Road Bike, Cycling Forums banner
1 - 18 of 18 Posts

·
Registered
Joined
·
336 Posts
Discussion Starter · #1 ·
Bought a used 2003 Cannondale R400 and swapped out the Cannondale Delta brakes 'cause they seemed crappy. Installed 2006 105 calipers. Some improvement, but not as much as I expected.

Anyway, there's a 1 millimeter wide (a little less than a millimeter deep) grove than runs along the braking surface on the Mavic CXP-22 rims. I've never seen a groove like this before on any rim. The braking surface is machined, and the groove is right in the middle. What is this groove for? I would think this actually takes away at least 10 percent of the braking surface and it should be a bad thing. It's a 650c wheel if that makes a difference.
 

·
Registered
Joined
·
336 Posts
Discussion Starter · #4 ·
Wear indicator? Seriously?

Howzitbroke said:
Yep. The sidewalls above and below the groove will wear down. When the groove is gone, it is time for a new rim.
I've seen a lot of bikes (not enough, I guess), and I've never come across this before. Was this one of those one-time hare-brained ideas that was abandoned, or is there some merit to this wear indicator?

This wear indicator takes away a signicant portion of the available braking surface. Could this be why the braking seems less than optimal?
 

·
Number 2 on the course.
Joined
·
4,405 Posts
NoMSG said:
This wear indicator takes away a signicant portion of the available braking surface. Could this be why the braking seems less than optimal?
Well, as your pads wear they will develop a ridge corresponding to the groove, thus increasing you braking surface and eventually giving you more surface than if the rim were flat. :D
 

·
Registered
Joined
·
336 Posts
Discussion Starter · #7 ·
Really?

rjw said:
Nope.

The groove will make no difference at all, friction is independent of surface area.
Not disputing you. It just seems counterintuitive...especially since (at least with cars) having more rubber on the road provides more traction.

If friction is independent of surface area, what impact does surface area have?
 

·
Banned
Joined
·
2,767 Posts
NoMSG said:
Not disputing you. It just seems counterintuitive...especially since (at least with cars) having more rubber on the road provides more traction.

If friction is independent of surface area, what impact does surface area have?
It isn't independent of area. The braking force, i.e the frictional force between the pads and rim, is a function of area. Pads apply pressure. Pressure x area = force. However, on the rim with the wear indicator (the rim in question) the area over which brake pressure is applied is essentially just the area covered by the pad on the same rim without a wear indicator. The brake pressure is applied, ideally, perpendicular to the rim, therefore ideally there will be no brake pressure applied to the "walls" of the wear indicator. In truth, the pads will deform some and apply some pressure to the wear indicator walls, but no one would ever notice the difference. Until the pads bed-in, braking will be slightly less effective, since the pad will have to wear a bit to contact the bottom of the wear indicator. I don't that a person would notice that, either.
 

·
Registered
Joined
·
39 Posts
alienator said:
It isn't independent of area.
Yes it is

The braking force, i.e the frictional force between the pads and rim, is a function of area. Pads apply pressure. Pressure x area = force.
Right equation, wrong circumstances.

The total force you can apply to the rim is determined by the force you apply at the lever, plus any mechanical advantage due to lever/brake design. With a smaller pad/rim contact area you apply more pressure per unit of area than with a large contact area, however the total force applied is the same.

Larger contact areas do have one potential advantage in that any heat generated is dissipated over a larger area.
 

·
Registered
Joined
·
668 Posts
I don't think I have ever applied max force in braking so even with smaller contact area I can always develop greater preassure to have the same brake power.
I also think that you'll notice diffrences between various rims and pads (because of diffrent friction coefficient) much before than between the same rim with that groove or without it.
 

·
Banned
Joined
·
2,767 Posts
rjw said:
Yes it is



Right equation, wrong circumstances.

The total force you can apply to the rim is determined by the force you apply at the lever, plus any mechanical advantage due to lever/brake design. With a smaller pad/rim contact area you apply more pressure per unit of area than with a large contact area, however the total force applied is the same.

Larger contact areas do have one potential advantage in that any heat generated is dissipated over a larger area.
Yup. You're right. After your response, I re-read what I wrote and couldn't figure out what I was thinking. Apparently I wasn't.:blush2:
 

·
Registered
Joined
·
1,860 Posts
Friction force and area.

rjw said:
The groove will make no difference at all, friction is independent of surface area.
If only it were that simple.

The standard textbook model of friction is:

Ff = Cf x Fn

Ff = Frictional force
Cf = Coefficient of friction
Fn = Normal force.

However, the actual interaction between two surfaces which causes friction is a very complex subject, with several contributors. For some materials in some simple situations, the standard model is very close. In other cases, it is not.

For example, in the case of rubber on asphalt (i.e. vehicle tires), surface area is very important variable for traction. For the same vehicle weight (i.e. same normal force) and same tire and surface materials, a fatter, lower pressure tire will have more traction, due to a larger surface contact area. There are many other cases where the standard friction model also breaks down. In general, for the friction materials used for braking, surface area is also a factor in the friction, and the larger the contact area, the larger the frictional force (all else being equal).
 

·
Island Hopping cyclist
Joined
·
530 Posts
The groove is required by the European Union's version of the CPSC (CE?). It is more cost effective to make them all that way then to deal with a batch for import, then a batch to meet EU requirments. When the groove is gone, time to replace the rim.
 

·
Registered
Joined
·
336 Posts
Discussion Starter · #16 ·
So all bike rims in the EU are that way? nm

DieselDan said:
The groove is required by the European Union's version of the CPSC (CE?). It is more cost effective to make them all that way then to deal with a batch for import, then a batch to meet EU requirments. When the groove is gone, time to replace the rim.
xxxxxxxxx
 

·
Doesn't like subtitles
Joined
·
3,951 Posts
This is the police!

Step away from the physics books!

Too stray from the physics discussion, I have DT Swiss 1.1 rims which have dimples along the center of the brake surface. Always wondered what they were.
 
1 - 18 of 18 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top