Author Topic: Rohloff Efficiency on a tandem  (Read 3380 times)

swc7916

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Rohloff Efficiency on a tandem
« on: June 19, 2012, 10:18:59 PM »
this topic has been split from    
Poor shifting on new Rohloff on tandem
- stutho


We have two tandems that are otherwise identical except one has deraiIleurs and the other has a Rohloff hub.  Both tandems are in-phase.  I have found that the Rohloff works best when the cadence is kept high; at least 80 rpm.  In most flat-to-rolling conditions, I have had no problems with the shifting.  The 7-to-8 shifts are problematic, but I expected that.  When going up a hill wiith the derailleur tandem I can delay shifting until we can hardly pedal anymore and still force it to shift.  With the Rohloff, I have can't let our cadence drop too much or downshifting becomes very  difficult.  I will say that the shifting has gotten better as we have put more miles on it and it also seems to be more efficient and quieter.  One thing that bothers me however, is that the Rohloff tandem is about 10% slower overall and it is much harder to accelerate.  Still, we like it and the derailleur tandem is used as a back-up in case the Rohloff tandem is out of commission for some reason.

Here is a link to photos of our tandem:  http://www.trilliumcycles.com/profiles/rohloff-tandem/rohloff-tandem.html
« Last Edit: June 23, 2012, 06:19:01 PM by stutho »

Andre Jute

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Rohloff Efficiency on a tandem
« Reply #1 on: June 19, 2012, 10:30:13 PM »
Here is a link to photos of our tandem:  http://www.trilliumcycles.com/profiles/rohloff-tandem/rohloff-tandem.html

What an intensely beautiful bike! Andre Jute

fleur

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Rohloff Efficiency on a tandem
« Reply #2 on: June 20, 2012, 03:27:55 PM »
We have two tandems that are otherwise identical except one has deraiIleurs and the other has a Rohloff hub.  Both tandems are in-phase.  I have found that the Rohloff works best when the cadence is kept high; at least 80 rpm.  In most flat-to-rolling conditions, I have had no problems with the shifting.  The 7-to-8 shifts are problematic, but I expected that.  When going up a hill wiith the derailleur tandem I can delay shifting until we can hardly pedal anymore and still force it to shift.  With the Rohloff, I have can't let our cadence drop too much or downshifting becomes very  difficult.  I will say that the shifting has gotten better as we have put more miles on it and it also seems to be more efficient and quieter.  One thing that bothers me however, is that the Rohloff tandem is about 10% slower overall and it is much harder to accelerate.  Still, we like it and the derailleur tandem is used as a back-up in case the Rohloff tandem is out of commission for some reason.

Here is a link to photos of our tandem:  http://www.trilliumcycles.com/profiles/rohloff-tandem/rohloff-tandem.html

That'a beautiful tandem !

I have three questions to better understand the 10% speed difference:
- how much miles do you have now on your Rohloff hub ?
- do you have the same wheel traveled distance on your Rohloff and derailleur tandem ?
- do you think that the big wheel weight difference between the Rohloff equipped wheel and the derailleur wheel can explain the speed difference (on a solo bike, it is well known that lightweight wheel improve the efficiency, going to a Rohloff means a big increase of the of the rear wheel weight) ?

swc7916

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Rohloff Efficiency on a tandem
« Reply #3 on: June 21, 2012, 04:20:16 AM »
That'a beautiful tandem !

Thank you.  We get a lot of attention with this bike

Quote
- how much miles do you have now on your Rohloff hub ?

About 2,000

Quote
- do you have the same wheel traveled distance on your Rohloff and derailleur tandem ?

I'm not exactly sure what the question is, but both bikes have 26" wheels.

Quote
- do you think that the big wheel weight difference between the Rohloff equipped wheel and the derailleur wheel can explain the speed difference (on a solo bike, it is well known that lightweight wheel improve the efficiency, going to a Rohloff means a big increase of the of the rear wheel weight) ?

No.  A lightweight wheel will accelerate quicker but once up to speed there should  be no difference.  My derailleur bike's rear wheel is not light either, it has a drum brake.  I think that the Rohloff is just not as efficient.  It doesn't coast as fast or as long and it's more work to accelerate and keep up to speed.

fleur

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Rohloff Efficiency on a tandem
« Reply #4 on: June 21, 2012, 07:09:25 AM »
- do you have the same wheel traveled distance on your Rohloff and derailleur tandem ?
I'm not exactly sure what the question is, but both bikes have 26" wheels.

