f-n-r chain conversion for speed

i was recently watching a video by a guy that uses an older style f-n-r transaxle, one with the vari drive separate from the transaxle and 5/8 keyed input shaft, in his racing mowers, he replaces the diff and drive gears with a solid axle, sprocket and chain leaving only the input, the collar that shifts into the forward or reverse and the forward gear, the shaft the gears and brake ride on is replaced with a 5/8 keyed shaft and a sprocket to drive the axle, since i'm planning a speed build and want around 40mph i was wondering how this would hold up, he said he uses a 1 to 1 pulley ratio, but i planned to use a go cart torque converter with the driven directly on the transaxle itself, i know they will break the input neck off if to much pressure is applied as seen in some of redneckcomputergeek's recent videos, but i don't think the torque converter would put that much pressure on the input neck, no more than the factory set up would anyway, as it stands now i have every thing to do it except the transaxle itself and the 5/8 shaft, i'm interested to know other members thoughts on this before i go to my parts guy and pick up a transaxle,

5/8 inch input is pretty hard to come by, usually splined input for the belt pulley. The internals will need major machining too, to switch from the splines to a keyed shaft, that is if the part isn't a one piece but multiple pieces. That center shaft that you are thinking of is usually a 1 piece hunk of hardened steel. If you got a lathe then that would make it pretty likely to do easily. Would cost a lot to do for outsourcing the part and still using a weak aluminum case to fit it in. Would be cost affective to fill it with oil and overdrive the piss out of it. Forgot the ratio of the f/n/r's but I think a 1 to 2 or 1 to 3 ration would make it useful and cheap.

I was going to do this years ago. I had machines one of the shafts but then I lost it.

prancstaman wrote:

5/8 inch input is pretty hard to come by, usually splined input for the belt pulley. The internals will need major machining too, to switch from the splines to a keyed shaft, that is if the part isn't a one piece but multiple pieces. That center shaft that you are thinking of is usually a 1 piece hunk of hardened steel. If you got a lathe then that would make it pretty likely to do easily. Would cost a lot to do for outsourcing the part and still using a weak aluminum case to fit it in. Would be cost affective to fill it with oil and overdrive the piss out of it. Forgot the ratio of the f/n/r's but I think a 1 to 2 or 1 to 3  ration would make it useful and cheap.

I don't know. Some FNR's have keyed shafts instead of splined. The Heckingers trans was keyed. Really really old, but it was keyed. They're pretty hard to come by from what I know, but they are around. Shaft seems to be a little smaller than 5/8 though. It looked more like a 9/16 shaft. I don't think it would hold up. I do know MST's have 5/8 keyed inputs and so do some 820s, but I doubt they would interchange with the FNR.

As Darkside said, RCG has been experimenting with his stash of MTDs. He's been doing some serious overgearing. 9" up front and 5" in the rear. Ripped the top off both FNRs. So I don't think overdriving would work. Not unless you brace up the input neck and all. And even then it would probably seize up since a portion of FNRs don't even have input bearings at all.


I would be all for this idea, but if you don't have a large stash of FNRs, I would be a little hesitant to start on this. Especially if you're only working with 1 trans. Not to be negative nelly here, but I just see a lot of frustration if it's only gonna be a one shot ordeal. Gotta have enough so that when one self-destructs, it's no big deal. I would need at least 4 FNR transmissions before I would be at all comfortable with experimenting with the idea. Ultimately it is up to you. But as far as I go, I would just need a bigger surplus of transmissions before I would start on this.

There's my 2 cents.

FNRs are so cheap and plentiful that I don't think he"ll have an issue finding a new one of he breaks it.

Like others said, getting the shafts machined is the hardest part. If you have a splined input shaft you could probably just have it turned down and sleeved up to fit your CVT driven pulley. Everything else will be custom machined and you'll not only have to pay for it, but provide some sort of print with accurate dimensions for a machinist to work to.

Some of the newest FNRs dont even have a proper spline. The last two I worked with had a flat spline, sort of like some mower blades mount with. So you couldn't really turn one of those down, you'd have to have a new one machined, and you don't want to pay for them to cut a spline.

https://www.youtube.com/watch?v=E04LsSnLwG0&list=WL&index=2
this is the video i was refering to in my 1st post, he didn't machine anything, didn't need to, he just replaced what was there with store bought parts