Wednesday, 29 February 2012

Dingle gearing

You might think that bicycle gearing would be a simple thing. There are only four variables: rear wheel size, rear cog size, front cog size and crank length. Surely Sheldon Brown has the answer. But then the rider comes into the picture... Consider this.

How many gears does a single speed bicycle have? I say at least two. Sitting and standing.

There is also a difference between Spinning - evenly applying force through the whole pedal cycle, usually with a fast cadence, and Honking - pushing hard on the down stroke and letting your foot come back around with the pedal, usually at a slower cadence.

If you want to think in terms of energetics, how much power is needed to move at a given speed, then factors such as the weight of the bike and rider, the energy lost to heat through friction and the angle and surface of the road come into play.

Do you remember Miguel Indurain and Lance Armstong riding in the Tour de France? Big Mig growling along like a V8 and Armstrong spinning like a Wankle rotary engine. They say each rider has an ideal cadence and you should use your gears to keep your rhythm steady over different terrain. But I'm not so sure about this. Years ago I was riding with my mountain biking mentor on a long climb into the Port Hills behind Christchurch. He pulled up beside me and said. "How are you going?... If you're getting a bit tired change up." "WHAT?"... "Change Up. Get out of the seat and honk for a bit. Give your legs a rest."

He was right. Bigger gear, slower cadence, different position, till I got tired of that, and then I could change back down and keep going without loosing speed. One of the cool things about riding a single speed bicycle is that you get a more varied work out over a range of resistances. From spinning quickly down hill to standing up and even using your back and arms to climb steeply. Fun and fitness over speed and efficiency.

But I did want to go fast.

I wanted to go as fast as I could and then get back up hill. That's where the idea of a  2-geared dingle bicycle fits in, and the questions deciding my dingle gearing set up were: How high can I practically run my high gear, and how low do I need to make my low gear to make the climb home?

When my big green jellybean bike came out of the box, it had 700C wheels with 700x25 tyres, 165mm cranks, a 44T chain ring and 18T rear cog. sheldon says that's a "gain ratio" of 5.1. Whatever that exactly means, I could climb from my house up the steep last pitch to the top of the neighbourhood hill. After a couple of weeks I could do it pretty easily.

Time to go to A BIGGER GEAR.

Let's start at the rear wheel.

It's got a flip flop hub.

One side has ISO 1.375" x 24 tpi thread, and is fitted with a 18T freewheel. The other side has, a section of 1.375" x 24 tpi threaded hub and a small section of left handed 1.29" x 24tpi thread, with a 18T fixed sprocket and a left handed lock ring.

The smallest freewheel that will fit on a 1.375" x 24 tpi hub is 16T. (Some BMX hubs will take a smaller freewheel.)

The 18T pheasant brand freewheel that came with the bike didn't want to go with out a fight. (See freewheel-destructive removal.)

So I took the fixed cog off the other side... you can google "chain whip" for yourself.

I took a punt that a 16T freewheel would fit onto the double threaded side of the hub... and it did. A Shimano SF-MX30 fit at least as nicely as the pheasant on the other side, and had notches for a removal tool.

So I had my first dingle set up. A 1x2 imperfect flipping half-dingle (see my earlier post "the idea of a dingle") with a 44/16 high gear and 44/18 low. The track forks took up the difference in chain length, with a small adjustment to realign the rear brake shoes.

Mostly I just rode the 44/16. Sheldon says that's a gain ration of 5.6. Which meant in practice that I could still get up the big hill, but I was glad when it was over. But still not fast enough down hill.

The 1x2 'Half-Dingle' set up gave me a choice of two different gears, but they weren't very different!
But with anything more than a 2 or 3 tooth difference in the sizes of the two rear sprockets, the change in chain length between the two gears would see the rear wheel slide out the back of the track forks in high gear. The advantage of a 2x2 dingle set up, is that it should be able to give a more useful range of gearing options while keeping the chain length the same in both gears.

The next step was to put a second chain ring on the front of the bike.