Was the sprocket ratio of the Rohloff chosen to provide the same sprocket ratio as the derailleur system according to the table given by Rohloff http://www.rohloff.de/fileadmin/rohloffde/produkte/speedhub/kettenuebersetzung/rohloff_shimano.en.pdf

il padrone

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Rohloff Efficiency on a tandem
« Reply #5 on: June 21, 2012, 08:43:12 AM »

No.  A lightweight wheel will accelerate quicker but once up to speed there should  be no difference.  My derailleur bike's rear wheel is not light either, it has a drum brake.  I think that the Rohloff is just not as efficient.  It doesn't coast as fast or as long and it's more work to accelerate and keep up to speed.

The weight of a Rohloff hub is relatively minor in the whole rotating mass equation. The most significant drag on acceleration comes from weight located further from the axle ie. the rim. A Rohloff with a light rim would probably give faster acceleration than a derailleur wheel with a heavier rim, even though its total mass might be heavier.

As far as efficiency loss is concerned, I have not been noticing it on my switch from the 26" Giant Sedona MTB tourer to the similar weight Thorn Nomad. My riding speed is comparable riding on the same street and conditions. I am not falling behind any of my riding mates. There is technical data (done by Rohloff I will admit) that shows very minimal efficiency differences (and in some gears eg. 11 the Rohloff is more efficient)


http://hubstripping.wordpress.com/speedhub-rohloff/


http://www.rohloff.de/en/technology/speedhub/efficiency_measurement/index.html
The wider band for the derailleur gears on this diagram reflects the greater variability of drivetrain efficiency with 1000kms of wear. The straight drivetrain of the Rohloff without a jockey cage shows much lower efficiency loss after 1000kms.
« Last Edit: June 21, 2012, 08:55:43 AM by il padrone »

fleur

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Rohloff Efficiency on a tandem
« Reply #6 on: June 21, 2012, 11:15:01 AM »
The weight of a Rohloff hub is relatively minor in the whole rotating mass equation. The most significant drag on acceleration comes from weight located further from the axle ie. the rim. A Rohloff with a light rim would probably give faster acceleration than a derailleur wheel with a heavier rim, even though its total mass might be heavier.

As far as efficiency loss is concerned, I have not been noticing it on my switch from the 26" Giant Sedona MTB tourer to the similar weight Thorn Nomad. My riding speed is comparable riding on the same street and conditions. I am not falling behind any of my riding mates. There is technical data (done by Rohloff I will admit) that shows very minimal efficiency differences (and in some gears eg. 11 the Rohloff is more efficient)


http://hubstripping.wordpress.com/speedhub-rohloff/



http://www.rohloff.de/en/technology/speedhub/efficiency_measurement/index.html
The wider band for the derailleur gears on this diagram reflects the greater variability of drivetrain efficiency with 1000kms of wear. The straight drivetrain of the Rohloff without a jockey cage shows much lower efficiency loss after 1000kms.

I don't agree totaly with this statement.  Yes, the closest to the center of the wheel, the less influence of the mass increase.  But the wheel mass increase induced by the Rohloff is huge = more than 1kg increase (a tandem hub is may be 400g or less, the total weight of the wheel is almost doubled with a Rohloff) compared to the rim mass difference (you can may be save 100 to 150g on the rim = 10 times less), so even if it is placed close (but not at) the center of the wheel, it should have an impact.

Do you know the source of the hubstripping data ?  Is it Rohloff, I didn't saw that curve on the the Rohloff web site.

swc7916

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Rohloff Efficiency on a tandem
« Reply #7 on: June 21, 2012, 03:20:20 PM »
The weight of a Rohloff hub is relatively minor in the whole rotating mass equation. The most significant drag on acceleration comes from weight located further from the axle ie. the rim.

In the case of the Rohloff hub there's more than just the acceleration of the wheel about the axle, there is also the acceleration of all the gears inside the hub; this is a point that most people miss.  Each gear has it's own rotating mass which adds to the rotation inertia of the wheel; you can feel this when trying to get up to speed from a stop.   Another factor is the Rohloff's full-contact seals; try spinning your rear wheel and see how quickly it comes to a stop compared to you derailleur bike.  Our downhill speeds with the Rohloff are noticably slower than with the other bike because of this.

