JohanNeeda

I have a folder with an IGH, Shimano C300-7R. Front chainring is 44T rear cog is 17T (factory standard). The gear ratio is 2.58. The Shimano recommendation is max 2.1.


Any of you have similar gear ratios? How does the IGH work in the long run. I know that IGH cannot be really forced as they get damaged, therefore my question.

rhenning

I would try it before modification of any kind. It may work perfectly for you. Crunching numbers and real world experience are not always the same thing. Let us know how it works. Roger

sweeks

I *could* be wrong here, but I think the Shimano recommendation is a *minimum*. A larger chainring will not over-torque the hub; however, a larger rear sprocket might. This was discussed here: https://www.bikeforums.net/bicycle-m...speed-igh.html
IGHs are very reliable when used within specifications and maintained properly.
Steve

Bonzo Banana

I think you have that reversed. You can have a larger rear cog or a smaller front chainring as this will put less torque on the IGH but require a higher cadence by the rider to maintain the same speed.

So for example if you had 46T chainring and 22T rear cog you are at Shimano's exact recommendation 2:1 but if you went for 38T chainring and 22T rear cog you would be at 1.73:1 comfortably below 2:1 so the hub would have an easier time of it. This is the recommendation for 26" wheels I think. I'm unsure how much gain ratio you can get away with 20" wheels or larger 700c wheels although I have seen a 700c bike with a nexus hub and I think the front chainring was only 33T. You'd think something like 50T x 19T would be ok for 20" wheels however that is a complete estimate on my part as I haven't calculated the circumference difference. Apologies if I have misunderstood anything.

https://si.shimano.com/pdfs/dm/DM-SG0003-06-ENG.pdf

fietsbob

2.1:1 is the low limit.. [Rohloff is 2.35:1] higher than that is no problem.. my bike friday 20" wheel is 53:16. Koga WTR, 26" wheel 38:16.. (at low limit)

over the road, ratio range, the same..

Torque strain against gravity, going up hill, is greater than the effort done on the flat, against air resistance to go faster..


[because It's gear ratio X wheel diameter a smaller wheel gives you a lower gear using the same ratio ]







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Tourist in MSN

Nope.

A smaller chainring means that you can put more tension on the chain, that would then cause more (not less) torque on the hub internals.

Bonzo Banana

The normal granny gear arrangement is the chain running between the smallest chainring at the front and the largest cog of the cogset at the back which is super easy to turn enables you go up hills at low speeds. Also on many cassettes and freewheels the largest rear cog can be aluminium or lower grade metal but the smallest cog will be highest grade or heat treated metal possibly a decent steel alloy. Wouldn't that indicate the level of strain and wear?

Admittedly I'm not really factoring how internal hubs work because I'm not too knowledgeable with regard planetary gears but assume it is similar situation to a derailleur based gear system.

Besides isn't it how Shimano configures nexus/alfine hubs if you have a smaller rear wheel (20") you can have a larger front chainring but if you have a larger rear wheel (700c) you need a smaller front chainring?

fietsbob

4th is almost 1:1 (0.989 :1) so 3 overdrive 3 reduction gears... I looked it up https://www.sheldonbrown.com/nexus7.shtml






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Tourist in MSN

When you said "The normal granny gear arrangement is the chain running between the smallest chainring at the front and the largest cog of the cogset at the back which is super easy to turn enables you go up hills at low speeds.", that is correct. But, keep in mind you need high torque to go up hills. I have spun out (lost traction) when climbing steep hills because my rear wheel torque was high enough to spin on the ground surface.

When a cassette manufacturer uses a softer metal for a larger sprocket, perhaps they do not expect that sprocket to be used as often? Or perhaps they wanted to make it lighter? But if you are using the smallest chainring on a bike you are maximizing the chain tension and that would put more torque on the sprocket than if you were on a larger chainring.

I can't comment on specifics on Shimano IGH hubs, the only IGH hubs I have are a Rohloff, Sram Dual Drive and a vintage Sturmey Archer 3 speed.

The point with a IGH is that if you put too much torque on the sprocket, it could damage the inner gearing components. That is why Rohloff sets minimum ratios for the chainring to sprocket ratio. For my weight, that ratio that Rohloff sets is 1.9, but I use a range of 2.25 to 2.75 depending on where I am and how much weight I plan to have on the bike, it is a touring bike.

The reason you use a larger chainring on a folding bike with small wheels is because if you don't you spin out.

Abu Mahendra

i run a SRAM DualDrive II with a 47T chainring and a 11-36T cogset. Lowest ratio is 47/36, or 1.3

Nightdiver

No, Steve has it right. As you gear lower you're relying on mechanical advantage to move your bike, instead of your own muscles. 2.1 is the recommended minimum ratio quoted by Shimano for their Nexus/Alfine hubs. But, not all the hubs are built/designed equally, and there are many Nexus/Alfine 8 users who run ~1.5 ratios with no issues. While the Alfine 11 is known to be pretty sensitive to high torque.

That's just a matter of creating equal development metrics between two differing wheel sizes. The smaller wheel bike requires a larger gear ratio to achieve the same speed at a given cadence.

edit: whoops. Just noticed that Tourist already said the same stuff.

JohanNeeda

Thank you Steve. Spot on! I have followed the link and the links in the link. I think the whole confusion small/large cog comes from the fact that everybody thinks from its own (biker) perspective: small cog = hard, large = easy. Nobody think from the bike's perspective :-).


Thanks a lot.

JohanNeeda

fietsbob

FWIW https://si.shimano.com/pdfs/dm/DM-SG0003-06-ENG.pdf

Schwinnsta

The largest force that can be put through the hub will be the one with the largest rear cog. Why, because of leverage. For all riders, all have a max force they can produce. When that force acts on the largest cog, torque is maximized for that rider. I would assume the engineers have knowledge of that force. Perhaps max force a very strong rider can make on a hill start. my 0.02

fietsbob

Actually that is just true, if the internal gears are in 2nd, the 1:1, hub shell and cassette moving as one.

but because of the Cassette/IGH combination
in 1st, internally reduction geared, that ratio is X 0.73:1. cog driver rotating at a different rate than the hubshell.