Hub gears

[benb]

As in this thread, I'm looking to get my wife a new bike for her birthday.
The one I have my eye on has a Shimano Nexus-7 speed hub gear.

How do I make sure that the lowest gear is going to be low enough for hills?
At the moment she can make it up the hills on our commute to work, with a bit of a struggle, in the middle ring at the front, biggest on the rear, on a fairly standard MTB.

Presumably, I can change the rear sprocket for a larger one, and bung a few extra links in the chain, if the lowest gear is too high.
She won't be bothered about the highest gear being too low, so that's not a consideration.

 

[mickle]

Chainring # of teeth (32?) divided by sprocket # of teeth (28?) multiplied by wheel diameter in inches (26"?) gives you gear inches. (29.7"?)

Internal hub will give you a '%of' number to decribe its low gear. One of the gears will be direct drive, other gears are described in relation to it. So top might be, say, 150% and bottom 66% of direct.

You adjust the relationship of your sprocket and rings to acheive the low gear you desire.

Simples...

 

[corshamjim]

You can use the Sheldon Brown gear calculator to check what ratios are possible. http://www.sheldonbrown.com/gears/

The largest sprocket available for Shimon Nexus (on SJSC website) is 23t. This in combination with the 44t chainring would give you a 33" lowest gear. If we suppose the MTB has a 38t middle chainring and 32t largest sprocket then this would give you a 30.9" gear. Based on that assumption, the Nexus will be a little tougher up the hill than the MTB.

You really need to check how many teeth are on the middle ring and largest sprocket rear of the MTB to compare properly, and maybe contact CremeCycles to ask them what sprocket they normally fit too. If it's a lot smaller than 23t then you might find your larger chainring won't fit under the chainguard. You could presumably fit a smaller chainring instead, but that might add more cost than you feel is worthwhile. It might be worth asking CremeCycles if they can offer that as an option. It's a nice looking bike btw.

 

[benb]

Thanks, I'll check the exact gearing on the MTB later.
I just need number of teeth, and size of wheel, right?

 

[MacB]

I find it easiest to llok at the range described in decimal format in relation to the chose cog/chainring combination. for example:-

The Shimano 7 speed ranges from 0.632 to 1.545 so this means that whatever your chosen cog/chainring works out to in gear inches, then your bottom gear is .632 of that and the top is 1.545 times as big.

Using the biggest Nexus cog of 23t as a starting point, it will also depend on what size wheels the new bike has, but based on 700c and a reasonably chunky 700x35 tyre. Say you go with a 2:1 ratio so 23t/46t then that's 54 gear inches, so your upper gear is 1.5 times that and the lower is 2/3, or 83 and 34 gear inches. Moving the ratio around works fine but whichever way you go you'll lose gears at the other end.

Say you wanted a bottom gear of 27 inches, then you'd limit the top end to 65 inches, personally I'd look for an alfine 8 speed rather than the Nexus 7. The 8 would give you 27 to 83 for example.

 

[benb]

I find it easiest to llok at the range described in decimal format in relation to the chose cog/chainring combination. for example:-

The Shimano 7 speed ranges from 0.632 to 1.545 so this means that whatever your chosen cog/chainring works out to in gear inches, then your bottom gear is .632 of that and the top is 1.545 times as big.

Using the biggest Nexus cog of 23t as a starting point, it will also depend on what size wheels the new bike has, but based on 700c and a reasonably chunky 700x35 tyre. Say you go with a 2:1 ratio so 23t/46t then that's 54 gear inches, so your upper gear is 1.5 times that and the lower is 2/3, or 83 and 34 gear inches. Moving the ratio around works fine but whichever way you go you'll lose gears at the other end.

Say you wanted a bottom gear of 27 inches, then you'd limit the top end to 65 inches, personally I'd look for an alfine 8 speed rather than the Nexus 7. The 8 would give you 27 to 83 for example.

Thanks, very useful.
Would other hub gearsets fit into the same frame?

 

[MacB]

Thanks, very useful.
Would other hub gearsets fit into the same frame?

yep, most of them are 135mm OLD, the cable runs for the gears are normally full outers and you can fit them with cable ties. The neatest option is to use the Rohloff specific ones that actually hold the cable in a little open/close latch bit, but they are the best part of £5 per pair

Oops, hang on, the Nexus may be a 130mm OLD - but if you're looking at a complete bike then there are several options with the Alfine 8. The 135mm is MTB standard and road is generally 130mm - the hybrid/tourer market varies between the two depending on what you go for. Personally I'd aim for something with a 135mm OLD as most hub gears are aimed at that size now.

 

[benb]

yep, most of them are 135mm OLD, the cable runs for the gears are normally full outers and you can fit them with cable ties. The neatest option is to use the Rohloff specific ones that actually hold the cable in a little open/close latch bit, but they are the best part of £5 per pair

Oops, hang on, the Nexus may be a 130mm OLD - but if you're looking at a complete bike then there are several options with the Alfine 8. The 135mm is MTB standard and road is generally 130mm - the hybrid/tourer market varies between the two depending on what you go for. Personally I'd aim for something with a 135mm OLD as most hub gears are aimed at that size now.

Thanks. I've emailed Chain Reaction, as they seem to be the only stockists of that bike. Hopefully they can advise whether customising it is something they can do, as I'd rather not have to buy an additional hub gearset.