I have a brand new b17 brooks that I was going to put onto a 72 px-10 but I just realized that the saddle rails are not the same distance apart.

I'm planning on keeping the original simplex seatpost that requires a larger distance between the rails.

Is this another case of weird french sizing? I guess so, but I wasn't able to find anything in the forums about it after a quick search.

I guess I figured that saddle rails were a pretty universal standard but the more I dig into bike parts the more I'm not surprised by this kind of thing.

Overall question, Is there a lot of variables in saddle rails?

How much difference are we talking about? AFAIK, the standard spacing is 44mm, and usually you'd just be dealing with "manufacturing tolerances". Back when, there was a special "narrow rail" B17 with a matching Campy post, but that was decades ago. And now that you mention it, one of my B17s seems to be a few (3-4) mm wider than the cradle on my FSA post. But I just squeeze the rails together til they fit. So, no, it's probably not another case of "French standards".

SP
Bend, OR

Rails can be 'sprung' with the aide of a spreader to 'jack' the rails apart. I have a spreader that came with a gear puller kit I got for car maintenance. I have only spread the rails about 10mm though. I don't know if a greater distance is possible.

Problem is that sometimes the rails spread unevenly so care must be taken. I found that re-positioning the spreader is necessary and great visual care be taken during the job to check the position of the rails and to see that the rails don't bulge in the middle. They will bounce back a little when the spreader is released.

I have found Brooks rails do have width problems with Campagnolo posts and often need adjustment to get them just right. I find it annoying if they don't line-up perfectly and are out by about 1 mm.

I have a '72 PX-10 with the original Simplex seat post and I experience the same issue when I've swapped out saddles. It's kind of a pain, but the rails will spread enough to install. As a matter of fact, I just swapped out a saddle yesterday, changing out a red Flite that I had been riding for a period correct Ideale 80. You would think the French saddle would fit right on, right? No way! Same problem, so I had to very carefully slide the darned thing on. Crazy, but you have my empathy!

Yeah, it isn't really far, probably about the diameter of a rails difference give or take a few mm(don't have a ruler handy as I typed this). And I probably will be able to spread the rails enough to compensate.

But I was curious if saddle rails were something everyone knew but the topic didn't come up much.

Like I said, I assumed they all had the same distance.

Thanks for the replies guys.

In theory they do, but...

SP
Bend, OR

"In theory, theory and practice are the same. In practice, they aren't."

I had a NOS Vuelta Bassano (531 rails) that I tried to put in a Ritchey WCS seatpost. I just ended up bending and letting the tightening of the clamp pull things in line. Not ideal, but it works. No worries.