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PostPosted: Wed Aug 08, 2012 11:11 pm 
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prendrefeu wrote:
Yeah, seriously. It's like WW has a forum full of non-weight weenies all of a sudden. WTF are you all doing here if you aren't interested in tuning and getting the most out of your components as the very basis of the forum? The CycleChat and Pro talk and all the other stuff is just icing on the cake.


I don't know how I missed this thread the first time around, but I was thinking about "modifying rims" myself... and so here it is.

I'm surprised at the negative responses right off the bat... none by anyone who had actually tried it. And I'm an engineer, and I'll tell you that IMO it isn't a stupid idea at all. Of course it isn't *practical*... ie you are never going to make these wheels *light* for the amount of work it will take, but it will be an interesting experiment.

Spoke beds are thick to take spoke loads, but are only highly stressed right around the spoke. There is a lot of extra material in this location in between spokes that can be safely removed... that is why many wheel manufacturers in fact do this. If you can't machine easily, then drilling will work... but this must then be covered with some sort of light tape or composite if you want it to make sense.

Sidewalls are usually quite thin, but there is a limit to how thin they can be extruded and still resist buckling when the rim is formed into a hoop. AC420 rims have sidewalls that are machined after the rims are extruded and formed. So you could also experiment with drilling holes on the sidewalls.

The "web" that joins the brake tracks together is quite thin already, but 36h rims seem to get along fine by perforating that quite a bit more than a 16h rim... plus the rim tape will cover that part.
...............

What prompted me to think of this is that I want to get a 16h Kinlin XC280 rim, and none of the distributors want to carry them. Can't get a blank and drill my own holes either, so I'd have to get a 32h rim and have extra holes to cover... or do something completely different.

If I have to cover holes anyway, then why not drill a bunch of holes to optimize the structure, and cover it all with a thin layer of carbon? Sure, a lot of work... but potentially a stronger, lighter, and prettier rim all at once...

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Posted: Wed Aug 08, 2012 11:11 pm 


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PostPosted: Thu Aug 09, 2012 12:12 am 
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I have thought about covering the holes by light tape but dropped the idea later. I am thinking about climbing instead of aerodynamic, why take the time to make it prettier if I am not going to sell it.
I know there has to be brilliant people here. :beerchug:
I would like to further lighten the wheels, R500 come with straight gauge spokes, do you think shaving the spokes into aero-shape with a file is OK. I will probably do it to the nds first and then the front wheel.


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PostPosted: Thu Aug 09, 2012 12:18 am 
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kai-ming wrote:
Phill P wrote:
..... show us the modified wheels!

I would rather take the time to ride it, C potatoes! :podium:

There is a reason I said C potatoes. Use the head please, giving static photos of a modified wheel is dead easy, proving it durable for my weight and riding style is difficult if not impossible.


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PostPosted: Thu Aug 09, 2012 12:35 am 
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Ok.

So, then:
-What is the current status? Have you finished drilling or modifying them?
-Can you provide a picture of the rim on a scale before drilling? After drilling?

That's all people are asking for, pretty simple stuff and very standard for this forum. Providing pictures for evidence/proof is looked upon very well here.
Thanks.

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PostPosted: Thu Aug 09, 2012 12:50 am 
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I agree, I would like to see some shots. I'm curious as well.

Otherwise this thread is going around in circles.

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PostPosted: Thu Aug 09, 2012 3:17 am 
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I think it's the OP's attitude that's makes the whole project sound off-putting... If there was some scientific explanation, explaining how he did it, how it was calculated, how it affects the ride, how it remains stiff/safe, I would be very interested to hear since I have a set of R500s on the commuter (not that I would do it.) People take a drill to a lot of things around here, including shifters and derailleurs.

Instead it's gone something like "I'm going to do it and you're all twats for questioning me.", coupled with a number of thinly veiled insults and a know-it-all attitude that makes this thread a trainwreck.

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PostPosted: Thu Aug 09, 2012 6:40 am 
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Thinking about it, I feel like running 16H in a 32H rim would make a stress point at the empty hole in between. You'd have two opposing forces pulling in opposite directions and a point in the middle with no material which would seem to form a weak point. Think of pulling on a rubber band from two ends and then poking a hole in the middle. Pull hard enough and it will always break at the middle sooner than a rubber band without a hole. Now how this is different than running a 32H rim with 32 spokes I'm not quite sure, because the hole is still there and the forces are still there or greater because of additional spokes. All I can think of is that you would need to run higher spoke tension with fewer spokes to keep the same level of trueness, thus increasing the stress on the unused hole.
The bottom line is that yeah, sure may e you could do some analysis and perhaps design a rim with holes or less material to save weight, but taking an unknown design with unknown material properties and drilling/dremeling material haphazardly and risking a spontaneous failure descending at 30mph just seems ludicrous. At the very least I would expect the balancing of the wheel to be screwed. Maybe you could estimate the strength limits and appropriate safety factors given the dimensions and standard alloy compositions, but the whole exercise seems to be overly convoluted for zero to no gain.
Tl;dr. Flex your engineering nuts and redo all the analysis the manufacturer already did to drill some holes just buy a *f##k* $40 light alloy rim.


