Sunday, 24 October 2010

Improving braking efficiency

My Mango velomobile has been a really wonderful thing to ride, but for one thing: the brakes. The Sturmey Archer drum brakes never seemed to quite have the stopping power required. An attempt to stop suddenly was met with a slightly too gradual deceleration and locking the wheels to skid was impossible.

Almost all velomobiles use these same brakes. The Alleweder, Go-One, Milan, Quest, Strada and WAW all also have the same brakes. So do many open recumbent trikes. It's been obvious to me for a while that there was a lot of variation in the performance of the brakes amongst different bikes fitted with these drum brakes. The problem was clearly not inherent in the Sturmey Archer drums themselves as at their best they work very well. I first assumed that the problem was variation in the brakes themselves, and tried changing them, but it made little if any difference. Other people reported improvements by changing the brake cable outers. As a result of this I took a close look at how the cables behaved on my Mango when I pulled the brake lever.

The brake cables on the Mango take the most direct route they could under the circumstances, but relative to the average upright bike it is still quite a circuitous route with a lot of relatively tight bends. The curvature of the cables changes quite noticeably when you pull on the brake lever, and that represents lost braking effort.

Also, these tight bends lead to more friction than normal between the inner and outer cables.

Together, these two things add up to an effect where the inner cable moves a greater distance at the brake lever than at the brake. Much of your effort in pulling on the lever goes into compressing the cable and overcoming friction rather than applying the brakes. They also mean that the springs in the brakes themselves are only just about strong enough to pull the cable back when you let go of the brake lever.

Potentially this could happen on any bike with any type of mechanical brakes, but because cable runs are typically less direct on recumbents than upright bikes, and less direct again on velomobiles vs. open recumbents, I suspect it is more frequently a problem on velomobiles, recumbent trikes and recumbent bikes than on uprights.

A couple of weeks ago I made a change to the brake cables on my Mango which significantly improved this situation. I can stop very quickly with my front wheels skidding. This is how it was achieved:

Good cables are the key to the transformation. I used low compression outer cables to solve the first problem, combined with Teflon coated inner cables for low friction.

Note that while gear shifter outer cable offers low compression it should never be used for brakes. It is constructed differently from brake cable and is not strong in compression. For this reason, it can fail under hard braking, resulting in little or no braking just when you need it.

A tip given to me by a colleague at the Ligfietsgarage a year ago was to remove the rough swarf on new brake cables so that they can rotate more easily within the brake levers. This reduces the chance of the brake cables breaking in the levers, as they often do.

Before and after use of the file.

And the other side...

The wheels are removed with a 5 mm Allen key, giving access to the brake mechanisms. It is a good idea to lubricate the pivots in the mechanisms, but make sure that no oil or grease gets onto the braking surfaces as this will drastically reduce the power of your brakes.

Low compressibility outer cable, with a low friction liner. To make a good job of cutting this you need the proper tool. For the Mango or Quest, the new outer cable needs to be 1.03 m in length.

The easiest way of making sure that the new outer cable takes the same route as the old, including through the hard to reach parts under the bridge, is to use the old inner cable as a guide.

I used a little white grease on the adjusters for the brakes, and also an additional nut to fit against the brake lever for additional rigidity at the lever end. These adjustments should be screwed all the way in. The wheels should then be re-fitted and the brakes adjusted so that they are just free when the brake lever is not pulled. This means that later adjustments can be made, even while riding, if the brake performance drops a little in the first few km after fitting the new cables.

Finally, refit tyres (in this case I have fitted Schwalbe Marathons ready for winter) and pump the tyres up to pressure (the Schwalbe pressure gauge gives an accurate reading of the pressure in your tyres).

Afterwards, it is much easier to pull on the brake lever, with much less obvious friction, the brake lever returns much quicker to being fully "off" when I release it, and the braking is very much improved. I can now lock the front wheels and skid to a halt, stopping in a much shorter distance than I could stop previously. This removes a slight doubt I had with the Mango over whether I would stop in an emergency situation. Here's the evidence:

A slow motion view of the skid. Note how in every frame the label on the tyre is in the same place:

The total list of parts that you need for doing this job is as follows:
All the parts can be bought individually in our webshop, or we can send you a kit of the parts you need with the brake outer cable already cut to the correct length, at a slightly lower price than buying the parts individually.

