What do you think about my frame geometry?

Hi all,

I'm planning my second bamboo bike, and I'd like a sanity check on the geometry. Note that I've only very roughly sized it using the rider height function on bikecad, so that might change a little once I've taken more measurements, worked out my centre of gravity, decided what cranks I'll be using, etc. ... the geometry should stay pretty much the same though.

I was unhappy with the bendiness of the rear triangle on my last bike so I'm going for 420mm chainstays, which is about as short as I reckon I can get them with room for 35c tyres and mudguards.

The BB drop is 65 which is quite a lot on the safe side 'specially considering I like very short cranks (I've got 140s at the moment but may get some nicer 155s because no-one makes decent shorter cranks), but having a high CoG is good despite what you might think (recumbent wisdom).

I've gone for a quite short trail of 45mm, on the basis that though I flatter myself, most of my riding is done at low (>20mph) speeds, so stability on quick descents isn't much of an issue, whereas low-speed maneuverability without wheel flop is. The idea is that with the steep front end and short(ish) rear triangle it should handle quite quickly, while the unique compliance of bamboo should mean it isn't too rough a ride. Might slacken the seat tube a little for some more comfort but I need to do some CoG sums first ...

Here are some pictures: (by the way, ignore the handlebars. I'll probably use Nitto moustache bars - I got as close as I could on bikecad but they're not as low as that really.

By the way, the marked 19mm half way down the ST is the distance between the ST and the tyre; the mudguards are mounted 7mm away from the tyre which I've always found just about fine, but there's room for them to go a bit further out.

I suppose the fact that no-one has commented means I've not done anything heinously wrong ;)

why such a big head tube?

Almost a polo bike there.

A few reasons for the long head tube, none of them super good, so I may change it now someone's noticed. Need to give it some thought:

1. Ceeway sell headtubes in 160mm lengths so it would save me the hassle of having to cut it down exactly straight and face it. And yes I know you should face it anyway ;)
   
2. A longer headtube means I don't need to use stem spacers (got the stem the wrong way around on the drawing as well, would be vertical for normal use and facing up as shown for longer more relaxed rides) but still not have an enormous seat-bar drop.
   
3. Having a short headtube is all very well and good but puts more strain on the joints (or so I've heard but when you think about it, it's just reducing the size of the lever, so maybe it reduces the stress ... who knows.) and means that only someone with a correspondingly short headtube will be able to use your fork in future.
   
4. I'm going to experiment with thinner bamboo tubes than is normal; usually 40-45mm tubes are used for the main triangle; I'm going to try more like 35. I'm 80% sure this will be stiff enough if the joints are good (my slightly noodly thick-framed bamboo bike has taught me that the joints are the big deal). But no-one (that I can find) has done that before; I guess it doesn't have the margin of error for commercial builders and there aren't that many hobby builders. If it turns out to be awful a longer headtube would let me graft in a second TT like on some of the bigger Rivendell frames. It would still end up weighing a respectable amount, though more than I plan at the moment.
   
   

How do you mean, almost a polo bike? Is it the lack of trail and the fat tyres? I think fat tyres are great; wish that I could get some of the Rivendell or Grand Bois gucci ones, but I'm happy with the Specialized Infinity ones I've got at the moment, apart from the weight. On thinner tyres I'm just rattling around the whole time and the placebo effect of feeling sportier is not enough to convince me that I'm going faster ;) as for the lack of trail I don't know whether I should play it safer. I've read some very convincing arguments in favour of low trail bikes, though many of them have been from the point of view of tourers using handlebar bags and 650 wheels. High speed stability is not an enormous issue, but the question is whether it would be too twitchy at normal speeds. I could lengthen the chainstays, but the rear triangle is quite difficult when you're building with bamboo...

Thanks for your replies.

Just the short chainstays and steep angles reminded me of polo frames, my polo bike was pretty similar but designed around 26" and with a few differences, 60odd trail and a small bb drop. Apart from the spinny gear, most people I know love riding their polo bikes around, they're a lot of fun.

I see, yeah.

I've now updated my design to fit my measurements, which are:

Inseam 86 0mm
Trunk 64 0
Forearm 390
Arm 70 0
Thigh 54 0
Lower leg 580
Ground to sternal notch 1510
Height 1800

I used the Competitive Cyclist fit calculator. I've used the 'French Fit' because although I do want to be fast and sporty etc., it's a lot easier to make a more touring fit sporty than make a very aggressive fit comfy for long journeys (which I intend to do from time to time)

That yielded an effective ST length of about 590mm, and an effective TT length of about 620mm. When I adjusted for the compact geometry, this is what I came up with:

Anyone notice any fatal sizing / geometry errors?

Thanks!
Ben

Edit: seems like a big gap between front wheel and the frame; think that's because of the relatively high stack of the On-One Smoothie headset I'm thinking of using. It's a MTB headset really, but I like the needle bearings to offset the instability of the short trail

in my opinion, thats some steep angles for a 'cruiser'.
of course, with current range of components you can make any frame do anything, but why struggle if you can make it good from scratch.

I would suggest 73 degrees for st and ht tops, and IIRC longer wheelbase will help at lower speeds.
bb quite low, but that might work for your specified purpose

Maybe I'll rein it in a bit, bump the trail up to 55 or something. Potentially doing lots of experimental stuff is not terribly bright, especially as my precision eyeball building technique cannot guarantee that it won't go even further the wrong way and end up twitchy as fuck.

Don't see why a longer wheelbase should help at lower speeds though, surely it's more of a high speed stability / lower maneuverability tradeoff? I've also decided to go for longer (easier to find) cranks despite enjoying my 140mm experiment, so need to hoick up the BB a bit, good catch.

(edit) PS: thanks for your opinion, it really does help - I know it seems a bit silly to design a bike to fit me, then ask other people what they think, but this is my first scratch-built geometry (just copied a Surly for the last one).

(have also decided to shorten the top tube because the results the Competitive Cyclist calculator seems way out of whack with the 58/59cm OTP frames that should fit me. 620mm is looong...)

Bike rationalised slightly, with some inspiration from Cinelli Mash, BMW Gangsta Track, various Surlys, and a Mercian or two:

Now with a higher BB and correspondingly slightly shorter chainstays (still with clearance for 35 tyres and mudguards), 73deg seat tube and 73.5deg HT for a trail of 55mm, which should be nippy but not too twitchy (decided that 45 was too little unless I had a front rack). And now with chainring and sprocket for verisimilitude.