Disc road frame for commuting

[barefoot]

Yep try a few spacers.

Yep, that's the plan.

I spent this afternoon pulling the SLX crankset and BB out of my MTB, measuring, comparing, mixing and matching to see if I can bodge something up with a MTB BB (with/without spacers) and road crank.

Verdict is... no. MTB crankset has long spindle (and longer spline on LH crank), so requires BB outboard faces to be 5mm further apart. The MTB bearings are 1mm narrower (11mm instead of 12mm), but the MTB unit uses 7mm of spacers (or chainguide brackets etc) to get the additional 5mm. The cups have longer threads, and the plastic pipe is shorter to suit. Even without the pipe (which really doesn't have much function), there's not much scope for getting a wider chainline with any road/MTB parts mix.

But, from getting intimate with a few Hollowtech BBs, I reckon I could get away with ~2mm less spline engagement on the LHS crank, which is really the main implication of adding a spacer behind the BB cup. So I'm now on the scrounge for a spare spacer from my MTB buddies (has to be cheaper and easier than getting one mailed over from Belfast!).

I leant a friend my conventional BB tool, so I couldn't pull the old square-taper bits out of the frames they're in to check whether that's going to be a solution (although sourcing a long-spindle BB isn't a big deal if necessary).

Also, the option of denting the chainstay isn't entirely out of the question. The contact point isn't as close to the bridge as I had thought. I reckon the chainstays have ample strength (they're big for the sake of stiffness), so I'm not too worries about that. Just the process of carefully crushing my beautiful new frame...

All part of the adventure. If I wanted to be sure everything would just work, I wouldn't be building up a custom Chitanium bike...

tim

[MichaelB]

Barefoot - what chainring combo are you running ?

[barefoot]

Planning on a normal 6750 compact - 50/34 rings. Can't go much smaller than a 34 looking for clearance...

[find_bruce]

If I was you I would think very very carefully before denting the chain stay. Once dented, it will be awfully hard to undent it. Are you confident that denting the tube will not result in work hardening ?

Titanitum is a fabulous material, but once work hardened it gets very brittle

edit glad to see you found a better way.

[barefoot]

Win!



I knew it was close to fitting without a spacer.

That one 2.5mm spacer gives me about 2.5mm clearance.

So I've now dremelled it down to 2.0mm.

The lock pin on the LHS crank doesn't quite drop in to its hole now, since the spline is 2mm less engaged than it should be. I'll just have to be careful not to accidentally ride without the pinch bolts done up.

Starting to look a little bit bike-y now:



(finishing off the rear wheel while the Mrs was out last night). A bit wonky with such a tall fork, but... meh, it's for riding, not looking at

tim

[gassyndrome]

That is a pretty spesh looking piece of kit. Count me in as intrigued.

[Crawf]

The lock pin on the LHS crank doesn't quite drop in to its hole now, since the spline is 2mm less engaged than it should be. I'll just have to be careful not to accidentally ride without the pinch bolts done up.

Mmm I reckon that could work it's way off even over a short time, def keep a close eye on it.

[barefoot]

The lock pin on the LHS crank doesn't quite drop in to its hole now, since the spline is 2mm less engaged than it should be. I'll just have to be careful not to accidentally ride without the pinch bolts done up.

Mmm I reckon that could work it's way off even over a short time, def keep a close eye on it.

Yeah, fair enough. It's not much of a pin to hold a crank on, but it really ought to be in its hole.

I'll enlarge the hole on the swing-tab thingy so that the pin can reach the hole and lock in properly.

tim

[MichaelB]

Yeah, fair enough. It's not much of a pin to hold a crank on, but it really ought to be in its hole.

I'll enlarge the hole on the swing-tab thingy so that the pin can reach the hole and lock in properly.

tim

Good idea.

Looking good so far !!

[barefoot]

That's the first iteration of a drawing with my geometry. Yes, it's a bit unusual, no it won't fit you

I cropped down a picture of my current road bike, for another thread, but thought I could also post it here to justify why I've needed to go for crazy custom geometry on this disc roadie.


That's a 53cm generictaiwanesealloy frame (which DBR call size "S / 53"). I'm 5'10", and most simple fitting charts suggest would put me on about a 56cm frame.

I need the small frame for the top tube length. Then I need a ridiculous stem to get the bars high enough (and quite a lot of seatpost to get the saddle high enough). This bike puts me in quite a low aggressive position - especially in the drops - certainly not up in the wind admiring the view. I'd probably add another spacer under the stem if the steerer was long enough to fit it (it was a second hand frameset, I didn't get the option of leaving the steerer long).

My other two drop-bar bikes - an On-One Pompino and a Long Haul Trucker - are both 54cm frames, and are both too long in the top tube. I use a 50-60mm stem on each of them to bring the bars closer, up on a big stack of spacers. The three bikes all end up with relatively similar bar position - the LHT is somewhat closer and taller than the DBR, and the Pompino is in between, but all of them are much closer and taller than I can get on a production frame using "normal" parts.

That saddle position on the DBR is universal on all of my bikes. Slammed as far forward as it will go, on a zero-offset post. I think I'd like to go a little further forward still, but I have no way of trying it.

So, my custom frame has a steep (75°) seat tube angle to bring the saddle forward, short top tube and long head tube to bring the bars up and back. I intend that I should be able to get the same fit position on the new bike as I have on this DBR (and the others), but with the saddle clamped mid-rail, and with a flat ~100mm stem. Like how normal shaped people fit on production frames.

From there, I expect to continue fiddling with my position... but starting from the middle of the adjustment range, rather than hard up against the limits where I am now on all of my bikes. I can tweak saddle position forward _or_ backward. I can swap in shorter _or_ longer stems (angled up _or_ down), I can add _or_ remove spacers. A range of adjustability that is unprecedented for me.

tim

[barefoot]

Tim, glad to see you still believe in Santa!

Bugger of a thing to wrap:

Snuck in beside the tree at the last moment:

Happy Christmas to me, from Santa


QOTD:
"Well, it seems Santa knows the rule about N+1, doesn't he dear?"

Said through clenched teeth, with raised eyebrows... but I think I got away with it

Now I'm out of the closet... it's time for the build, then riiiiiide time

tim

[Reman]

Tim, glad to see you still believe in Santa!

QOTD:
"Well, it seems Santa knows the rule about N+1, doesn't he dear?"

Said through clenched teeth, with raised eyebrows... but I think I got away with it

Now I'm out of the closet... it's time for the build, then riiiiiide time

tim

Mate, if you're still alive to tell the tale, then you got away with it!

The perfect crime

[barefoot]

Also now I'm out of the closet - and since this one really isn't for commuting so doesn't belong with this thread title - I've split my posts off on to a new build thread:
viewtopic.php?f=66&t=59797

tim