Re: [650B] Mafac Raid Straddle Cable Length

[Jan Heine]

Straddle cable height makes a huge difference on low-profile cantilevers. On these, a lower cable both lengthens the effective brake arm length and increases the mechanical advantage. So it's a win-win, and it's also the reason why wide-profile cantis often don't work well with wide tires and fenders, because you cannot set the straddle cable low enough.

On wide-profile cantis, straddle cable height makes much less of a difference, because in this case, lowering the straddle cable shortens the effective brake arm length. This counteracts the increase in mechanical advantage. On really wide-profile brakes, like the classic Mafac, the two effects cancel each other, and straddle cable height has no effect at all, except that you stress the straddle wire more with a lower straddle cable. There as an article in the "Brake Special" Bicycle Quarterly (Vol. 7, No. 2) that showed these effects with drawings.

On the Mafac Raids, straddle cable height also has very little effect of mechanical advantage and braking power. So you can set it wherever it works best. Make sure that there is enough room to the cable stop so that even when the brake pads wear, the straddle cable hanger won't bottom out.

Jan Heine
Editor
Bicycle Quarterly

-----Original Message-----
From: Brad
Sent: Feb 15, 2015 1:59 PM
To: 65...@googlegroups.com
Subject: Re: [650B] Mafac Raid Straddle Cable Length

It is not exactly the same because the arms and the pivot point are in slightly different relation.  However the same concepts apply.
As a practical matter an inch either way wont make a huge difference.   If your not banging into the cable hanger, don't worry about it.
Are you using the Mafac cable hanger or something different?   I have tried others and the Mafac one
http://www.velo-pages.com/main.php?g2_view=core.DownloadItem&g2_itemId=29935&g2_serialNumber=2
works best.
If you need an adjusting barrel use an inline one.


The really long straddle cables used to go around seat tubes on mixte frames probably require some math skills on implementation.

And last but not least- most of the braking actually happens at the front brake.  If it is the other way around your front wheel is pulling you as you skid.

On Sunday, February 15, 2015 at 3:26:45 PM UTC-5, Darren Darren wrote:

NickP, the Raids are center pull not cantilever.  Unless, you are saying straddle cable set up is the same.

D

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[ted]

I think it is simpler to remember the vertical component of the tension in the straddle cable doesn't depend on its length, and that the horizontal component increases as you shorten the straddle cable.

As long as the ends of the brake arms are above the pivots, lowering the straddle cable will increase the mechanical advantage.

The higher the ends of the brake arms are, the larger the lever arm for the horizontal component of straddle cable's tension and the larger the effect of lowering the cable.

[satanas]

Changing the straddle cable height and, to a lesser degree, arm width can be really helpful in getting cantis to function as desired. The ability to do so, and thus to get it wrong, is also probably the greatest single reason why so many people hate cantis. Shimano supplied shops with a set of plastic guides when they came out with their narrow profile cantis, and these really helped make things easier. Later on, Shimano's V-brakes solved lots of setup and clearance problems, and made many people's and shop's lives easier still.

FWIW, with 23 mm outside width rims I have had significantly more braking power with wider profile XT M732 brakes than with narrower profile XTR M900 callipers, when used with drop bar levers and with the straddle height adjusted to suit in each case. (The XTR cantis are fine when paired with flat bars and their matching levers.)

Perhaps the crucial thing with linear pull brakes was the increased cable travel from the lever, enabling decent leverage with less sponginess and less critical adjustment compared with traditional cantilevers.

Later,

Stephen