Total cost for a Mango or Quest (1.03 m outer cables) is €16.50 + postage (+ 19% tax if you live within the EU). If you've a different bike/trike/velomobile with weak brakes, please check what lengths of cable you require and let me know. I'll add them to the menu.
Read my review of the Sinner Mango Velomobile.


Zyzzyx said...

I need to give that a try on my Quest. Two years now and I've never been satisfied with the brakes. Been contemplating upgrading to the 90mm drums, but with the much lower cost, I really should try this first.

Though I still may change over to the 90mm drums, my 70s have a tendency to have a pulse and be extremely grabby, especially when its wet. Right when you really don't want brakes acting up.

dr2chase said...

If you have a Dremel, that's another lovely tool for cutting brake cables, and being sure they have a nice flat end.

I drizzle a little light oil down my brake cables, that seems to help. It's probably not a problem on a velomobile, but if you get water in a brake cable, and then it freezes, it doesn't work well at all.

David Hembrow said...

Zyzzyx: The 90 mm brakes are of course a nice option, but they're heavy, and require rebuilding the wheels. I'm not sure that the best will be made of them without also having better than standard cables. The better cables will of course work just as well with 90 mm brakes as with 70 mm.

Dr2chase: I'm running these cables dry, just to see what happens. Like you, I've also used light oil in the past. The brake cables I took off my Mango had Shimano's "special" white grease for brake cables on them, but it sadly didn't help a lot in this case.

Brent said...

The end of the film is pretty funny.

I'm curious whether hydraulic cables would make any difference.

dr2chase said...

David: I've also tried grease, and decided that it was too viscous for that long a run of cable in a tiny confined space (the cable housing).

I also hit all the hinge and joints and pins with a few drops of oil. I use a solder-flux applying tip; very narrow, I can put down one drop at a time and not overdo it.

Brakes themselves are disk, useful on a cargo bike, with hills, and rain and snow. Adjusting them is an at-the-wheel job, however, which might not work so well for you, not sure.

David Hembrow said...

Brent: Mangos are also available with Hydraulic disk brakes. However, drums need a lot more maintenance than drums, especially in winter conditions, and are more time-consuming to work on.

Dr2chase: It wasn't any old grease that I used, but the "special grease" for brake cables that Shimano sells. I don't think it is actually very different from "any old grease," though, apart from the price.

Kevin Steinhardt said...

The brakes on my Challenge Hurricane are scary powerful; granted they are disc brakes and I have no idea how to brake properly. At least with V-brakes, you just pull the lever all the way back to the handlebars and hope that you don't hit the car in front. I have crashed (read as 'tipped' my Hurricane quite a few times in the last month or so; you brake, both wheels lock up, I guess I instinctively lean, turn forty-five degrees while still travelling forward at considerable speed and ...tip. It's a very anticlimactic experience. Not sure what this has to do with Mangos, but still; if the infrastructure wasn't so awful back here in Cambs, I'd certainly consider one. Say: does it have like shelves or spaces to put things inside the cockpitty section?

David Hembrow said...

Kevin: That sounds painful. Be careful !

There's a lot of space inside the Mango, but not shelves. They'd get in the way if what you were carrying didn't happen to fit in them. Mine has a basket, though.

Unknown said...

I like to gently grind the ends of my brake cable outers so they have a perfectly flat end. You have to go gently so you don't melt the plastic cover, but the end result should be more rigid, I think :)

I might well be ordering some cables for my Windcheetah, which also has reducing performance from its front hub brakes.

Kai said...

Remember to check the torque arm of drum brake is securely fastened.

If this bolt loosens up and fall away that's a disaster!
If you then even touch the brake lever, it will cause drum brake main body to rotate with the wheel causing it to violently pull brake cable around axle, causing barek to lock up. and possibly cable can pull the steering column and cause the wheels turn suddenly with one wheel locked up.

You can imagine what will happen if that happens at full speed. It happened to me