| Yup, changeing BB's due to different tires is a good thing.
Look at it this way :
Consider what your favorite BB heights are for a roadbike, a CX bike and a mountainbike. I bet a fortune it's something along the lines of 275mm, 285mm and 300. Pretty close, am I not?
Now, a road, CX and mtb tire will typically be about 23mm, 30-40mm and 50mm high, perfectly adjusting the BB height to the tires/terrain to be conquered.
Now just make sure the BB drop is perfectly corrected for the used fork and headset, as otherwise BB heights will be off for all three "rides".
Equasion :
- rims are 635mm total diameter, or 317,5mm radius
- Set your requested BB heigth for a certain tire (300mm BB, 50mm tire). Axle height : 317,5+50mm=367,5mm
This would bring you to a BB drop of 67,5mm.
Double check : your road BB height would be 317,5+23-67,5=273mm
Average CX BB : 317,5+35-67,5=285mm
Just use the BB drop that brings you to your ideal actual BB height, considering your riding style and preferred cranklength.
A short 29" chainstay will be about the same as what's used on most CX bikes, so you'd be fine on that part, 440mm is shortish, 450mm is the standard for custom builds. Slacker seatangle->more chainstay needed.
What bar type will you be using? Flats, drops, or switching in between both? What I did for my custom CX, to leave the door open to flats, was to get a toptube longer than my road bike (585mm), but shorter than my favorite mountainbike (616mm). I use 130mm stems on both. I went for a 600mm TT and 100mm stem, some 45mm shorter in total, which I think is about the average difference you'll be looking at. My CX would be less racey than I'd prefer, should I ever use it with flats, but it'd be ridable and even racable.
You could go for a TT to have you require a 100mm stem with flats, having you end up with a 50mm cross stem which may even not be all that bad afterall, since the bars effectively the stemlength to begin with.
Hard to make it ideal for both bar setups, but it you like long mtb stems and short CX ones, you're on your way to biking heaven for sure!
Providing well thought-out, a bike as you picture is very realistic. It will even work if you don't tell the builder he's making you 3 bikes for the price of one, just get the angles and lengths ride first time out.
Look at your bar height. A suspension corrected rigid fork will make your bars end up quite high from the ground. Will be great if that's your type of riding, to me it poses some small difficulty adjusting a 29" bike to my needs. Unless you're really tall, 110mm headtube will be all you need. Shorter may even be better, as long as your builder can make it strong and stiff enough.
If you don't want front suspension at all, any builder will be able to make you a short fork with enough clearance for a 29" wheel plus mud. as long as the unicrown is shaped smartly, a 400mm and up fork can be just fine.
Just for the record, I had my Cx built to be just like my beloved VooDoo Bokor hardtail, 72/72. Even with the drops, racing it a year to a few days ago, I felt "at home" immediately. I had hardly tested it in training, but diving into some high-speed turns trying to stay unlapped by Sven Nijs, thre were no unexpected surprises.
Good luck getting your Superbike! |
(
