I have an older Trek 720 that currently runs a 7 speed cassette. I believe the rear spacing is 135 because I put one of my newer road bike wheels into the stays and was much narrower. What I'd like to do is swap the 7 speed freehub body with a 9 speed body so that I can use the older wheelset(they are much more heavy duty). Everything is shimano and hoping I can make the swap without any issues. I'm hoping for some comments on whether or not I can do this?

If this is a Shimano brand hub, you may be in luck. I believe (someone please confirm or deny) that the freehub/hub interface is the same on a wide range of Shimano hubs (if not all) is the same. So odds are that you can switch to a 9s freehub. You may need/want a longer axle, and will probably need to re-dish the wheel, so it's not a simple plug and play swap.

Also, going from 7s to 9s will involve some expense related to switching an incexing system, so check the total cost of the change before starting in.

+1. If it's a run-of-the-mill Shimano 7-speed hub, I'd put a fair amount of money on this conversion working. (Pictures always help.)

One caveat is that very early Freehub bodies had a different attachment than the later ones, not sure if Shimano had worked that out of their system before introducing 7-speed. Also, don't try to use a Dura-Ace Freehub body, as they were often DA-specific and not compatible.

https://sheldonbrown.com/k7.html

I've read that some of Shimano's cheapest rear hubs also used an incompatible attachment method but I can't think of the model number -- hopefully Bill Kapaun chimes in on that.

- Scott

You will need an 8sp freehub to make this work and possibly a longer axle. The 9sp and higher use a slightly different freehub body. I think this is what you need. https://www.bikeparts.com/search_resu...p?ID=BPC121337 Someone else might be able to verify that it will work.

Chiming in-
I don't have any idea of all the compatibilities, but-
I know the new RM-30 for example, will NOT fit an M-570. (or old EXAGE hubs)
I tried to put the 7 speed body onto an RM-30 hub to make an extra strong rear wheel. (along with an OC rim)
I was going to use 8 of 9 on 7 for my grocery getter.

Just use the 9 speed hub and add the 4.5mm spacer.
Many NEW 7 speed are doing exactly this. (maybe all?)

https://www.amazon.com/Wheels-Manufac.../dp/B001CJVLWO

You DON'T get a bag of 5! Best to check with your local LBS. They "should" have them. Mine seldom does though.

You can buy a complete RM-30 for < $20 vs just a body for $28.

https://www.amazon.com/Shimano-FH-RM3.../dp/B004HJ73XC

The RM-30 is an odd-ball hub with a different freehub body.

So?

I doubt that it will fit his unless it is the same hub. https://techdocs.shimano.com/media/te...9830800934.pdf
It most likely looks like this. https://techdocs.shimano.com/media/te...9830608927.pdf

The entire hub is < $20 vs your free hub body only for $28.

Go ask your neighbourhood bike shop, sometimes they have old sets lying around that their customers traded in. Customers who upgraded to disk brake wheelsets or who may have damaged the front wheel but bought a new set (front & rear), or those who feel that the stock wheels are for whatever reasons - too heavy, too slow, too ugly, etc has upgraded a new pair. Some of these are lying in some corner of the shop taking up space, so they usually will let you have it for cheap price if you are one of their regular customers. I have given my regular customers derailleurs, shifters, brakes from my spare parts bin at nominal price.

The only one I've come across that's been any different(I never see D-A stuff) has been the RM-30, or if there are any more out there that use the same shallow star as opposed to the deeper splines.
There are usually some differences between spacers and seal configuration, so even if another body will fit straightaway on the hub, it's often easier if you can get a donor hub and transplant axle, cones, seals, spacers and all in one go.