Saturday, June 30, 2007

Equipment - Reconstruction Time Again

One of my favorite albums back in the day was REM's Fables of the Reconstruction. I listened to it all the time, that and all the other REM songs I had on my cassette tapes. Great music for long drives and long rides - eventually REM songs all sound the same, sort of like the way PowerGels taste on a 6 hour ride. They just merge into one.

I thought of the album (well, now I have it on CD too) as I'm looking to rebuild my Power Tap wheel before the upcoming second New Britain Criterium. You see, when I think of rebuilding wheels, I think of my cycling "peak" in the late 80's through mid 90's, and when I think of then, I think of the music I listened to at the time. REM is somewhere at the top of the list.

I've been puzzled by the high weight of my Power Tap rear wheel. Okay, it has 2.0mm straight spokes (I confirmed that). It has a clincher DT 1.1 rim (probably double eyelet, not sure though). Either 465 grams (double eyelet, more durable) or 415 grams (single eyelet, less durable), either way an anchor of a rim. Brass spoke nipples. The wheel just seems a lot heavier than it ought to be.

Long FYI - double versus single eyelet. An eyelet is something that (in this case) rings the spoke hole in the (aluminum) rim. If you drilled a hole in bare alumnium and put a tensioned spoke/nipple unit through it, the fragile aluminum would crack quickly, especially if you had to turn the spoke nipple at all. In carbon rims they either put a lot of carbon or a thin washer under the spoke. You can put washers between a spoke nipple and an eyelet too, if you want a "perfectly" built wheel.

An eyelet, made out of slippery but heavy brass, spreads the stress of the spoke tension evenly around the rim hole, allows the spoke nipple to turn relatively easily, and makes the rim that much more durable. Single eyelets simply ring the spoke hole on one rim wall. Double eyelets are sort of cone shaped and connect/reinforce the two rim walls, making for a stiffer, stronger, but heavier rim.

The problem with a single eyelet rim (basically a ring around the spoke hole) is the eyelet still stresses the aluminum to the point where the eyelet will pull through sort of quickly. The advantage is the light weight and the fact that the spoke nipple turns on brass, not the aluminum rim.

A double eyelet is much stronger but adds weight - up to 20% more weight on the lightest aluminum rims. A small bonus is when you build a wheel, the tube connecting the two rim walls prevents the spoke nipple from disappearing into the gap there. When that happens on a single eyelet rim, it can be a frustrating exercise in rim shaking to get the errant nipple out.

Hm. I suppose you could drill out the sides of the ferrules (the tube part of the eyelet) to reduce the amount of material - Mavic did this with some of their wheels, very cool. I'll have to experiment with that in the winter when I'm feeling heavy and trying to find ways of optimizing my bike.

Okay, enough about eyelets.

Oftentimes rim weights were taken before eyelets added (I don't know how anyone justified that). So a 280 gram rim would actually weigh 320-350 grams.

To verify my perceived wheel weight angst, I checked out a source I turn to when I need some actual weights - the weight weenie site.

What surprised me (I should say "What surprised me again" - since I'd researched this before and forgot what I found) was that the straight spokes and brass nipples added about 120 grams to the wheel weight as opposed to DT Revolutions spokes. In 45 gram increments (that 1/10 pound increment I spoke about earlier), that's about 0.4 pounds, a little less than that but that's about right. The rim, assuming a double eyelet rim, is about 100 gram heavier than a comparable tubular. That's another 0.2 pounds.

So, if I get a normal tubular rim, DT Revolution spokes (2.0-1.5-2.0mm), alloy nipples, I'll have myself a wheel that's about 1/2 pound lighter. With the additional 50 to 70 grams of weight saved on the tire, tube, and rim strip, I figure the rear wheel will be 0.6 0r 0.7 pounds lighter.

That's actually a significant amount of weight off of one wheel.

And all of that would be away from the center of the wheel. Lighter weight, yes, but a significant reduction in the wheel's moment of inertia.

In other words, a better wheel for the jump.

I wish I'd gone a bit less conservative and bought a 28H Power Tap wheel - I have a couple 28 hole Campy Record Crono rims just dying to be laced up. Lighter rims (by another 45 grams), fewer spokes, and less air resistance. I have to admit that the latter is not significant but a wheel with a wall of spokes simply screams out, "I'm not aerodynamic!" 28 spokes seems a more optimal number for box section rims.

28 hole rims have what I consider to be a significant advantage over 32 hole rims. When you lace up a 28 hole rim so that the valve hole is surrounded by parallel spokes (so you have room to pull the pump head off without slicing the back of your hand), the four spokes around the rim seam all pull the seam together. The spokes help support the seam. On a 32 hole rim, the seam is surrounded by two sets of four spokes pulling the seam apart - your beautiful spoke tension is working to wrench that seam apart. I've always felt 28 hole rims were easier to build just for this reason.

After gushing praise on 28 hole wheels, you might wonder why I bought myself a 32 hole Power Tap. Originally I wanted to lace on a 24 hole Zipp rim. I've built several rear wheels like this - 32 hole hub, 24 hole rim, use 16 spokes on the drive side, 8 spokes on the non-drive side. I end up with something akin to Campy's G3 spoke pattern. Works well on normal rear wheels.

The problem is that the Power Tap hub works on torque delivered from the flanges to the hub axle. It demands even spoke counts on either side, both with crosses. Unfortunately I only learned of this after I bought the wheel.

So for now I'll make do with the 32H. And we'll see how it goes.

2 comments:

trueno92 said...

hey aki,

well... i decided to move on to REAL wheels and away from my well loved REV-X.

my site, pistarice.blogspot.com now has pictures of my repaired 16 spoke rear cosmic carbone!

i had, as well as many others, cracked eyelets and a spoke that collaped part of the rim where the tension was only relying on the inside eyelet. Not sure if the early mavic double eyelets were infact brass, or aluminum. since they cracked so cleanly, i am not sure.

I decided this could be repairable and give myself a true/ridable wheel again, and with the addition of a slightly longer spoke, I could repair my cracked eyelets with those from a donor, Mavic Open pro. the open pro has significantly shallower eyelets.

so now the inner, collapsed, eyelet is no longer in use and the tension is only being used from the outside eyelet, brass, from the open pro.

http://www.bikeforums.net/showthread.php?t=553249&highlight=cosmic

Aki said...

Interesting. I like that reconstruction idea.

Not sure of the Mavic eyelet construction either, but some were black plastic (my friend's carbone had black plastic ones).

I like the idea of resurrecting wheels like this. Very cool.