bark_eater 

I've got a couple 21" nice beater frames (picked up a Trek 410 yesterday) looking for a new purpose in life. I ride 23" frames that tend towards a "French Fit" due to short legs and a long back. 56mm frames would probably be the best technical fit, but I read somewhere that a "Cyclocross" bike might be sized down from a road bike. So assuming I need get a longer seat post to have 6 or 7" showing above the top tube what would the front end look like? I'm not interested in having a full extension Technomic stem on a bike that would see "better off on a mountain bike" terrain, nor will I ever ride a bike with a "slammed" stem. I'm sure this has been covered at some point in the hundred years or so of cyclocross history. I'd love to hear what the "correct" set up is. Thanks, Woody

dddd

For cyclocross, it will be important to have the front wheel more forward than for road, due to mainly a tendency for the bike to tip forward when slowing for barriers or rough descents. There is also the need for clearance between your toes and the fatter front tire, but this is secondary.

So a slightly-smaller road frame is just a bad idea from my experience trying an old Windsor road bike at a CX event. I literally put that bike back in the truck after one practice lap, fortunately brought my regular cx weapon along so I rode that instead.

If you can get good fit and handling with a shorter stem, say 90-100mm, and the bike's geometry features a slack headtube angle, then some road bikes can work well enough for CX racing. Something like an old Super Course or pre-1973 PX10 can work really well but not in a smaller size. A lot of the less-expensive models of Japanese bikes featured relaxed headtube angles and can also work well for CX. Touring models as well, but tend to be heavier and any longer than 45cm chainstays becomes a hindrance to climbing traction and to performance.

bark_eater 

The little Trek has a 72 degree head tube and 45mm of rake the seat tube is 73.5 degrees. So a set back seat post would be a good idea? The top tube would be 10-15mm shorter than a bike that "fits" with the stem pretty much level with the seat. Does lowering the stem bring weight forward? Or doses a a higher and wider "dirt drop" bar and stem setup bring the weight back?

dddd

Since cyclocross requires the rider to make frequent accelerations fron low speed, a more-forward saddle placement is ideal, since this reduces the "heave" effort of getting off of the saddle into an aggressively-forward sprinting position.

21" is about five cm smaller than 23", so I would say that is too big of a frame size reduction. I ride about a 57cm road frame and my CX bikes have been 58cm, though 56cm could have worked out.

The Trek's slack headtube angle and steep seattube angle are both favorable to your small-frame dilemma. Knowing the Trek's toptube length is key, and if it's near 56cm I think it could work well. I would advise against using longer than a 110mm stem or a lot of saddle setback.
You could still do up a build to test out the theory here, and just maybe it works out for your riding conditions.

bark_eater 

The top tube on the trek is 55cm. On my 23" road bike, I just changed the stem to a 90mm Technomic down from a 110mm stem that was to long with the bars above the saddle. The bars are now a bit below the saddle and the fit is better. So I think I can duplicate the contact points. I guess the follow up is how much handle bar drop and width would be appropriate?

dddd

At this point it would be useful to know what your road bike's tt length and seattube angle are, as both affect the frame's reach dimension.

ddeand 

From what I understand (and from my experience), quite often CX bikes are sized down one size. I have two CX bikes - one is a 56cm frame and the other is a 57cm frame. I generally ride a 58cm or 59cm road frame. The reason CX bikes are sized down is because the bottom bracket height (ground to bottom of the BB) is is greater - I assume for obstacle clearance. Beyond the seat tube measurement, my bikes are pretty similar to my road bikes, although they feel like the top tube is a bit shorter. E.g., my favorite CX bike is labeled 57cm seat tube and 57cm top tube. It feels like a 59cm bike height-wise, and the 57cm tt seems about right. While the cyclocross geometry defines that type of frame (with the elevated bottom bracket), I suppose you can get close to that feel depending on the components you use (seat, seatpost, stem, handlebars). You just won’t have CX clearance with most road frames. And, of course, you may have tire clearance limitations if you need to mount fatter tires. Have fun with the project!

bark_eater 

Road bike, a 23" 1989 Centurion DLX has a 56cm top tube and I think 73 degree's on seat and head tube.

The 21" Trek 410 has a 55cm top tube, a 72 degree head tube and a 73.5 degree seat tube, so the reach will be getting shorter as the handle bar is raised.

bark_eater 

I found these pictures at:RAW PRE-WAR CYCLOCROSS

All three riders are using a different frame fit. I think the Trek would end up like the top bike which is relatively small with a high stem.

bark_eater 

Heres the "look" I am instinctively avoiding, but I dont think I'd tell this guy he's doing any thing wrong.
from:https://www.bikeforums.net/classic-v...s-bikes-2.html

dddd

The trek frameset will come within a half cm of your Centurion frameset's reach dimension, since the Trek's 1/2-degree steeper seat tube effectively adds a half cm to the reach.
So 55 + .5 = 55.5cm, just a half cm shorter top tube reach.

Another thing about cx bike fit is that these bikes have a shallower headtube angle, which is going to handle best with a shorter stem length than on an equivalent road bike having a steeper headtube angle.
So the otherwise-similar (in terms of TT length and ST angle) CX frame may end up feeling like it has a shorter reach to the bars, which it does.
However, if your road bike already has a shortish 9cm stem, the CX bike (or your Trek) will almost certainly handle fine with a 9 or 10cm stem, and can thus arrive at the same good fit.

Loose Chain

The reason you should size down for a typical cycle cross bicycle is because they have a high bottom bracket. The high bottom bracket shoves the resulting required saddle height way up there. If the bicycle has a level top tube then the next consideration is that cycle cross bicycles also typically have a low top tube height for generous standover and mounting and dismounting to run and carry the bike on rough terrain. The low top tube (assuming a level top tube type) then results in a low bar height because of the short steering/head tube. Therefore, cycle cross bicycles, even those a little toned down like the Surley Cross Check will have an aggressive set up and a high saddle to bar. As well, this often results in skewed frame sizes because the seat tube is short because the top tube is low.

All this comes too the manufacturers stated frame sizes are off on a cycle cross bike, measuring the same way a road bike is measured, c/t or c/c, the seat tube is shorter compared to the effective top tube. The top tube is the true frame size you should look at if you are looking for a real cycle cross frame. If you are converting an old road bike, it will be a fun endeavor but not a real cycle cross because the frame geometry is not the same, so I would size it as usual.

Track bikes or kind of the same way because they have a high bottom bracket (to allowing pedaling in turns) but unlike a cycle cross bike they do not usually have a low top tube because they are not constrained by the need to mount and dismount in rugged and uneven terrain but still they will have a high saddle to bar top as a result.