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20mm rims with 28spokes? 32 spokes? or 38mm with 20 spokes?

Who are you looking at for the rims?

If it's farsports then I stand by my earlier point....

This 50mm is meant to be 410-30g: http://www.farsports.cn/ViewProduct.aspx?id=576

Are there any other cheap carbon clinchers that come much under that? Quick look shows 20mm being 395-415g...

...if spokes are c. 5g then you should be be able to drop the front to something like 16h and still be lighter. Then 24h rear - maybe in some funky half radial lacing patern using a 36h(????) hub.

http://www.farsports.cn/ViewProduct.aspx?id=536

Item No. FSW20-C
Depth: 20mm
Weight: 405g+/-10g

http://www.farsports.cn/ViewProduct.aspx?id=533

Item No. FSW38-C
Depth: 38mm

Weight: 430g+/-10g

http://www.farsports.cn/ViewProduct.aspx?id=576

Item No. FSP50-C

Depth: 50mm

Weight: 420g+/-10g

As smallfurry said it's not worth under-building the rear, With 50mm and the right spokes I'd *guess *24h would be cool. On the front I guess just work out what the lowest spoke count you can go with a 20mm rim is... if 16 exists I guess it's possible... but I'd still go with those FSP50-C's things in a 28/16h to Tune hubs (as per 'tester's suggestion), internal alu nips, veloplugs, and ultremos.

I'd also add that deeper rims = shorter spokes = less weight.