Changing the gearing on an Avanti Metro 8c

[Biffidus]

I want to lower the gearing on my wife's Avanti Metro 8c. It has an 8-speed Nexus hub with a fixed 38-22 drivetrain. I would like to lower the gearing significantly as she never goes above 5th gear and has trouble with hills. The problem is that 22T is about the biggest rear sprocket available (23T being the biggest, but harder to find) and the front is apparently fixed to the cranks so it might require new cranks.

By my reckoning I'd be looking for a 26T front sprocket. Any ideas on the best way to proceed? Has anyone tried something similar? Can I avoid replacing the cranks somehow?

[rkelsen]

It depends on the bolt circle diameter of the chainring. If it's 130mm, then 38 teeth is as small as you can go without replacing the cranks.

[Biffidus]

It isn't a chainring as far as I can tell. No BCD as it is a single piece sprocket attached to the cranks and axle.

I haven't taken it apart yet to see how it attaches.

[MattyK]

Pull the cover off and get us a photo of the chain ring. Might be a BMX style thing...

You can get a 24 t sprocket in 1/8" width but that is less than a 1-gear downshift

Currently she has a 25" low gear. A 26T ring will give her 17" of satisfaction (and a 52" top gear)

[rkelsen]

If the crank & chainring is all one piece, then you'll have to buy a whole new assembly.

I'd look for something second hand.

[Biffidus]

Here is a shot taken from underneath - I am going to have to take it apart to get better photos.

[rkelsen]

New crank required.

[MattyK]

I disagree. (Plus you would be hard pressed to find a single-ring crankset that can take such a small ring)

The chainring looks like a stamped steel piece, it will be separate from the crank arms - possibly a 3 bolt pattern (looks like I can see one nut on the arm at about 11 o'clock). You will however probably have to remove the right crank to replace the ring, and you would probably need a custom replacement eg from Cycle Underground.

If you're going to custom fab (and you probably need to), you might also consider just custom fabricating a larger rear 3-spline sprocket instead. (Assuming there is clearance on the chainstay) 38/32 would get you the same gearing as a 26/22.

[find_bruce]

Have you considered advising the GLW to HTFU ?

Ok moving on to comments that may actually assist, biggest rear cog I have seen commonly available is 24 teeth - available for a 3/32 chain from sjscycles.

The IGH forum on MTBR has plenty of information from people who have run low ratios. The lowest I have seen is 29 x 26

It seems a 24 or 26 teeth sprocket may require you to bend the cassette joint to clear the sprocket. Haven't seen anyone going anywhere near as big as a 32 at the rear. Some rough calcs indicate a 32t sprocket will be something like 130 mm diameter

To avoid the expense of custom manufacturing, you may need to go both directions with a 24 t sprocket and 32 t crankset

Have you thought about coming from another direction to make hill climbing easier with an electric assist - may even work out cheaper as well as keeping the GLW happy

[MattyK]

Looking at my bike - yes a 32T sprocket might hit the cassette joint. Dishing it inwards or bending the cassette joint out should alleviate this.

Basked on that MTBR link people clearly make custom sprockets.

[Biffidus]

Have you considered advising the GLW to HTFU ?

No. There are good reasons why that's not an option but I'd rather not go into that here. We had an electric assist bike before this one but it was heavy and harder work than a decent IGH. A decent quality kit might be an option in future but I was hoping for a simple solution so she can cycle up the short hills near our house.

I'm going to take the cranks off when I get time over the next few weekends and see how the front gear is attached. Hopefully it will be something I can then go hunting for replacement parts online.

[find_bruce]

Mate I had assumed you had a good reason for the seriously low gearing you were considering which is why I tried to be funny. Sorry if it didn't come across that way.

While you are looking at the bike, have a good look at the rear sprocket- if you can get it to fit, a custom sprocket as suggested by Mattyk may be the simplest solution, although you may need to move the chain guard.

[rkelsen]

I reckon it'd make more sense to watch ebay or gumtree for a cheap 2nd hand square taper triple crank and remove the two big rings before installing it.

[Biffidus]

How about BMX components? Are they compatible with other types of bikes?

[mitzikatzi]

To fit a small chain ring get a crankset like Truvativ AKA GXP Chainset You can remove all the chainrings and run a spiderless cog.

You can also do this with some older square taper shimano cranks.



that is a 28t chainring

[MattyK]

How about BMX components? Are they compatible with other types of bikes?

Far as I can tell - not likely. They seem to use different approaches to bottom brackets, rather than the 68mmx1.1375732238263" you probably have. They use cranks arms with a pin on the inner face that engages a single hole on the chainring, so you would have to swap BB, cranks and chainring.

Pretty sure you will be able to pull this off with just a custom chainring. Show us pics with the chainguard off.

[find_bruce]

Have you had any luck as yet biffidus ?

[Biffidus]

Not yet, here's a couple of photos of the crank and sprocket...



... and one of the crank itself... is there a way to get that plastic cap off without damaging it?

[MattyK]

Hmm, still hard to figure it out. Looks like it's pressed on to the RH crank on some splines, I can't see any other fastening method holding it in place.

[Biffidus]

Indeed. I tried prying the plastic covers off the cranks so I could remove the crank itself but I didn't want to break it.

[Comedian]

Just for a different take on things.. and not knowing the tyre size... could you fit some smaller and lighter tyres? That would lower the effective gearing (slightly) and a lighter/faster tyre might help her around and up the hills. You might have some laying around you could trial it with. Just a thought...

[rkelsen]

The plastic covers should unscrew.

The chainring is pressed on. You can't buy replacement rings for this type of crank.

I still reckon that something like this is probably the cheapest way out:

http://www.ebay.com.au/itm/MOUNTAIN-BIKE-42-34-24-TRIPLE-ALLOY-CRANK-COTTER-LESS-CHAIN-WHEEL-CRANK-SET-/261102391985

Just use the small ring.

[Biffidus]

Comedian: Smaller tyres won't do much - I want to move it down by about 2-3 gears which is a significant change.

rkelson: The plastic bits seem to just rotate in place - I suppose they could be damaged. The bike shop suggested I might need to replace the cranks as well.

[Comedian]

Comedian: Smaller tyres won't do much - I want to move it down by about 2-3 gears which is a significant change.

Mate I take that on board. I know you mentioned you'd tried the electric thing ...

But our story is I bought my wife a lovely Cannondale Adventure with an IGH to ride to work on. She tried it a few times. But the hills... it was just too much with all her gear in it etc. I rode the thing and it took my commute from like 30-35 minutes on my hybrid to 45-50. So, I got her one of these and she hasn't missed a commute since. Her commute is about 35 which is fine and she doesn't even need a shower at the other end. With the geared motor hub she gets up the **really** steep hills that allows her to bypass the worst of the traffic out our way... another win.

[Biffidus]

Ok, finally got the crank off:

It appears to be a Prowheel brand chinese crank and the sprocket seems to be permanently attached.

Edit: the local bikeshops I tried were no good. Anyone know where I can get a square-taper crankset (preferably a double rather than a triple) that will take 64 or 74mm BCD chainrings? I'd rather not replace the bottom bracket.