Revelation or Sheer Lunacy?

[neil_p]

It's commonly suggested that the Rohloff hub doesn't weigh any more than 2 deraileurs + chainset + cassete etc.  However it is also regularly suggested that one disadvantage is that the weight of the Rohloff hub is all within the wheel, and that "rotating mass counts double" (or something along those lines).  This is therefore used as an argument against Rohloff and in favour of deraileurs.

Thinking about this logically, and viewing the video on youtube.com of the internal workings of the speedhub, I believe this isn't strictly true...

The chain connects to the rear sprocket which rotates, in turn spinning a series of cogs which in turn rotate the hub casing to which the spokes and rims are attached.

Firstly, there seems to be quite alot of "metal" which isn't rotating - the axle, and various casings around it all seem to remain stationary whlst everything else work around it.

Secondly, only some of the cogs are being used at any one time, some appear to remain stationary so aren't having to be moved via pedal power.

Thirdly even the cogs that are in use aren't strictly spinning around the axle.

I would argue (based on my non-existant knoweldge of physics and engineering) that most of the weight of the speed hub is not rotating mass at all. Yes the gearing system adds a certain amount of friction, but doesn't add noticeably to the amount of weight needing to be rotated.

I would love to hear people's thought's about this.  If I'm completely wrong it would be great to know!  []

Neil

[pdamm]

As I understand it, the extra significance of additional weight in the wheel increases the further the weight is away from the centre of the wheel.  The impact of this effect is that it is harder to accelerate a bike with a heavier wheel than it is to accelerate a bike of the same weight but the wheel is lighter.  This is because to get a bike to move faster you must not only accelerate the whole bike horizontally (wheels included) but also get the wheels to spin faster.  Since the bits on the outside of the wheel are moving faster than the bits at the centre, adding weight to the outside of a wheel makes it harder to for the wheel to spin faster than adding the weight to the centre of the wheel.

Adding say 500g to a wheel in the form of a heavier tyre will make the bike harder to accelerate than having the additional 500g attached to the frame in say the form of half a litre of water.  Having an additional 500g siting in the axel will have much the same effect as the extra half litre of water on the frame.  Since the Rohloff is mostly at the centre of the wheel it will have only a small additional effect.  

However, as you point out, due to the different movement of the individual components within the hub a wheel with a Rohloff will have a slightly different “rotational inertia” compared to a wheel of the same weight distribution where all the parts are fixed.  But because all the parts are small and near the centre of the wheel any effect would be small.

Another theoretical aspect of the additional weight in the wheel is the performance of the suspension in a fully sus MTB.  The “smoothness” of the ride (other things being equal) is determined by the ratio of sprung to unsprung mass.  Since the unsprung mass is a lot less than the sprung mass removing say 200g from the unsprung mass has a much bigger effect than adding it from the sprung mass.  In a fully sus MTB having a Rohloff in the back wheel increases the unsprung mass compared to a deraileur system.  Hence theoretically a fully sus MTB with a Rohloff would give a rougher ride than a comparable bike with deraileurs.  I haven’t ridden a fully sus MTB with a Rohloff and been able to compare it to a similar bike with deraileurs so I don’t know if this effect is noticeable at all, but I would be interested to find out.

[bike_the_planet]

quote:Originally posted by pdamm


As I understand it, the extra significance of additional weight in the wheel increases the further the weight is away from the centre of the wheel.  The impact of this effect is that it is harder to accelerate a bike with a heavier wheel than it is to accelerate a bike of the same weight but the wheel is lighter.  This is because to get a bike to move faster you must not only accelerate the whole bike horizontally (wheels included) but also get the wheels to spin faster.  Since the bits on the outside of the wheel are moving faster than the bits at the centre, adding weight to the outside of a wheel makes it harder to for the wheel to spin faster than adding the weight to the centre of the wheel.

Adding say 500g to a wheel in the form of a heavier tyre will make the bike harder to accelerate than having the additional 500g attached to the frame in say the form of half a litre of water.  Having an additional 500g siting in the axel will have much the same effect as the extra half litre of water on the frame.  Since the Rohloff is mostly at the centre of the wheel it will have only a small additional effect.  

However, as you point out, due to the different movement of the individual components within the hub a wheel with a Rohloff will have a slightly different “rotational inertia” compared to a wheel of the same weight distribution where all the parts are fixed.  But because all the parts are small and near the centre of the wheel any effect would be small.

Another theoretical aspect of the additional weight in the wheel is the performance of the suspension in a fully sus MTB.  The “smoothness” of the ride (other things being equal) is determined by the ratio of sprung to unsprung mass.  Since the unsprung mass is a lot less than the sprung mass removing say 200g from the unsprung mass has a much bigger effect than adding it from the sprung mass.  In a fully sus MTB having a Rohloff in the back wheel increases the unsprung mass compared to a deraileur system.  Hence theoretically a fully sus MTB with a Rohloff would give a rougher ride than a comparable bike with deraileurs.  I haven’t ridden a fully sus MTB with a Rohloff and been able to compare it to a similar bike with deraileurs so I don’t know if this effect is noticeable at all, but I would be interested to find out.

