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Old 05-25-08, 09:57 PM   #1
Bumbliwa
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Araya PX-45 rim

I recently built my first wheel with an Araya PX-45 rim that I bought used and a Shimano hub. However, once I got the wheel built I realized the braking surface was concave. In researching this on the net I found that Araya built some rims intentionally with concave braking surfaces. Does anyone know if this particular rim was built this way or is it just worn out and should be replaced?
Thanks so much in advance......Bumbliwa

Oh, and by the way it's a 700c wheel....

Last edited by Bumbliwa; 05-25-08 at 10:51 PM. Reason: addition
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Old 08-14-17, 03:51 PM   #2
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Old thread but since the question comes up occasionally... yup, the Araya PX-45 rim (link to Araya Japan archives) was made with concave braking surface, as were several other Araya rims from that era, the early 1990s. The theory was that it provided better braking with less weight but some compromise to strength. The Araya catalog still recommended stronger double wall non-concave rims for more demanding downhill and other mountain biking activities.

My 1993 Univega Via Carisma came with the original Araya PX-45 rims. The bike was essentially marketed as what would later be called a hybrid. Good single wall rims advertised for light duty riding. Mine have held up to gravel, chipseal and rough rural pavement and some light off road riding. The rear rim was warped in a sideways skid on loose gravel over a freshly chipsealed road -- no crash, just a sideways skid and sudden stop. A bike shop mechanic was able to straighten the rim in a few minutes. Rides just fine.
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Old 08-14-17, 08:57 PM   #3
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I just can't wrap my mind around how that would be a benefit. Maximum bending stress is on the surface of a beam, and making it thinner brings the point of maximum stress closer to the neutral axis, reducing stiffness. The thicker upper and lower would add nothing because the reaction force is at the thinnest point.

Might as well just make the whole sidewall thin.

Which is what we see in modern rims.
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Old 08-14-17, 10:23 PM   #4
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I just can't wrap my mind around how that would be a benefit. Maximum bending stress is on the surface of a beam, and making it thinner brings the point of maximum stress closer to the neutral axis, reducing stiffness. The thicker upper and lower would add nothing because the reaction force is at the thinnest point.

Might as well just make the whole sidewall thin.

Which is what we see in modern rims.
There must be something to that "concave rim" thing, as Araya makes the VP-20 to this day.

I wouldn't use one, though -- I like being able to judge rim wear by feeling the sidewalls for concavity.
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Old 08-14-17, 11:45 PM   #5
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oh, a company does it so there must be something to it. sure. wink wink.
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Old 08-15-17, 12:24 AM   #6
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Yeah, who knows whether concave rims offered any real advantage, or whether it was a fad like Univega's cool looking bi-axial oval frames.

The tricky bit is finding brake pads that fit properly.

I started out with Kool Stop Eagle 2 pads on this particular bike. Good pads, easy to toe-in (the plow tip is self aligning), but tricky to align perfectly to clear the tire while maximizing contact. The pads were just a bit wider than I'd like for the Araya PX-45 rims. And too thick to clear the fork enough to easily drop the wheel with slightly oversized tires.

So I switched the front set to Jagwire Mountain Sport 70mm pads for cantilever brakes. Perfect. Thinner, to fit the rim perfectly, snugged into the concave surface. Curved to match the rim. Longer than the Kool Stops. Overall more effective brakes than Kool Stops for this particular rim. I kept the Kool Stops on the rear wheel.
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Old 08-15-17, 10:02 AM   #7
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I have Araya VP-20 rims on a 1995 mountain bike with concave braking surfaces. Some of Trek's Matrix wheels also had this. Trek's materials said it was to help fight brake shoe dive. I'm not sure how effective it would be at that, but I guess it makes sense in theory.
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Old 08-15-17, 10:47 AM   #8
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Quote:
Originally Posted by Jiggle View Post
I just can't wrap my mind around how that would be a benefit. Maximum bending stress is on the surface of a beam, and making it thinner brings the point of maximum stress closer to the neutral axis, reducing stiffness. The thicker upper and lower would add nothing because the reaction force is at the thinnest point.

Might as well just make the whole sidewall thin.

Which is what we see in modern rims.
Perhaps the intent was to increase the surface area of the brake track?
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Old 08-15-17, 10:52 AM   #9
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Quote:
Originally Posted by Jiggle View Post
I just can't wrap my mind around how that would be a benefit. Maximum bending stress is on the surface of a beam, and making it thinner brings the point of maximum stress closer to the neutral axis, reducing stiffness. The thicker upper and lower would add nothing because the reaction force is at the thinnest point.

Might as well just make the whole sidewall thin.

Which is what we see in modern rims.
Concave = more braking surface in the same amount of physical space = faster stopping. Although with modern brakes this seems hardly necessary.
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Old 08-15-17, 06:26 PM   #10
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Yeah, it's the mating surfaces. Those Jagwire pads I mentioned snuggle nicely with the PX-45 rims. Definitely better braking than the Kool Stops on the front wheel. I don't know that I actually need better braking on the rear since it mostly leads to skidding. I can already skid and dig into gravel and grass on the rear wheel with the Kool Stops and three finger pressure.

My Centurion Ironman has the original Araya CTL-370 rims, ultralight anodized with slightly convex braking surface. Trouble is, with the Suntour GPX group sidepull brakes there's no practical way to adjust the pads other than height and angle. No toe-in, etc., none of the adjustments we've come to expect, love and hate from good linear pull and canti brakes. I can see the pads only making partial contact with the rims. In two months I don't see any significant pad wear to indicate they're gradually mating with the rims.

OTOH, it doesn't seem to matter. The Centurion's brakes will lock up easily, usually with a one finger pull on either lever, no squealing. So while they could be improved in fit, it wouldn't necessarily result in better braking in actual practice.
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