CX frame fit?

[d_douglas]

I am looking at a new CX frame. I don't race, so it would be for all season, all road riding. I do sometimes ride offroad on it as well.

I have been looking at the Moots website and wonder about their fit approach. My custom road frame has a 175mm HT and for a near equivalent frame from Moots, they use a 155mm HT (a 57cm or so).

Is this difference something to be concerned about? Is the HT on a CX bike shorter to account for the longer legs on the fork? (i.e.., 395mm vs 370mm?)

I want my bike to a) ride nice and b) look nice. I don't want to have a positive rise stem on my CX bike, nor a stack of spacers, so I want to sort out whether this is a suitable fit for me, or whether I should either go custom (too expensive at that point - there are others that do custom for less $$), or just live with an extra 10mm spacer to account for the lower stack?

I know there are many factors at play here, but can anyone summarize the short stack HT for someone who is not in need of a flat-out racing position...


Thanks for shedding light on this darkness!

[Uncle Jam's Army]

I can't offer much guidance for you as I don't have a CX bike (yet), but I pondered similar questions regarding the Moots Psychlo-X geo chart and I believe Moots will give you a 1 cm taller head tube for no additional charge. Any more than that and I believe you have to go custom geo I believe, which they will do for a price.

[KonaSS]

Yes, the shorter headtube is to offset the longer fork legs. Think of the distance between the center of the wheel and the top of the headtube when comparing between the cross bike and the road bike.

[echelon_john]

this is why stack is an important (and increasingly used) number in bike fit. if you compare the stack of your current bike to the 'cross bike you're considering you'll be able to judge how the front end/handlebar height might be different and plan accordingly.

yes, as confirmed above, 'cross bikes will generally have shorter head tubes corresponding to the taller cx forks.

[d_douglas]

OK, so that makes sense. Now, does anyone here have a newish Moots Psychlo X (not RSL! - the BASE model) in 56cm? Could they measure from the fork D/O centre to the top of the HT (or the headset, if it is a King)? Just the point to point measurement - as accurately as you can?

This would be a valuable measurement for me.

Thanks, folks....

[echelon_john]

that measurement is only part of the picture; you'll need the bb drop as well.

OK, so that makes sense. Now, does anyone here have a newish Moots Psychlo X (not RSL! - the BASE model) in 56cm? Could they measure from the fork D/O centre to the top of the HT (or the headset, if it is a King)? Just the point to point measurement - as accurately as you can?

This would be a valuable measurement for me.

Thanks, folks....

[d_douglas]

I think my road frame is 70mm drop. Moots BB drop is 61mm. What does this mean? All things being equal, I will be perched 9mm higher on the CX than on a road bike?

PS I am going to shell out for a pro fitting in a local shop here - first time ever. I figure it is worth it, right? They are Moots fans (and sell a few) so they will know their stuff, though i think they do more Road and MTB and less CX, but they'll figure it out...

[echelon_john]

Yes--you'll be 9mm higher off the ground for a given saddle height measured from the BB on a bike with 61mm drop than you will on a bike with 70mm drop.

But what's more important is that, for a given front axle-to-handlebar measurement, the bars will be higher in relation to the BB on the 70mm drop bike than the 61mm drop bike.

Stack is the vertical measurement from the BB center to either the TT/HT junction (used sometimes) or the BB center to top of the HT (more common).

So if you compare the stack heights of two bikes it will take into acct any difference in BB drop and give you an accurate ideas of front end height range in relation to saddle height. It doesn't take into account reach. For reach you want to figure out what the saddle setback is on both bikes (i.e. dropping a plumb line from the saddle tip and seeing how far that falls behind the BB). This will be determined by seat tube angle.





I think my road frame is 70mm drop. Moots BB drop is 61mm. What does this mean? All things being equal, I will be perched 9mm higher on the CX than on a road bike?

PS I am going to shell out for a pro fitting in a local shop here - first time ever. I figure it is worth it, right? They are Moots fans (and sell a few) so they will know their stuff, though i think they do more Road and MTB and less CX, but they'll figure it out...

[tuscanyswe]

Yes you will be 9mm higher up in relation to the bars if all other things beeing equal. So 9mm more drop from saddle to bar if you use the same riding height on your cx as your roadbike.