Without getting toooooooo technical, rotational mass is all about what physicists call 'moment of inertia' - the rotational equivalent of mass and is proportional to m x R^2, where m can be considered as the weight of the rotating object (in this case the rim, tyre, spokes and rohloff hub). The other bit is the R^2 - the radius squared of all these bits of weight.

In simple language, this means that bits on the rim of the wheel (ie rim itself and tyre) have far more effect on the rotational acceleration of a wheel (and therefore the bike) than bits at the centre of the wheel (ie Rohloff hub).

And as you said, some of the bits in a Rohloff don't rotate anyway - others rotate within the hub itself (planet gears).

So I would suggest that weight saving at a rim's periphary (lighter rims and tyres?) is of far more relevance than at the hub. As the old riders used to day "an ounce off the wheel rims is worth a pound off the frame"

Cheers

[bike_the_planet]

quote:Originally posted by pdamm


As I understand it, the extra significance of additional weight in the wheel increases the further the weight is away from the centre of the wheel.  The impact of this effect is that it is harder to accelerate a bike with a heavier wheel than it is to accelerate a bike of the same weight but the wheel is lighter.  This is because to get a bike to move faster you must not only accelerate the whole bike horizontally (wheels included) but also get the wheels to spin faster.  Since the bits on the outside of the wheel are moving faster than the bits at the centre, adding weight to the outside of a wheel makes it harder to for the wheel to spin faster than adding the weight to the centre of the wheel.

Adding say 500g to a wheel in the form of a heavier tyre will make the bike harder to accelerate than having the additional 500g attached to the frame in say the form of half a litre of water.  Having an additional 500g siting in the axel will have much the same effect as the extra half litre of water on the frame.  Since the Rohloff is mostly at the centre of the wheel it will have only a small additional effect.  

However, as you point out, due to the different movement of the individual components within the hub a wheel with a Rohloff will have a slightly different “rotational inertia” compared to a wheel of the same weight distribution where all the parts are fixed.  But because all the parts are small and near the centre of the wheel any effect would be small.

Another theoretical aspect of the additional weight in the wheel is the performance of the suspension in a fully sus MTB.  The “smoothness” of the ride (other things being equal) is determined by the ratio of sprung to unsprung mass.  Since the unsprung mass is a lot less than the sprung mass removing say 200g from the unsprung mass has a much bigger effect than adding it from the sprung mass.  In a fully sus MTB having a Rohloff in the back wheel increases the unsprung mass compared to a deraileur system.  Hence theoretically a fully sus MTB with a Rohloff would give a rougher ride than a comparable bike with deraileurs.  I haven’t ridden a fully sus MTB with a Rohloff and been able to compare it to a similar bike with deraileurs so I don’t know if this effect is noticeable at all, but I would be interested to find out.

Without getting toooooooo technical, rotational mass is all about what physicists call 'moment of inertia' - the rotational equivalent of mass and is proportional to m x R^2, where m can be considered as the weight of the rotating object (in this case the rim, tyre, spokes and rohloff hub). The other bit is the R^2 - the radius squared of all these bits of weight.

In simple language, this means that bits on the rim of the wheel (ie rim itself and tyre) have far more effect on the rotational acceleration of a wheel (and therefore the bike) than bits at the centre of the wheel (ie Rohloff hub).

And as you said, some of the bits in a Rohloff don't rotate anyway - others rotate within the hub itself (planet gears).

So I would suggest that weight saving at a rim's periphary (lighter rims and tyres?) is of far more relevance than at the hub. As the old riders used to day "an ounce off the wheel rims is worth a pound off the frame"

Cheers

[neil_p]

Thanks for that, you've supported my hunch with actual science - bonus!  So, in summary, when people say that a Rohloff has all the weight of the drivechain in the wheel, and is thus bad, they're wrong.  Hurrah for Speedhubs!

[jawj]

Well, they're not quite wrong...

While the inertial effect of the hub is indeed negligible compared to the rim & tyre, I think people's beef with the Rohloff is that it concentrates all of the weight of the drivetrain in one point, i.e. a point that's quite far back on the bike thus affecting the fore/aft balance of weight.

This is indeed noticeable on a Rohloff-equipped bike, but I've not been affected by it. When I put a Speedhub was on my Orange P7 the extra weight at the rear was counteracted by the rather portly, but brilliantly performing, Marzocchi forks at the other end. The Speedhub is now on a Raven Nomad which has a not-lightweight rack bolted on to the back and more often than not is carrying a shedload of stuff so  the backend is heavy, no matter what!

[avdave]

Yes it does move weight to the back of the bike but in comparison to the effect of going from rigid to suspension forks it is insignificant in the handling of the bike, in fact if anything on an mtb it shifts the balance back towards the middle of the bike and should therefore improve handling.

[jawj]

... unless of course you're riding a mountain bike with rigid forks...

I tried my P7 with some rigid Pace RC31s on (not for very long as the bike was designed for much longer forks and the handling was impossible, as detailed in another thread on here somewhere) and the weight was very much at the rear of the bike.

But, as you say it, with the suspension forks on the Speedhub definitely helped with the balance.