Single Speed plus Granny - will it work?

[ChrisEyles]

First the idea - since it might sound stupid I'll add some background as to why I want to achieve this next post.

I want to convert my 90's rigid Marin to SS, with the option of a granny gear for long steep climbs.

I currently have a triple 42/32/24 chain ring set-up, and an 11-34t (8-speed) cassette.

I'm thinking of converting this to a 42/23 SS gear, with a 32/34 granny option.

The total tooth count is very similar for the two gears (65 vs 66) so the ideal chain length will be essentially the same for the two gears, given that I'll have to use a tensioner (was planning on simply using the rear derailleur) in any case as the drop-outs are vertical. It should also be perfectly possible to get a good straight chain-line in both gears.

Switching between the gears would be done by hand, and the rear derailleur would be trimmed to suit using the barrell adjuster on a captive bit of gear cable. Basically a "dingle speed", but with a much bigger gap between gears than is usual in such set-ups.

Will it work? The main problems I can forsee are

1) the tooth profiles on the chain rings and sprockets won't retain the chain well enough and I'll drop the chain a lot without a front mech

2) the torque exerted in the 32/34 granny gear might cause the sprocket to really dig into the cassette hub body and knacker it up

 

[Smokin Joe]

1) I don't see what problem you'll have with dropping the chain _ that happens when you shift and you won't be doing that.

2) Lower gears reduce the torque to the rear, they don't increase it (The reason your rear wheel is less likely to spin twiddling up a wet hill rather than grinding your way up).

 

[ChrisEyles]

Why do I want to do this?

I've always enjoyed riding single speed a lot, but shied away from trying it on a MTB because all my rides involve long steep climbs where any sensible SS gear would mean a lot of walking.

I've also had two MTBs for a while - one "best" bike (previously a retro FS, now a slightly more modern HT) and my trusty old Marin that's lived at work for the past few years. After changing jobs it's now back at home and I haven't really been riding it - there's not really any reason to choose it over the HT.

So I started toying with the idea of a SS conversion, and started paying attention to the gears I use the most on my HT. This turned out to be 30/17 or 30/18 (depending what cassette I've had) for general riding, but I also noticed I spent a LOT of time in pretty low gears, like 22/28 or even 22/32, spinning slowly up steep climbs. Other gears are either used for 1) short periods of time eg when levelling out of a climb 2) when I'm lazy and down-gear when I don't strictly need to 3) fast road sections.

Now a granny as low as 22/32 doesn't really go with the SS ethos of grunting away on the pedals, and when I was running a single ring up front I could get up pretty much everything with 32/34 (it was hard work sometimes, but satisfying).

So yeah - a 42/23 gear for most of the time (all the time if I ever find a ride without a brutal Devonshire slog in the middle of it!) and a 32/34 granny for long steeps. The 32/34 is sufficiently low that there's no temptation or point to twiddling around manually shifting gears as you go, which would defeat the point of it being a single (dingle?) speed.

 

[ChrisEyles]

1) I don't see what problem you'll have with dropping the chain _ that happens when you shift and you won't be doing that.

2) Lower gears reduce the torque to the rear, they don't increase it (The reason your rear wheel is less likely to spin twiddling up a wet hill rather than grinding your way up).

I can see the logic in 1) - but in that case why can you buy SS specific chain rings and sprockets? Is there any real advantage to them? I wouldn't mind splashing out on a couple of rings but unfortunately my lovely vintage STX crank has a stupid shimano special design where the big ring is mounted on the middle ring rather than the spider!

For 2) maybe I'm using the wrong term (I know torque is rate of change of angular momentum but I find that hard to visualise). Surely a gear with a big mechanical advantage is going to dig into the cassette body more though?

 

[Ian H]

If you're riding on the road then I'm slightly puzzled. If you are keeping the derailleur, then why not just use to change gear?

I find 43x18 fixed is okay for most Devon hills.

 

[ChrisEyles]

No, this will definitely be for off-roading, with road connections between bridleways only where necessary. Should probably have mentioned that - the lower gearing is necessary as otherwise you just stall when you hit a bump while climbing.

