Tuesday, April 7, 2015

An economical super compact road crank.

Shimano "RSX" triple road crank re-purposed into a wide-range compact double.
I'm still trying to find a lower priced compact double which will give me the ability to run a 46/28-30 tooth combination. I did buy a V.O. 50.4 mm "Rando" crank some time ago. While I do love the gearing it allows, it has proven quite finicky to install. The most successful installation is my latest, on my '82 Mercian Olympic which features fairly long chainstays, a 120mm, 5 speed rear wheel, and an old Campy Nouvo Record front derailleur. I couldn't make it work well on my new Mercian.

I also tried converting an old Shimano STX 94mm BCD MTB crank. This wasn't entirely successful because the shape of the inner face of the spider pushed the chainrings too far outboard.

I have been reading about Sugino's recent super compact cranks, the OX801d and OX601d. These double cranks feature a 74mm BCD and a 110 BCD chainring. This would provide the gearing I'm looking for in a more modern crankarm shape, allowing me to use a normal Front Derailleur and my 135mm 8 speed rear wheel. Unfortunately, these cranks are also staggeringly expensive! $490 for the 601!

Looking over the pictures of these cranks, it seems to me that you could build your own with an ordinary 110/74 triple crank. I have a couple lying around so I thought I'd try it.

My test subject was a Shimano RSX triple I bought from Nashbar years ago. This is a decent enough crank which had a ramped and pinned chainwheel on the outer side, the inboard granny gear was a simple, plain chainwheel.

A look at the inner side, the large ring was reversed to allow the chainwheel bolts to be recessed a bit.
 Basically, what you're doing here is removing the middle chainwheel and replacing it with the outer (46 tooth) one. You will need single wheel chainwheel nuts and bolts since you're now converting the 110mm ring into a single. You also need to have the chainwheel bolt nuts(?) recessed a bit or they could interfere with the chain.

Another view. One advantage with reversing the ring is that I can use the unworn side of the teeth!
Since this was an experiment I used the original outer ring. I had to reverse it to get the recesses on the correct side. This will mean the ramps will be negated but I'm not too worried about that since this will be a double and shifting shouldn't be much of an issue. I also had to remove the little pin which catches the chain in the event of an overshift. Sadly, this pin was not threaded into the ring, just a press fit so it can't be reused.

I had to install a longer bottom bracket in order to have the chainrings clear the chainstays. A 118mm Shimano UN55 did the trick. The bolts fixing the small chainring come fairly close to the chainstay, but they do clear it and a miss is as good as a mile!

A 118mm bottom bracket provided just enough clearance.
I had to adjust both the front and rear derailleurs for acceptable shifting. The front changer had to be lowered a few millimeters and the side to side movement adjusted for the longer BB spindle. The rear short-cage Tiagra can handle the 18 tooth difference in the front sprockets, but only after I adjusted the "B" screw, turning it all the way in to increase tension on the chain. Indexing was unaffected.

The one issue which remains is that the chain contacts the little pins on the reversed big ring when the chain is on the small ring and the 5 outer rear sprockets. It clears the pins fine in the 3 inboard positions, which will probably be the only ones I use with the granny anyway. At any rate, I'm not too concerned because I will eventually replace the large ring with an un-pinned and un-ramped one which should eliminate the problem. I expect there may be some interference in the outermost sprocket positions even then, but I would never use those combinations.

My initial test ride was a success! Shifting was good and the gearing is more to my liking. The 46 tooth outer ring gives me a nice selection of gears for almost 90% of my riding, (I use all 8 sprockets with the big ring), and the 28 tooth granny offers a "27-inch" low gear which is good enough even for touring loads. Shifting was reliable and quick.

A view of the drivetrain.
There have been some cranks offered which have the same idea: Rivendell offers a nice Sugino crank which is essentially a triple with a chain-guard in place of the outer ring for only $146. The only thing I don't like is the really low gearing offered: 40-26, for my road riding that would be too low, I prefer the 46-28 combo. Velo Orange offered a crank with close to that combination, (46/30) for their Polyvalent at one time, but I can no longer find that on their website.

In the future, I will replace at the outer ring with a plain Sugino chainring that has no ramps or pins. Eventually, I will try to find a nice set of new arms to build a brand new crankset with this gearing.

Update: I have replaced this crank with a similarly modified Sugino Messenger: See: Another stab at a subcompact crank.