Torque and gears and design limits. How to calculate ?

[KirstinS]

I have tried to search web but not doing a good job. Any help appreciated . Another kiwi I think has a particularly good understanding based on previous posts

Basically I have an alfine 8 hub and a bbs02b. And currently a 16t rear sprocket and a 42t bling ring up front

How on earth do I calculate wether I’m over stressing the alfine (at least to design spec , we all know it’s over designed and near bullet proof)

The settings on my bbs rarely pull as much as 500w. So less than a bbs01 at full tilt (around 600w)

But I’d like to know as my mates dad owns huge chunks of Essex . So private , unrestricted madness would be legal. As long as I go back to normal settings before I use it again

Just don’t want to blow up my alfine ....


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[Nealh]

I not 100% but even with power drive it is down to gearing that determines any failure 42 /16 combo is well under gearing stress level, 2.1 is recommended so 42/21 would work.

http://www.gear-calculator.com/
But doesn't allow for motor power.

Over to you AK .

 

[anotherkiwi]

I'm going to have to change my reading habits!

Shimano recommends 19, 20, or 21 for a 42 tooth chain wheel. So 16 is fine, 22 and above are no-no. Tech notice is on their site.

 

[KirstinS]

I'm going to have to change my reading habits!

Shimano recommends 19, 20, or 21 for a 42 tooth chain wheel. So 16 is fine, 22 and above are no-no. Tech notice is on their site.

Thanks , but isn’t it something to do with the watts that go through any given ratio ?


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[anotherkiwi]

It is the torque that the gearbox can handle. Or rather the torque it can't handle so 42:16 will have less torque than 42:22 throughout the 8 speeds.

 

[Nealh]

I read somewhere that they generally are over engineered and watt wise they can handle 1500w which none of us can do even standing on the pedals.
As already mentioned K it is torque that kills them via wrong gearing. 2:1 ratio or more is recommended so for a 42t front use a max 21t sprocket.

I have just put my old BBS01 from 2013 on my ht mtb with alfine 8 and have fitted a 48/32t double up front, chain line is a bit crap and front changes up and own ok without any issues so am going to give it ago.
With the 32t gearing torque will be a bit high with a 20t sprocket currently fitted though I have had no issues previously with this set up using a front hub motor, the 32t I only used occasionally.

Alfine 8 ratios 0.527, 0.644, 0.748, 0.851, 1.000, 1.223, 1.419, 1.615
Divide sprocket T size by each ratio above to get prexact gear size equivalent.

Sprocket size to approx. cassette gearing.

16t = 10/30 10,11,13,16,19,21,25,30.
18t = 11/34 11,13,15,18,20,24,26,32.
19t = 12/36 12,13,16,19,22,25,30,36.
20t = 12/38 12,14,16,20,24,27,31,38.
21t = 13/40 13,15,17,21,25,28,33,40

 

[Trevormonty]

I read somewhere that they generally are over engineered and watt wise they can handle 1500w which none of us can do even standing on the pedals.
As already mentioned K it is torque that kills them via wrong gearing. 2:1 ratio or more is recommended so for a 42t front use a max 21t sprocket.

I have just put my old BBS01 from 2013 on my ht mtb with alfine 8 and have fitted a 48/32t double up front, chain line is a bit crap and front changes up and own ok without any issues so am going to give it ago.
With the 32t gearing torque will be a bit high with a 20t sprocket currently fitted though I have had no issues previously with this set up using a front hub motor, the 32t I only used occasionally.

Alfine 8 ratios 0.527, 0.644, 0.748, 0.851, 1.000, 1.223, 1.419, 1.615
Divide sprocket T size by each ratio above to get prexact gear size equivalent.

Sprocket size to approx. cassette gearing.

16t = 10/30 10,11,13,16,19,21,25,30.
18t = 11/34 11,13,15,18,20,24,26,32.
19t = 12/36 12,13,16,19,22,25,30,36.
20t = 12/38 12,14,16,20,24,27,31,38.
21t = 13/40 13,15,17,21,25,28,33,40

Don't be surprised if hub dies.

 

[KirstinS]

I read somewhere that they generally are over engineered and watt wise they can handle 1500w which none of us can do even standing on the pedals.
As already mentioned K it is torque that kills them via wrong gearing. 2:1 ratio or more is recommended so for a 42t front use a max 21t sprocket.

I have just put my old BBS01 from 2013 on my ht mtb with alfine 8 and have fitted a 48/32t double up front, chain line is a bit crap and front changes up and own ok without any issues so am going to give it ago.
With the 32t gearing torque will be a bit high with a 20t sprocket currently fitted though I have had no issues previously with this set up using a front hub motor, the 32t I only used occasionally.

Alfine 8 ratios 0.527, 0.644, 0.748, 0.851, 1.000, 1.223, 1.419, 1.615
Divide sprocket T size by each ratio above to get prexact gear size equivalent.

Sprocket size to approx. cassette gearing.

16t = 10/30 10,11,13,16,19,21,25,30.
18t = 11/34 11,13,15,18,20,24,26,32.
19t = 12/36 12,13,16,19,22,25,30,36.
20t = 12/38 12,14,16,20,24,27,31,38.
21t = 13/40 13,15,17,21,25,28,33,40

Ahh

I think the penny has finally dropped ! Thanks for the full and patient explanation !


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[anotherkiwi]

I really wanted a nexus 8 (Alfine without disk mount) and the only way I could work it was with a twin chainwheel:

- low geared one right on the limit specified by Shimano for the local "hills"
- high geared one for flat going

And more number crunching made me come to the conclusion that the Rohloff was the best answer to this problem...

 

[KirstinS]

I really wanted a nexus 8 (Alfine without disk mount) and the only way I could work it was with a twin chainwheel:

- low geared one right on the limit specified by Shimano for the local "hills"
- high geared one for flat going

And more number crunching made me come to the conclusion that the Rohloff was the best answer to this problem...

Without question ! It’s something I’ve always hankered after but it’s a bit out of financial range more me

One day though !


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