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Why is braking so much better on a new rim?

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Why is braking so much better on a new rim?

Old 09-17-18, 03:46 AM
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TallRider
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Why is braking so much better on a new rim?

I just replaced my 12-year-old front wheel with Sun M13II rim, with a newly-built wheel with H+Son TB14 rim, because I wanted a wider rim.
It's striking how much the braking is strengthened by the new rim. I wouldn't expect such a difference. Same brakes and pads. Only factors I can think of:
  • machined braking surface of the TB14
  • different aluminum alloy and coefficient of friction with the pads
  • old rim surface may have gunk on it (but I'd cleaned it with acetone and scoured it recently)
  • angle of contact might be slightly better on the wider rim, although with a single-pivot sidepull brake I don't think the angle would be meaningfully changed
Thoughts? I've never thought machined braking surfaces should matter much, especially after some riding and braking, but perhaps that's relevant (or is until I've ridden and braked on the rim for a bit)

Last edited by TallRider; 09-17-18 at 03:50 AM.
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Old 09-17-18, 04:23 AM
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I guess with use the brake surface get's polished so the friction coefficient gets lower. When trial cyclist used rim brakes, they sandpapered it.

Maybe your new wheel's rim is more rigid so less braking force is lost by flexion when the brakes press it
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Old 09-17-18, 08:24 AM
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Originally Posted by FullSpeedAgain View Post
I guess with use the brake surface get's polished so the friction coefficient gets lower. When trial cyclist used rim brakes, they sandpapered it.
Maybe your new wheel's rim is more rigid so less braking force is lost by flexion when the brakes press it
I hadn't thought of the surface being smoother mattering - machined braking surfaces are smooth in the direction of travel for rim against paid. But that might play a role as well.

I highly doubt that either double-walled rim is flexing inward enough under braking force be create a noticeable difference. Maybe this happens with some single-wall rims, but I suspect that mechanism is rare in recent decades.
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Old 09-17-18, 09:28 AM
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Actually my Mavic SSC, CD treated rim (A Hard Anodizing) braked better
once I Scoured the anodized surface off and exposed the base aluminum

[EX 721, 559 size FWIW]





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Old 09-17-18, 09:59 AM
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Originally Posted by fietsbob View Post
Actually my Mavic SSC, CD treated rim (A Hard Anodizing) braked better
once I Scoured the anodized surface off and exposed the base aluminum
[EX 721, 559 size FWIW]
The H+Son rims I have are just machined bare aluminum; I don't have the hard anodized version. Should have mentioned that.
So I'm comparing bare aluminum to bare grooved aluminum and wondering what is so different.
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Old 09-17-18, 10:07 AM
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groove? if only 1, when it's gone replace the rim,
you wore it down to the level of wear indicator..
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Old 09-17-18, 12:20 PM
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No, I just mean the tiny grooves/channels of a machined brake track.
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Old 09-17-18, 10:07 PM
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Turns out there's more gunk on the old narrower rim than I'd expected - I cleaned it off with an acetone rag tonight, and I got plenty of aluminum/rubber dust, the residue of braking.
Plus the narrower brake pads, which had worn in for a wider rim, didn't have much contact area on the narrower rim
So maybe there's no real difference here after all.

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Old 09-17-18, 10:48 PM
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The wider rim makes the brake pads hit sooner when you put on the brakes. This changes the point in the brake lever travel. This means you probably get more power with your hands in that position. In other words the pads hit the rim when the lever is father away from the handlebars. This gives you more to squeeze against.
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Old 09-17-18, 11:27 PM
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Its because of the wider rims
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Old 09-18-18, 08:44 AM
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Originally Posted by josephn1990 View Post
Its because of the wider rims
What about wider times could matter to braking force, beyond pad contact angle?
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Old 09-18-18, 08:46 AM
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Originally Posted by 2manybikes View Post
The wider rim makes the brake pads hit sooner when you put on the brakes. This changes the point in the brake lever travel. This means you probably get more power with your hands in that position. In other words the pads hit the rim when the lever is father away from the handlebars. This gives you more to squeeze against.
I opened the caliper qr after installing the new wheel, which puts the brake pads the same distance away from the rim as with the narrower rim and closed qr. So that issue is accounted for.
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