I also rode fixed for a bit, 44/19 I think. Going up was OK, but going down was pretty scary on some of the steeper hills! In the end I decided I prefer a freewheel.

No way would I try fixed off-road

 

[ChrisEyles]

Also thinking that usually the largest 6 sprockets on the 8 speed cassettes I've used are all joined together with pins, which I guess spreads out the force/torque over the splines of multiple sprockets, whereas on the set-up I'm proposing all the force/torque would go through the splines of a single sprocket.

Normally I'd just go ahead and try it anyway, but the rear wheel I've got on the bike at the moment is quite nice and I don't want to mess up the cassette body!

 

[Threevok]

It will only mess your freewheel body up if it's alloy. My steel one does get "marked" but not ruined

There are single speed solutions available for alloy hubs, but I would also suggest using a wide profile sprocket, like a Surly

I currently run 30/22 on the singlespeed for MTB purposes

PS : reading your post again, I think you may be referring to a "Dingle-Speed"

 

[mickle]

I did a similar thing a few years ago, single chain ring with two sprockets and a singleator chain tensioner set up to allow for the difference in chain length between the two sprockets. I ran two jockey wheels and shifted digitally (that is to say with my fingers). It worked OK. Never threw the chain.

 

[Drago]

Multiple chainrings? May as well ride a regular bike but not change hear.

 

[ChrisEyles]

@mickle that's encouraging. I don't think I'd have worried about dropping the chain so much if not for the availability of all these fancy chain guides, narrow wide chain rings etc etc. Guess the marketing has been working its magic on me...

@Threevok good point, I'm not sure if the cassette body is steel or aluminium on this hub, I'll have to check. Are you using a regular cassette sprocket on your 30/22 set-up? If it's an alloy body I don't think I'll manage to find a dedicated SS wide based 32t sprocket somehow!

30/22 is pretty low, but having pondered over all this for a while now I can see why you went for it as a single gear for off-road. I think that's about the highest I could push up the steeper climbs I ride (though I couldn't sustain it for as long as some go on for). Must be a PITA on the road sections!

 

[ChrisEyles]

@Drago and @Ian H yeah, I know... if the derailleur's still there anyway why not keep the shifter (or even the front mech & shifter too)... Hopefully I won't sound like too much of a hipster if I admit to preferring the idea of the simplicity of doing away with them. Plus it will stop me wimping into the granny unless I *really* need it. I'm thinking most of my rides involve at most three long steep climbs that would require use of the secret weapon granny gear... or since a SS bike would probably mostly get used for shorter blasts probably less.

 

[Threevok]

@Threevok good point, I'm not sure if the cassette body is steel or aluminium on this hub, I'll have to check. Are you using a regular cassette sprocket on your 30/22 set-up? If it's an alloy body I don't think I'll manage to find a dedicated SS wide based 32t sprocket somehow!

I am using dedicated SS sprockets on a 10 speed freehub (with spacers). You may be able to find a dingle-speed to suit your needs, not sure if they go up to 32t or not, but because they are doubled up they are wider - solving the width problem.

30/22 is pretty low, but having pondered over all this for a while now I can see why you went for it as a single gear for off-road. I think that's about the highest I could push up the steeper climbs I ride (though I couldn't sustain it for as long as some go on for). Must be a PITA on the road sections!

The climbs and flats here are very muddy here at the moment, so 30/22 suits it well. I have been using the same bike for the commute in the summer but I was running 36/14

I've been toying with the idea of Dingle speed myself - have a search and see what you think

 

[ChrisEyles]

Ah, I see, dinglespeed sprockets are two sprockets joined together with the correct (slightly wider than usual) spacing to match the chain rings? I didn't know such a thing existed! I'll have a look and see what I can find, thanks.

 

[Threevok]

I cant seem to find normal splined dinglecogs anymore

However, this may help

http://www.velosolo.co.uk/shopsingle.html

(Andel (S*rly) Cro-Mo Singlespeed Cog)