Sask has drilled out his Coker rim and reported the weight savings at ~75g: Coker Rim
A year ago I bought a Coker wheel and replaced the hub with a Nimbus 48h ISIS for use in a KH36 frame. When I had everything taken apart I weighed the rim only at 2lbs 9.5oz, which is very close to Sask’s 1175g pre-drilled weight.
I agree the most important part to save weight from is the outside edge, but I still reckon losing 12 spokes would be more noticeable than 75g from the rim. Not saying it’ll make NO difference, just seems like a lot of effort for not a lot of return. I’m sure most drilled rims are made that way just because people think they look cool.
I’m a bit bored at the moment. It’s too much rain outside, wind and cold. I should have been outside riding a lot more. To elevate the enthusiasm to new heights, I have decided to go for a crazy Rim-drill-project.
If a Stealth 36” rim is reduced about 72g with 36 holes of size 7/8”, it should be reduced with about 150g with the “double hole” pattern shown in the figure, if each single hole is drilled as 1”.
I think single holes bigger than 1” might weaken the rim to much concerning the remaining “height” of the rim, seen from the side. On the other hand, the difference here between a single hole and a “double hole” won’t be that big. I’ll go for a “double hole”, testing if that is possible, instead of confirming that a 1” single hole is ok and still wondering if a double could have been an option.
The idea is to keep the inner wall. It’s thin and I won’t save that much by drilling there. And, it will be important that the inner wall is as strong as possible, because it is the main part remaining to keep the stability sideways.
I plan to drill the holes in a brand new Stealth2 rim.
Due to the risk of building a wheel that might collapse, I am planning to put it into low-speed flat-road service, first. That won’t be difficult, concerning the weather outside here nowadays…
(And, yes, I’m very confident that I will manage to build a wheel. The question will be which size of bump makes it crack.)
Figure showing: a) 1” double-hole, and b) 7/8” single-hole rim drill pattern:
Keeping the inside surface will make it easier to deal with for things like rim tape… that becomes a bit of a pain with large holes in a rim. The outer surface is much thicker (about three times I think) so that’s where you need to cut out first.
I’m guessing you are experienced with machining and know how to do two overlapping holes without it getting awkward and snagging on the edge of the first when you do the second one? I don’t know enough about it to know how to manage that.
Well, yes and no. My equipement was obviously not stable enough to do the overlap without a steering pin hole, so I had to first drill two small holes in both the inner and outer surface of the rim. This made it possible to drill (or saw) the overlapping holes, but I still had difficulties to get them exact where they should be due to the drawing. If I should do this over again, I would invest some more time in buiding a better jig.
And, surprisingly much need for sanding the edges afterwards.
After sanding, it looks like all the edges is fine. No cracks.
The weight reduction is about 151g. It feels light, but obviously not that stiff any more…
We will see. I can hardly wait to build the wheel. I’ll start now.
The Wheel is now round! With a spoketension of 800 Newton. I had some difficulties with the joint, as the rim was about 1 mm from round at this point, pointing outwards. I’we experienced this with some rims before. The solution had to be a bit higher tension on some of the adjacent spokes. Normally this should not be a problem, but since this rim might be weak, it might do a difference. Do anyone know some trick to flatten a bumpy joint in such a way that all the spokes might have exact the same tension?
The main problem have been to get my new NightRider tyre to clinch. It seem to fit tight to the rim, but with air pressure it have been a pain to get it even. I guess that this do not have anything to do with the fact that the rim is drilled. So, the rim is round, but the tire is not (bumping a millimeter or two at one point). I’ll have to do some more work here…
After a test-run it’s possible to say: yes it is runnable, but, yes its a bit soft. And, yes, it feels light!
Testing with 110mm crankarms, no brakes, 2.5 bar in a new Nightrider tyre, spokes at a tension of 800N, it looks like this: http://player.vimeo.com/video/52806178 (Testing: low speed flat, bumps, down-hill, high speed, up-hill)
As it is possible to see, the response to the push on the pedal, is to slow. Much power is lost in bending. But at high cadence, without giving power, it flies.
The rim bends. I’m not sure of that bending is a direct result of pushing the pedal, or the weight distribution of my body. Anyway, I think it is a result of low spoke tension. So, I guess the question is about maximizing the spoke tension without making a taco of the wheel.
The rim went for 75 km with a spoketension at 800N. Then I ripped off the tyre, and trued it at 900N. It feelt better, quicker response when pushing the pedal. But, on tape it do still not look good: http://player.vimeo.com/video/53279538
It even look worse than at 800N, at slight up-hill. The reason might be that I ride just a bit slower this time, but anyway, it still bends to much.
But at hight speed, it really flies!
In miles/hour I guess it is around 20 in both test videos.
Both those videos were interesting. Good job with the camera setup. It would be interesting to compare that to a rim that has not been drilled. I don’t know how much side-to-side flex is normal. I guess you can feel it when you pedal, so you can tell that it’s worse than usual.
BTW, the forward view in the first video was pretty scary for me since you keep accelerating way past the point where I start to worry about falling.
All you need is a wider frame and you could easilly go faster
A lighter tire and running tubeless would be easier and safer, I dropped > 600gm going from the Nimbus Nightrider and 36" tube.
I watched that video and the whole time I was wondering if the rim was going to catch the frame and send you flying, and at a speed that you would not be able to run out of; you’re crazy to ride that wheel.
Maybe machining the interior wall, like Surly did with the Large Marge XC?