodies had rusted away, and they were the best of the British cars of the period but still not as good as a prewar Bentley or Rolls. One item I remember particularly, because it irritated me every time I came to an R-type I had in "perfect original condition" (and sold on when I discovered I could only keep it that way by not driving it if one drop of rain fell) was the mu-metal corroding on the thinly plated door handles, and the preceding Mk VI was worse. A Land Rover splitting it's driveshaft by design was a clever excuse for poor quality control but at the time the manufacturing and materials sourcing in Britain were simply not there for fancy tricks like that one, though there was plenty of Heath Robinson talent left over from the desperate war years.

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Sure, Buffet, but if you do break off those nylon pins, everyone will know you for an abuser of innocent hub gearboxes and probably make you send your wheel to Germany -- expense! delay! summer gone! -- to get those nylon bits out and fit new ones. You'll get no sympathy anywhere. There's a reason people who've known about the shear pins for a decade or more -- don't shear them off.
« Last Edit: August 29, 2021, 07:30:31 PM by Andre Jute »

JohnR

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #326 on: August 29, 2021, 10:27:12 PM »
A Land Rover splitting it's driveshaft by design was a clever excuse for poor quality control but at the time the manufacturing and materials sourcing in Britain were simply not there for fancy tricks like that one, though there was plenty of Heath Robinson talent left over from the desperate war years.
It was a 1980 Land Rover and the driver carried a spare because it was a known weak link and took no more than 1/2 hour to replace (remove hub caps, push out the broken half shaft, push in new half shaft, replace hub caps) - an era when drivers needed to be mechanics when you're working in the African bush. It seems that the Rohloff shear pins weren't designed for such easy replacement.   

Andre Jute

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #327 on: August 30, 2021, 08:45:11 AM »
It seems that the Rohloff shear pins weren't designed for such easy replacement.   

Wait until you discover there are nine of them.

JohnR

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #328 on: September 01, 2021, 05:54:55 PM »
I've just reminded myself of what's written about Rohloff hubs at https://www.sheldonbrown.com/RohloffHub.html and note that there's plenty of discussion of gearing ratios. Some of the text is quoted from Andy B, perhaps from a early version of "Living with a Rohloff".

JohnR

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Re: What's your Rohloff combo (chainring, cog)?
« Reply #329 on: September 02, 2021, 04:57:17 PM »
It seems that the Rohloff shear pins weren't designed for such easy replacement.   

Wait until you discover there are nine of them.
Now that you've posted the link to https://matt.signorini.id.au/?p=198 in another thread I see that the shear pins are relatively accessible. However, I struggle to imagine that I'm strong or heavy enough to put sufficient load on the hub to break these, even if I combine the smallest available chainring with the largest available sprocket.