TurbineBlade

HI,

I'm getting a Nexus 8 speed equipped bike very soon and was wondering whether it would be acceptable to change the stock 38 X16t gearing to 38 X 19T...? I've read that IGHs have a narrow range of "acceptable" gearing combos based upon stress to the hub. Strangely, some seem to say that higher gearing is less stressful to the IGH, and then others (like Sheldon) have said that lower gearing is less stressful.

What is the inside scoop on this?

Thanks,

fietsbob

Not specified in the owner's flyer that came in the hub box?
or in Shimano's library of PDF copies of that sheet on Line?

their US Phone number is (949) 951 5003.. CDN (877)377 5211

I know Rohloff is clear on that: 16/38, 15/36, 17/42..
anything higher is fine, so the 16/53 on my bike friday
is better than the 16/38 in the 26" wheel trekking bike.

DCB0

Lower gearing (easier gears, so your legs spin faster to go the same speed, smaller chainring or larger cog will obtain a lower gear) is harder on a hub... the torque you are able to put into the pedals is increased by the gearing and the high torque is what is suspected of damageing hubs.

Here is what Shimano has to say about it:

"It is recommended that you set the chainrings sothat the gear ratios are approximately 2.1.

Example: F36T – R16T, F38T – R18T, F46T – R22T"

(from https://www.shimano.com/media/techdoc...9830638061.pdf)


This 'recommended' ratio has been the subject of much discussion with respect to the newer Alfine 11 speed hub, as some take this recommendation to mean 'ratio below which you can expect your hub to barf tiny gears and springs across the road,' and some take it to mean 'ratio that will give a reasonable ratio for a city bike, but go ahead and try whatever gears you have in your parts box.' Unfortunately, some the Alfine 11 hubs seem to be barfing their tiny gears and springs across the road regardless of ratios and input torque, so it is difficult to tell which group is correct. Happily, the Alfine 8 and Nexus 8 hubs seem to be far more robust and many people are using them with various ratios and they are holding up. I personnally have an Alfine 8, which I am told has similar internals tot he Nexus 8, and I use a 2:1 ratio (44:22) which is identical to what you are proposing, and it has held up fine for the past three years with no maintenance. Also I weigh 260 lbs and I stand up to torque my way up hills all the time and nary a whisper of complaint form the hub.

I think you should try it.

TurbineBlade

Okay, that gives me something to think about. Actually, I'm not sure that using the 19t cog is something I'll actually do....because the low gear doesn't actually decrease by a large degree regardless. I would probably have suitable gearing for the kind of riding that I do with either gearing, so I'll definitely think about it and maek a decision on it before the bike arrives.

Thanks -

fietsbob

Ah 1:2.1 ... so 19:40 is the combo within spec, 19:38 is just a bit over the line.
38:17 better.. just on the line..

TurbineBlade

Actually - I still don't get why lower gearing with an IGH is harder on the hub...? I believe it, but I don't understand why.

fietsbob

Over-torquing..
like using a foot long 1/2" breaker bar on a 5mm bolt.

The hub itself is already a collection of reduction gears..

In the case of Rohloff #11 is the 1:1, and only the 3 above that are overdrive gears,
So the other 10 are all reduction gears 1~7 are a double reduction gear,
as the 7 speed hub is used twice.. 7 is high in low range, 8 is low in high range.


Nexus/Alfine it's 5th of 8 that is the 1:1 direct gear..

krome

What is your wheel size? The input ratio for an IGH can vary greatly depending on the wheel size. Are you putting it in a folding bike with 20" wheels or a commuter bike with 700c wheels?

I'm running a Nexus 8 (SG8R-36) on my 650b bike. I use Sheldon Brown's gear calculator https://sheldonbrown.com/gears/ to play with the numbers. I started out with and currently have 48-24 (2:1). I also have short horizontal dropouts so I want to maintain the distance very closely, so I use the calculator at Fixed Innovations (Fixmeup!) https://eehouse.org/fixin/index.php to give me my chainstay distance. I wanted to run 46 x 19 to get a higher top end. However, to maintain chainline I have to install the cog dished in, this is not doable with the 19t cog due to a mechanical seal interference. I'm now planning on trying 46 x 22 because this will maintain the chainstay distance and give me a slightly higher top gear than my current setup. I'd like to go 44 x 20 but I don't want to get a chainring and a chain guard also. I started with 48 because I was originally going to go with a 1x8 derailleur setup, and had the chainguard. I went with the Nexus 8 instead, and so I used a 24t cog because I had it and knew I could gear it up more with smaller cogs. I want to go to a 46t front, because my chain slaps the chainguard and it is annoying).

The current gearing and 46x22 are really nice for mildly hilly terrain and city riding. I want it to be higher geared like 46x19 for long distance riding. I might put in an FD and two chainrings (with tensioner), but I'd like to avoid it.

So, the question is, what is your desired gear-inch range? Do you have a derailleur equipped bike or experience to compare with?

As far as protecting the hub, you can go to a higher ratio than 2:1 (higher numerically like 2.1:1). Going the other way, I'm not sure what the limit is, but you risk too much input torque. Chain tension will be lower with larger chainrings and cogs, even if the ratio is the same. 48 x 24 is the same as 38 x 19 as far as ratio, but the 48 x 24 should have lower chain tension for a given input torque (at the crank). Lower chain tension should mean less stress on the bearings. Larger chainrings and cogs weigh more but they should wear longer.

DCB0

The chainrings and sprockets on your bike multiply the torque produced by your feet pushing on the ~175mm cranks. Most setups involve a larger chainring with a smaller cog – like 48:21 or 36:18 – and this reduces the torque by a factor of the ratio (a 2:1 ratio like a 38:19 divides the torque by 2). If you go to a ‘smaller’ gear, like a 1.5:1 or 1:1, the torque is reduced by that amount… divided by 1.5 or 1, so is reduced less. The torque after this reduction is what the IGH internals see, so if you do not reduce it enough, the input torque can be too high.