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PostPosted: Thu Aug 09, 2012 9:36 am 
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Rims operate in compression, not tension. So the un-used spoke hole would not suffers from cracks because it is a stress raiser. It would act as a buckling of bending stress raiser. So Long as the rims are stiff enough to be laced as 16 hole then the spoke hole in the middle would be a minor issue. I'm thinking 30mm rim sort of thing here.

I'm just concerned that shimano use lower profile rims which with 16 spokes are at thier limit anyway. Removing extra material in the middle makes for an unpredictable result. Mavic and Campy wheels with machined middles have the rims designed for this in mind.

The OP is likely a lot less heavy than me (easy prediction to make...) and clearly more daring, I wouldn't ride fast around Hong Kong!!

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PostPosted: Thu Aug 09, 2012 6:07 pm 
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kulivontot wrote:
Thinking about it, I feel like running 16H in a 32H rim would make a stress point at the empty hole in between.


Compare this to the "paired-spoke" wheels that have a section of rim that is 1/7th of the circumference unsupported. Plus the stress is very small compared to the extreme point loads where the spokes attach.

Many people seem to think that the rims are already engineered and paired down to the minimum. This is simply not true. The wall thicknesses for most are determined by trial and error, and machining after the fact is expensive. There is obviously a lot of extra material.

If I wanted to design the ultimate strong/light/durable rim, I'd look at making a rim with a thicker than normal aluminum extrusion, and then machine an optimized truss-like structure followed with a carbon covering.

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PostPosted: Thu Aug 09, 2012 6:10 pm 
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kai-ming wrote:
I have thought about covering the holes by light tape but dropped the idea later. I am thinking about climbing instead of aerodynamic, why take the time to make it prettier if I am not going to sell it.
I know there has to be brilliant people here. :beerchug:
I would like to further lighten the wheels, R500 come with straight gauge spokes, do you think shaving the spokes into aero-shape with a file is OK. I will probably do it to the nds first and then the front wheel.


please tell me this is all an exercise in satire.
...
seriously.


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PostPosted: Thu Aug 09, 2012 6:12 pm 
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WMW wrote:
If I wanted to design the ultimate strong/light/durable rim, I'd look at making a rim with a thicker than normal aluminum extrusion, and then machine an optimized truss-like structure followed with a carbon covering.


At that point, you are better off with a full carbon rim. The alloy isn't an advantage anymore.

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PostPosted: Thu Aug 09, 2012 6:15 pm 
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motorthings wrote:

please tell me this is all an exercise in satire.
...
seriously.


I think he's all talk.

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PostPosted: Thu Aug 09, 2012 8:59 pm 
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ergott wrote:
At that point, you are better off with a full carbon rim. The alloy isn't an advantage anymore.


Maybe... but not in every way. Carbon rims appear to be very similar to aluminum ones, with constant thickness on the sidewalls and for most at the spoke bed as well. Making a more complex structure would be difficult for carbon because you can't machine it effectively. Then there is the braking issue.

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PostPosted: Thu Aug 09, 2012 9:15 pm 
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I would think the limitation on light weight Al rims is how thin you can consistently extrude the side walls.

CF you can just just a few layers in the side walls then thicken up the spoke bed and brake tracks fairly easily.

Because the rims operate in compression (except directly around the spoke holes) buckling of very thin side walls could be an issue. But if you use a nice curved profile that should make a bit more buckle proof.

Pair spoke designs do suffer from slightly more buckle issues between the spokes, also if you break a spoke they go further out of true because it is further before you get to a spoke to resist the side load of the remaining spoke on the other side.

If you look at light weight rims like stans, they are very soft and need more spokes to support them, so thinking there is more weight to be saved from a rim just by making the walls thinner clearly has its trade offs. OK stans are low profile, you could add some stiffness back by going deeper, but that is going to add weight back again. From what I've read recently the latest batch of stans rims have added weight back into the spoke bed.

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PostPosted: Thu Aug 09, 2012 9:53 pm 
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Curved thin sidewalls dimple when spokes are tensioned. If the sidewalls had a truss structure underneath, they could be lighter and have much better resistance to this.

I think the Kinlin XC280 size and shape would be a much better starting point vs the Stan's rim.

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