Designing a custom built 650b/27.5 frame, looking for advice / opinions

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sukebe

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I am currently finalising my custom design for a ChiTi XC/Trail hardtail to be built around a 120m fork (likely Reba RL). I want the bike to get back into some XC racing but still provide "trail fun". I have based my design on the Kona Explosif and Pivot Les with 120mm fork.

I'm 189cm with 196cm reach and 35.5" inseam. I weigh 86kg naked and am an aggressive technical XC rider. Below are the numbers I've come up with:

HTA 68.5 degrees (static)
STA 72.5 degrees
635mm ETT length
425mm chain stays
44mm BB drop (306mm BB height with 700 tyre diameter)
520mm seat tube
130mm headtube
443mm reach, plan to run a 70-80mm stem with 730mm flatbar.

what do you think?

As a first time custom frame, I've got a few questions that I'm not sure what way I should go. Below are the tube dimensions I've been provided by the factory drawings, all tube thicknesses are 0.9mm.

I thought to use a 31.8 seat tube with a 27.2ID Ti or carbon post for comfort flex. Right choice or should I go for tubing with 30.9ID?

34.9mm toptube and 44.5mm downtube.

19mm dimeter seatstays, 25.5mm oval to 22.2mm round to 19mm taper chainstays. Both will have S-bends.

Given my weight and riding style the frame needs to be stiff, but I want it to still have as much of that buttery feel of titanium as possible. Will the above tube dimensions provide this?

Regarding drop outs, is it worth going 12x142 or just stick with 135QR?

It will be built as a 1x10 but I'm toying with the idea of dropouts that can be geared or SS. Is it worth the more complex dropouts just so I can "maybe run SS at some point in future"?

Is internal routing worth the hassle? I would run a full outer on the rear derailleur cable if external. I'm not too bothered about internal if its just for the purpose of cleaner lines.

Thanks in advance for any advice or ideas you might have!
 
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bikeyoulongtime

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sounds like a fun project!

my .02 - slacken the head angle by a degree, steepen the seat angle by a degree. This is based on riding an on-one 456 for a lot of years and thinking 'well this is awesome, but maybe a little slack and the seat is too far back'. These proposed alterations would make for angles (HT 67.5, ST 73.5) exactly in the middle of your XC beast and the 456 - so easier to climb than the 456, but a little longer and more stable to descend than a standard XC machine.

a steeper seat tube angle would, of course shorten the reach a little. a slacker head angle will, of course, lengthen the wheelbase a little and possibly make tight uphill switchbacks harder. My current ride has HTA 66, STA 74, ETT 625mm (I'm 187cm, ape factor +). It is very nice, but not for smashing up hills fast...

having fixed a few internally routed bikes now, I'd say it is definitely not worth the hassle.

And, at the end of the day I may be talking out of my arse, and after all it is your custom dream machine. But you did ask :))
 
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sukebe

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Thanks for the suggestions bikeyoulongtime.

I've test ridden a kona explosif which has a 68HTA and 73STA and felt like good 'trail' numbers, though I was hanging off then back of the seat as 73 is too steep for my long femurs. My current FS bike has a 72 SA and I still need a lay back seat post.

I'm pretty confident with the geometry, but have no experience with building so am unsure about the frame tubing details. Anyone got experience with frame building? Will these tube dimensions provide the ride quality I'm looking for?

Hey knuckles, you ride naked and are from melb western burbs. Hmmmm, did you used to work with K Fumberger??
 
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BKMad

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You might want to specify a minimum tyre clearance for the rear end as well. With shortish chainstays I think that the detailing where the chainstays join the BB needs a bit of thought to make sure a decent sized tyre still fits.
 
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