Stiffness difference between super light carbon rim and very heavy alu?

[Alexandrumarian]

I have a set of Vento wheels sitting unused and I was thinking about rebuilding the rear with a Schmolke TLO 30. It is only 20 spokes but I guess thanks to the very heavy and solid rim, I had zero issues with the original wheel despite my 95Kg weight. With a bit of luck and maybe a set of washers I might even be able to reuse the spokes too (old rim is 27.5 depth vs new 30)
One concern I have is with the stiffness of the new rim. At only 270 grams and 20 spokes it might really be a bad idea for my weight. Any comments? Thanks!

[whosatthewheel]

A rim is in essence a circular pipe. The stiffness of a pipe is by and large down to the section of the pipe. So a wider/deeper rim will be stiffer than one that is not. Internal structure of the pipe matters too (I beam shaped extrusions for instance) Thickness of the "pipe" walls is somewhat less important and material is probably the least important when it comes to stiffness.

Roughly speaking, carbon composite has a density which is 70% that of aluminium. so if a rim with a given section and wall thickness made of alloy weighs 400 grams, then the equivalent in composite will be just under 300 grams.

Also, a clincher has some "extensions" to the main pipe section, which weigh a lot but don't contribute much to the stiffness of the structure... I am assuming your 270 grams rim is a tubular?

[kavitator]

95kg
270gram rims
only 20 spokes

- not recommended

go with deeper and heavier rim (stiffer)

[bm0p700f]

the risk will the rim might crack even with washers. those light weight carbon tubulars are not stiff. They need 28 spokes at the back really. 24 is fine for some but not all and not the OP probably. I am 10kg lighter and I would be building a 28 spoke rear.

With a 20 spoke rear you need stiff spokes and stiff rims but i like stiff wheels.

[Multebear]

I have a set of Vento wheels sitting unused and I was thinking about rebuilding the rear with a Schmolke TLO 30. It is only 20 spokes but I guess thanks to the very heavy and solid rim, I had zero issues with the original wheel despite my 95Kg weight. With a bit of luck and maybe a set of washers I might even be able to reuse the spokes too (old rim is 27.5 depth vs new 30)
One concern I have is with the stiffness of the new rim. At only 270 grams and 20 spokes it might really be a bad idea for my weight. Any comments? Thanks!

This is a bad idea on so many levels.

1) Why would you rebuild a low/mid level wheelset with carbon rims?

2) Why would you rebuild a perfectly fine wheelset?

3) Why would you rebuild a factory build wheelset?

4) Why do you want to go ultra lightweight when you're a beast at almost 100 kg (don't get me wrong, I'm close to the same weight)?

5) How do you think you'll achieve stiffnes with lightweight parts and your bodyweight?

IMO you're dreams are totally out of reach. First of all, with your weight, why do you even consider going sub 300 grams rims? Those kind of rims are not made for big guys like us. I dream of jerseys size XS, but they aren't made for me. Period.

It's fine to be a weightweenie. But the level of weightweenie'ism is decided by your weight.

And even if the rim will hold you, the low spoke count will force you to do a lot of retruing as mentioned by others.

Lose 20-25 kg, and you'll have a decent chance at that project. And if you insist on riding sub 300 gram rims, then don't build them on a Vento wheelset. Spokes will most definitely not fit, and the rims deserve better hubs.

[Alexandrumarian]

Thanks for the input!

The original 20 spokes wheel, although not used a lot has served me very well. I never felt brake rub although i like the pads very close and it has remained perfectly true. It is 10 radial 10 2x, large DS flange, Campy spokes are pretty close in size to Aerolites. 910 grams!

What I am really after is a third wheel for my Bora 35 tubular set. I often switch between 12-25 and 11-32 cassettes which is annoying. Scavenging the hub and spokes from the unused Vento would be one cheap option. The other cheap option, and definitely safe and stiff is to scavenge the Record hub from another set of wheels I built myself and pair with a cheaper, deep section Corima rim. This will not be very weenie (750g) but for flats will not matter.

Both options are crappy considering i would disable perfectly fine wheels but the third, expensive option (24 hole Schmolke with WI T11) is out of the budget for the foreseeable future.

[Alexandrumarian]

@multebear

Thanks for the reality check slap Yeah I am trying to lose weight but it is not too easy. I have a big frame, not just fat

The Ventos came stock with my first road bike. They are gathering dust for 2 years now, and selling would not bring anything useful. They worked perfectly well until it got into my head that i am too heavy and eventually they will start to fail and so I built a 32 rear 28 front set which also worked well and was lighter. Then I got into being a weenie and (reluctantly) bought a set of Boras this year. To my surprise they seem to bear me without much cringe so far... so again I was getting bolder.

But probably the best thing is to wait a little longer, buy all parts new and do the deep section with Record option (see above)