Quote
There is technical data (done by Rohloff I will admit) that shows very minimal efficiency differences (and in some gears eg. 11 the Rohloff is more efficient)

Maybe, in a lab at a constant speed with a constant load driven by a motor, but the power output of a cyclist isn't constant - it pulses.  Each pedal stroke is an acceleration, which I don't think the data accounts for. 

Cambirder

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Rohloff Efficiency on a tandem
« Reply #8 on: June 21, 2012, 05:31:04 PM »
Personally I have found no noticeable difference in downhill coasting speed between my Rohlhoff equipped Thorn and my derailleur equipped Dawes Galaxy.

fleur

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Rohloff Efficiency on a tandem
« Reply #9 on: June 21, 2012, 05:33:05 PM »
In the case of the Rohloff hub there's more than just the acceleration of the wheel about the axle, there is also the acceleration of all the gears inside the hub; this is a point that most people miss.  Each gear has it's own rotating mass which adds to the rotation inertia of the wheel; you can feel this when trying to get up to speed from a stop.   Another factor is the Rohloff's full-contact seals; try spinning your rear wheel and see how quickly it comes to a stop compared to you derailleur bike.  Our downhill speeds with the Rohloff are noticably slower than with the other bike because of this.

Maybe, in a lab at a constant speed with a constant load driven by a motor, but the power output of a cyclist isn't constant - it pulses.  Each pedal stroke is an acceleration, which I don't think the data accounts for. 


None of this is true.

The number of gears used in each speed is limited and in 11th there are none, it is direct drive therefore the very good efficiency in 11th.

The loss in downhill is also very limited, if it was huge as you say, the hub would become hot (remember that lost energy is converted into heat) which isn't the case.

For the additional energy needed at each start due to the Rohloff, compared to the energy needed to accelerate your own bdy and the bike it is not a lot.

swc7916

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Rohloff Efficiency on a tandem
« Reply #10 on: June 21, 2012, 07:17:46 PM »
None of this is true.

Whoa, you really jumped on me here.  Before you start calling me a liar or tell that I'm imagining things, let me elaborate....

Our two tandems are from the same builder and are set up identically except for the drivetrain.  The saddle and handlebar adjustments are the same, the crankarm lengths are the same, the wheel sizes are the same, the total weights are within a couple of pounds of each other and we are running the same tires at the same pressure (Serfas SECA 26x1.25 @ 100 psi.)  We have a 9-speed cassette and triple crankset on the derailleur tandem with a range of 25 to 120 gear-inches.  The Rohloff tandem is geared 50x15 which gives us a range of 23 to 122 gear-inches.  I keep my drivetrains clean, the brakes are not dragging, and the chain is not too tight.    We have ridden the same routes with both tandems, so I can compare both my impressions and actual measured speeds over those courses.  Of course, the captain and stoker are the same.

When we took our test ride with the Rohloff tandem, one of the first things that I noticed was that our speeds were slower with the same level of effort.  It has gotten better over the past couple of thousand miles but we're still not as fast.  On downhills the derailleur tandem feels like a runaway freight train; the Rohloff tandem does not. We have to pedal in order to approach the same speeds that the derailleur tandem reaches just by coasting. 

It is a fact that in order for the wheel to rotate at a different speed from the cog that gears are rotating inside the hub, more in some gears than in others. These gears have rotational friction as well as rotational inertia and it requires additional input to get and keep these gears turning.  This is most evident in the lower 7 gears - Where do think the noise comes from?  It's been my experience that the Rohloff works and shifts better at higher cadences and lower pedal pressure.  I've also observed that the harder we pedal, the more it seems to resist accelerating.

I'm convinced that the additional weight of the Rohloff hub in itself is not a significant issue.  Perhaps the fact that we are propelling a total bike + riders that approaches 400 pounds with the force that two people can apply shifts the total efficiency down.



 

Danneaux

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Rohloff Efficiency on a tandem
« Reply #11 on: June 21, 2012, 07:51:47 PM »
Hi All!

