Conversion from single speed to mutli-speed
 

We were considering upgrading our 1970 Schwinn Twinn from single speed to multi-speed. The Twinn is coaster brake in back and center pull in front. There are no Qrs. Shimano's Nexus hub http://bike.shimano.com/catalog/cycl...=1131371615095
has appeared in few posts. We would have a new wheel built for the hub as the oem wheel is in nearly perfect condition. Is the conversion difficult, and are there reliability issues? Any feedback on the sg-7r46 brake system?

I had the hub on a single bike and I liked it. I don't know how it would handle the load on a tandem! The roller brake that comes with the hub is not a very strong brake. Even on the single bike it would take very long to stop with just that brake. For a tandem, I would definitely get a rim brake as main rear brake and use the roller brake (maybe) as a drag brake.
Your tandem probably has horizontal dropouts or at least some kind of chain tensioner. Therefore it is well suited for the internal gear hub and the conversion is easy. :)

We have an internally geared hub on a Bike Friday tandem. It works fine.

One question is the axle spacing on your bike. The hub you're looking at is 130 mm. I'm guessing your bike might have 120 or 126mm spacing.

You probably can cold set the drop outs to accomodate the difference. However, if you're not familiar with doing that, I'd have a shop do it.

I put a 7-speed Shimano Nexus on the 1973 Schwinn Twinn my girlfriend and I recently bought. I have a lot of experience with the Nexus. I worked on a Moon Buggy project at Arizona State; we built human powered, tandem, 3 or 4 wheel, folding, off-road recumbents and used the Nexus as transmissions.

The gear range on the Nexus is just about perfect for the Twinn, the low gears make climbing bearable/possible and you can really get it going in the high gears. The other cool thing is that you can shift even when you are stopped and not pedaling.

The Nexus should be available with a thumb shifter or a grip shift. The thumb shifter is much better for the Schwinn bars but they are harder to find.

The roller brake is good considering the weight of the bike but it has to be set up correctly. It is an improvement over the stock coaster brake. It has a mushy lever feel but that seems to be normal for this brake. Also, I'm using the stock brake lever since I like the vintage look (and parking brake button), different levers do improve it some.

On the Twinn I did have to spread the rear dropouts to 130mm but it is pretty easy with the right tool.

I'm using the 21 tooth cog, it is possible to get other sizes if you want a slightly higher or lower gear range. The cogs are dished and can be flipped over to get the chainline right.

As for reliability I haven't had too many problems with them. On the Moon Buggies we over torqued them sometimes. But we were using significantly lower gears, the Buggies were heaver, and on some of them there was one Nexus for both drivers. This would only damage the drive cog which is easily replaced.

Getting it setup is pretty easy, the cog is held on by a snap ring, and over this there are three pieces that hold the cable and the cable nut, and rotate to shift gears. This cable nut has to be attached a precise distance from the end of the cable. You then use the barrel adjuster to line up two red marks when you are in 4th gear. The roller brake is bolted on the other side and there is an arm that attaches to the chainstay.

If you have any more questions let me know. I can post some pictures also if you want.

--Guz

It sounds to me like you'll have to buy the hub, get the wheel built, cold set the frame, figure out the brakes and the shift cable routing. It still might be a little bit iffy.

So how much would a brand new lower end Raleigh tandem cost where you live?

FWIW, Sheldon has a rather comprehensive web page addressing the Nexus hub to include conversion of existing frames: http://www.sheldonbrown.com/nexus-mech.html

Please post any photos of Nexus installations. Details are appreciated. I have another tandem ('dale mt800) but want to preserve the simple fun factor that the Twinn has. Our Twinn tends to be a 'loaner' that newbie riders will jump on. My concern was whether the Nexus could handle the torque exerted on hills. Guz, I overlooked the chainline. Thanks for mentioning that.

Based on my previous experience, it will handle the torque. Our Moon Buggies were usually 150 to 200 lbs and we were using significantly lower gears (much higher torque). We did ocassionally over torque them but even when we did, it didn't damage the Nexus itself. What happens is, the small dimples that hold the cog on the Nexus will shear off, all you have to do is replace the cog.

We have been putting a lot of miles on our Nexus equipped Twinn for the last few months. It has worked really well for us as beginning tandem riders. It makes the Twinn much more enjoyable which is the whole reason we got one.

--Guz

I converted my twinn to a "6 speed" by replacing the cranks with modern square tapers, bmx conversion bottom brackets, a down tube shifter and a new rear wheel with a 6 speed cog. It works great and was a much cheaper (for me) option to the nexus. At the time of the build, I was told by several sources that the nexus would fail under load. After reading this I don't know... sounds like the nexus would hold up. I just wanted to make this alternative suggestion, it don't look as pretty, but it rides like a champ!

The stock spread of the rear dropouts is 100mm. How do you widen that symmetrically to 130MM?

I am not certain the rear drops at only 100mm seemed more like 115-120 as I recall. I put a 126mm hub in there with a 6 speed suntour freewheel. I just squeezed it in. It fit fine with no problems. Previous to this wheel it has the original coaster brake wheel (a strumey archer). The front drops are around 100mm maybe that's what you're thinkin?

The dropout spacing on my Twinn was definently more like 120mm also. I used a Park frame tool to widen the rear triangle.

--Guz