Thinking about getting a flatbar version of the planetx xls. Seams like a good deal for £899.

Anyhow its for mrs earl who is 5’2.

According to the geo chart, Size small is for 5’3 to 5’7

Currently she rides a 15″ mtb with ETT of 555mm toptube + 80mm stem = 635mm reach
The xls in Small has a ETT of 516mm and 100mm stem. = 616mm reach

To get the same reach she would need to go for the Large frame which doesn’t sound sensible at all.

Can someone explain it to me?

If its the same frame as the drop bar version then it will have a shorter ETT for a given size than an MTB.
You need to consider seat tube length for standover. You’ll be able to tweak the geo with a layback post / different stem combo, but if in doubt, you’d be better off getting your mrs to throw a leg over one before buying.

Small sounds right for 5’2″.

Careful comparing the ETT on different frames as it depends on seat angle too.

As said, CX will be shorter reach than the equivalent mtb. And MTB reach depends on bar width and backsweep too.

Should we be trying to replicate the same reach on the flatbar cx as the mtb? or is shorter the correct fitting?

Should we be trying to replicate the same reach on the flatbar cx as the mtb? or is shorter the correct fitting?

Depends – instinctively, I’d be looking for a longer reach on a more road-biased bike to get a lower/faster position, but it depends on what it’ll be used for. You do really need to consider seat tube length too.

Thanks – I’ll try incorporate the seat tube into it tonight and see what pops out.
I’m guessing taking a frame designed from drop bars and sticking a flat bar onto it is not ideal.