The rear wheel gearing was a relatively simple matter with only a few, standardised components that fit together easily. But the chain ring, attached to the cranks, fitted to the bottom bracket, the whole crankset is a different matter. There a number of different ways these parts can go together and even Sheldon Brown can't tell you what will happen if you try to mix and match, although he was trying right up to the end. (This page is a listing of various bicycle crank sets, and the bottom brackets that fit them. Sheldon solicited information from readers to collect this data but that project is on hold until we set up a new mechanism for collecting and posting additional data. This is the information he colected:)

I got a 52T chain ring on eBay with some 170mm cranks. These had the same bolt circle diameter as the crank set on my bike 110mm. I couldn't mount 2 chain rings on the 170mm cranks because the inner ring wound up touching the chainstays, but I could put the 52T ring on the outside of the 160mm cranks and a 44T ring on the inside... and it would just fit.

But this gearing doesn't really work. The trick with a dingle is to keep the chain length the same in both gears. The 52/16 high gear has a total of 68 teeth on both cogs. The 44/18 low gear has 62 teeth. The difference is so much that the rear wheel falls out of the back of the track forks if you try to set it up in the low gear. The only way I could change the gears on this bike was to carry around a second chain. The Un-Chained Dingle! Of course I dropped the joining link in the grass and spent half an hour trying to find it. Which kind of turned me off this set up.

I liked the big 52T chain ring with the 16T rear freewheel. But I couldn't fit a bigger inner chain ring without it rubbing on the chainstays.

To make more room for a bigger inner chain ring, I needed a new bottom bracket with a longer spindle. I'd never played around with bottom brackets before, but this one was easy. With a screw-in, sealed cartridge bearing, all I needed was a tool that fit, and to be careful not to cross the threads. I put in a new cartridge with a 122mm spindle and a 50T inner chain ring.

This gave me a "perfect" dingle set up. High gear is 52/16 with 68 teeth, low gear is 50/18 with 68 teeth and the rear wheel ends up in exactly the same position in each gear.

Of course 50/18 is not a particularly low gear. (Sheldon says a gain ration of 5.6, the same as the 44/16 set up.) If I wanted a lower, low gear, having decided to go with the 52/16 high gear, the next lower dingle combination with a total of 68 teeth was 49/19. This is where the theory of dingle gearing runs into some practical problems. I don't think anyone actually makes a 49T chain ring with a 110mm BCD, and you might have to search a bit to find a 19T freewheel. I didn't want to go and get parts custom made, so the next usable gear combination is 48/20 which gives sheldon's gain ratio of 4.9, a bit lower than I actually wanted.

If all those numbers are a bit confusing, you should look at this table from  ;-) .

And besides...

I've been talking about a 'flip flop dingle', with a separate freewheel on each side of the hub. Turning the rear wheel around to change gear is not really too difficult, but it would be more convenient to have both rear cogs on the same side of the wheel. This introduces more practical difficulties.

Surly make a "Dingle Cog" in sizes 17/19, 17/20 and 17/21, but only as a fixed cog, and I wanted a freewheel. I found this ACS CLAWS 2 speed freewheel on line,
 but it wasn't on the acsbmx website, which makes me wonder if they've stopped making them, and the shop couldn't give me the 16/17 size, only the 15/16 which fits a smaller threaded hub than mine. It's probably possible to customise a freehub cassette, but I wasn't sure how to do it and I didn't really want to go to a whole new wheel. So that really only left one option, a white industries DOS ENO.

Most freewheels are semi-disposable items, engineered to fairly low specs and intended to be replaced as soon as they give any trouble. You can get a cheap one for under $20. But this beautiful piece of machinery, made in the USA, is finely machined, with a serviceable sealed bearing system. Even if it costs 5 or 6 times as much... that's not really a lot of money ... I convinced myself. So I asked the guys at gear brisbane to get me one.  It comes in two sizes 17/19 and 16/18, but no 20T option, so it made sense to find a set up for my low gear using the 18T rear cog. 

 An imperfect dingle set up, where the two gears have slightly different chain lengths and changing gears means the rear wheel moves slightly forward or back in the track forks to compensate.

Sheldon Brown's 'tool tip' on cotterless crank removal makes the process sound almost erotic. As he says, the crank puller is "a joy to use"

Oddly similar to my other favourite tool the tap re-seater.

I gently eased off the right crank and spider, hurriedly stripped it's cluster and impulsively mounted a 48T chainwheel!

And there it was, my 52/16 - 48/18 imperfect flipping dingle.

The 52/16 high gear is massive. I fly down hill and shoot across the flat land in an insane blur. The 48/18 lower gear is a sensible alternative. I can ride to work, climb hills and get home from wherever I might end up.

And I got an email today to say my White Industries freewheel was at the shop. Can't wait to get it on.

No comments:

Post a Comment