So you got 9mm less bb drop there. You got 20mm shorter ht than usuall = 29mm more drop. But the fork is 25mm longer than usual so that makes up for most of it. Usually you have a cable hanger that needs to be fittet on top of the headset so thats another 10mm or so if comparing to a roadbike. Since you will be looking at discs you will have to ad the 4mm still missing in form of a spacer. Or you could use a stem chart and c if the slacker ht angle of the cross wont make your favorite stem a few mm higher than if mounted on a roadbike. Will be pretty close..

[echelon_john]

PS if you're not racing, and simply using the cx bike as an all-around, all surface bike, you probably want basically the same position as your road bike, assuming you're happy with that. But maybe with the option to have the bars 1-2cm higher and 1-2cm closer to you if you want to try that later; some folks find that adjustment gives them more confidence on the dirt, which is fine as long as there's still enough weight on the front wheel.

[John H.]

Have someone who understands bike geometry mock up your road bike and perspective cross bikes on bikecad.
Then you can see what the differences are.
As others have stated fork length and bb height will affect relative drop.
You also may want to consider toptube length. People usually go a bit shorter for reach on a cross bike. No big deal if you already use a pretty long stem- you can just go 1cm shorter in stem. But if stem is already on the short side you may want a shorter toptube.

[d_douglas]

This is a can o worms. It's a fun can o worms, however.

Thanks for input, all.


Are there any 56cm Moots PX folks out there?

[tuscanyswe]

I can measure my 56.5

What measurement do you need dident understand. Also i think moots will make you a longer ht free of charge if you need it.

[d_douglas]

I can measure my 56.5

What measurement do you need dident understand. Also i think moots will make you a longer ht free of charge if you need it.

Hi Bjorn:

That would be great. Is yours a stock frame? If so, as I understand it, John has suggested to measure from BB centre to top of HT (or headset if you use a King HS). That would give me an idea of the proportions relative to the size of my Speedvagen.

Thanks a lot! I'll PM ya and correspond offline.

[tuscanyswe]

hmm i will measure that for you but are u sure thats what you want? Stack is the height from bb to top of headtube measured in a vertical plane. Its not the diagonal measurement from bb to top of headtube cause that would be affected by toptube lenght. But i will measure for you if you want :)

Heres a drawing of a 56.5 i think might help you get a better picture.

http://farm9.staticflickr.com/8164/7527302154_de7eeec0d2_b.jpg

[echelon_john]

measure ground to BB and ground to HT top and subtract the BB #; that will give you stack.

hmm i will measure that for you but are u sure thats what you want? Stack is the height from bb to top of headtube measured in a vertical plane. Its not the diagonal measurement from bb to top of headtube cause that would be affected by toptube lenght. But i will measure for you if you want :)

Heres a drawing of a 56.5 i think might help you get a better picture.

http://farm9.staticflickr.com/8164/7527302154_de7eeec0d2_b.jpg

[tuscanyswe]

So i measured 86cm from ground to top of headset.
Ground up to center of bb measures 27.5 wich would give a stack of 86-27.5 = 58.5

Hope this helps.

[d_douglas]

So, my Speedvagen road measures exactly 60cm in stack INCLUDING the King HS assembly.

Bjorn did the same on his 56cm Psychlo X and came up with 58.5cm.

This means that my BB / HS relationship is greater than his CX bike, which is as close to my size as possible.

Lowish BBs seem like a good idea to me, as I am not in need of anything that can clear barriers.

Can anyone add to this calculation - does this mean the 56cm with a 145mm HT (yes, correct, I thought it was a 155mm HT, but it's 145mm) would be my ideal fit?

Thnaks for more insight!


Darren

[tuscanyswe]

The difference between 2011 56.5 and the current 56 is minimal.
0.5 cm seattube lenght.
0.5 degrees slacker headtube.
0.6 mm in effective toptube lenght.
The stack will be the same on both models since bb drop and headtube lenght are the same (58.5cms)

ideally so far you would prolly want a 70mm bb drop (9mm more) and 15cm headtube (5mm more) to get the same stack as on your vagen. If this is the ultimate goal.

Then theres the lenght of the TT. Whats the st and ht angles on your wagen and the effective toptube?