This is a topic I am following closely and with particular interest. I deliberately selected a derailleur-equipped Thorn, and have had no direct experience with the Rohloff models. Nevertheless, I am fascinated by them and user impressions as well as empirical data comparing their efficiency.

Though we all may have different impressions, it is still very useful to see the range of experiences -- particularly so for those of us who don't have the hubs and are considering the drivetrain. After all, individual user impressions hold true for those who experience them. Sometimes they can be triangulated with empirical data...other times, not. They are no less important or valid either way.

Rohloffs, like all Internally-Geared Hubs, do have some internal friction losses. It appears Rohloff's careful machining and use of a proprietary-formula oil bath reduces those losses compared to IGH lubricated with grease, where the frictional losses are initially very high due to the viscosity of the grease. Those get better with time as the grease breaks down into oil at the mating surfaces. At the same time, the Rohloff has taken a great deal of care with their seals, and they are effective in part because they are self-lubricating to a degree, resulting in a fine misting of oil at and around the interface. Compare all that to a clean, fresh derailleur drivetrain and there's some striking differences and similarities at various points.

Overall -- as a function of wear and poor maintenance -- the differences are much closer. There are some relevant posts at this closely-related, earlier thread: http://www.thorncycles.co.uk/forums/index.php?topic=4073.0

Best,

Dan.

swc7916

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Rohloff Efficiency on a tandem
« Reply #12 on: June 21, 2012, 08:20:54 PM »
Dan:

My experience is that the Rohloff seems to impose a constant drag on the system.  With derailleurs on level ground I feel like I'm "chasing" the pedals; with the Rohloff I'm always pushing.  I think it's because the Rohloff doesn't coast as well and loses momentum between pedal strokes, and I believe that is why it works better at high cadences.  However, there are a number of things that I like about it:

In the higher gears it is quieter than a derailleur.  Shifts are quick and positive.  You never get a dropped chain.  You can shift without pedallng (I love this one.)   Because you always have a tight chain (no chain slap) the drivetrain stays cleaner and you don't really need a chainstay protector.  It's lower maintenance (I was always cleaning, adjusting and lubricating the derailleur system.)  My wife likes it because there's no front derailleur (she hits it with her calf when we're in the large chainring.)

It may seem like I'm griping about it, but we prefer the Rohloff tandem and ride it to the exclusion of the other one, which we keep as a backup.

fleur

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Rohloff Efficiency on a tandem
« Reply #13 on: June 21, 2012, 09:32:32 PM »
Whoa, you really jumped on me here.  

I am sorry, it wasn't what I was willing to say.

I have used a solo bike with a Rohloff and there was not such a difference.  Neither on the flat nor in downhill.

But on top of my subjective experience, it is just not possible that the hub reduces significantly your speed in downhill like a kind of brake that is continuously braking.  Imagine how much energy it must dissipate to reduce the speed of a tandem that probably weight 150kg (bike + two cyclists) or even more.  If it is the case with your Rohloff, it must really become hot just like brakes become hot if you brake continuously in a long downhill.

The same apply on flat roads.

Any equipment that has a low efficiency must dissipate/evacuate the lost energy as heat what will make it warmer, the lost energy doesn't dissapear, it takes another form.
« Last Edit: June 21, 2012, 09:35:56 PM by fleur »

il padrone

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Rohloff Efficiency on a tandem
« Reply #14 on: June 22, 2012, 06:49:44 AM »
In the case of the Rohloff hub there's more than just the acceleration of the wheel about the axle, there is also the acceleration of all the gears inside the hub; this is a point that most people miss.  Each gear has it's own rotating mass which adds to the rotation inertia of the wheel; you can feel this when trying to get up to speed from a stop.   Another factor is the Rohloff's full-contact seals; try spinning your rear wheel and see how quickly it comes to a stop compared to you derailleur bike.  Our downhill speeds with the Rohloff are noticably slower than with the other bike because of this.

I'm afraid this is all smoke and myth. I refer you back to that efficiency comparison graph. It shows the net outcome of the whole wheel in operation ie. includes all losses from gear rotation and any seal friction.

If your speeds with the Rohloff tandem are lower I'd suggest you have a look at a few other factors. Brakes rubbing? Gear ratio and range the same or different? Position on the bike? I don't know what it is, but there is no way a Rohloff hub should give you a 10% lower speed - that's a huge cutback.