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View Full Version : Reliable FNR gearbox for SAE app?



atomicjoe23
06-29-2009, 02:53 PM
What is a reliable FNR gearbox that isn't too heavy but will stand up to the 10HP B&S engine on a 350-400 lb. buggy?

I've been trying to read as many of the FNR gearbox threads in the Driveline forum as I can and I've got a list of companies, but it seems that most of them are for higher HP buggies and they are gonna be too heavy and overbuilt (is there such a thing???) for the HP limitations of the SAE Baja series.

I think I am giving in and trashing the manual transmission that we used last year. . .it was a great idea and it wasn't expensive, but we were definitely down on top speed last year and there's no such thing as an older 2-stroke dirtbike with a 6-spd. tranny (not any that I have ever heard of anyway). . .and even if there was I don't know that one extra gear would make up the difference for us. . .part of the problem was probably our heavy buggy, but I think that it will be easier to fully enclose the engine bay to keep the CVT dry than it would be to find a centrifugal clutch and shift linkage that will function reliably for the transmission that we have. . .not to mention the lack of reverse. . .

. . .I still prefer a manual gearbox, but I think for the SAE competition and our budget that an CVT and FNR gearbox are gonna be the way to go.

Thanks for the help!!!

Gene
06-29-2009, 09:38 PM
Have you looked at Stak 4x4?

atomicjoe23
06-29-2009, 10:32 PM
Nope. . .I know of them from the truck word, but didn't know they were in the business of smaller stuff. . .have to check it out.

www.stak4x4.com (http://www.stak4x4.com) right???

atomicjoe23
06-29-2009, 10:37 PM
Checking the website out now and saw the Minibuggy transmission section. . .thanks for the suggestion! Hopefully they aren't too expensive.

atomicjoe23
06-29-2009, 10:41 PM
Just checked it out. . .a little pricey and I'm guessing no differential action since it doesn't say anything about it. . .like the fact that it's set-up for VW CV's, but the gear ratio seems pretty high to me as well. . .seems like most people on the forum have said that they are very happy with ~4.85:1. . .the Stak is about double that. . .maybe a good thing for a little SAE B&S engine. . .that's not my area of expertise though. . .

. . .the price thing may not be that big of a deal since all I really need to buy this year is a CVT, FNR gearbox, and tubing (shouldn't be more than $600 for the tubing because I'm gonna build a smaller buggy this year than last and we were under $600 last year). . .a new seat would be nice but not necessary and I was kinda wanting to get some UTV wheels/tires instead of sport ATV tires but I can definitely re-use the ATV tires. . .

minibajaman
06-29-2009, 11:37 PM
Just checked it out. . .a little pricey and I'm guessing no differential action since it doesn't say anything about it. . .like the fact that it's set-up for VW CV's, but the gear ratio seems pretty high to me as well. . .seems like most people on the forum have said that they are very happy with ~4.85:1. . .the Stak is about double that. . .maybe a good thing for a little SAE B&S engine. . .that's not my area of expertise though. . .

No need for a diff on off-road race cars. The gear ratio is probably pretty close to what you need; most Baja cars are running anywhere from 7:1 to 10:1 with a CVT. 4.85:1 is only for bike powered mini buggies and is in no way comparable to a Baja SAE car. You can do the calculations pretty easy to figure out what gearing you need for a top speed of say 35 mph. Or look around on here, I posted a spreadsheet that does the gearing calcs for you several times before.

atomicjoe23
06-30-2009, 08:48 AM
Cool thanks for passing the info on. . .gearing calculations aren't something that I've had to do a lot of before. . .

. . .thanks for the info on the Baja ratios as well!!!

I know that not having a diff last year severely affected our turning radius, you could tell just by pushing the buggy around. . .it was really easy to push until you turned the wheels and then it was all we could do to keep our momentum up with manpower.

dillon_b12
06-30-2009, 10:49 AM
No need for a diff on off-road race cars. The gear ratio is probably pretty close to what you need; most Baja cars are running anywhere from 7:1 to 10:1 with a CVT. 4.85:1 is only for bike powered mini buggies and is in no way comparable to a Baja SAE car. You can do the calculations pretty easy to figure out what gearing you need for a top speed of say 35 mph. Or look around on here, I posted a spreadsheet that does the gearing calcs for you several times before.

While I tend to agree with the diff. statement on high horsepower bike engine based buggies or pro-lites, pro-2s, etc...(off-road vehicles with power), I prefer having a diff. in our car. We are going to convert ours over to a spool soon to test it but as of right now I prefer the diff. to all our older cars which all have spools. It is worth it to me if only for the ability to easily push the car around and to be able to do a slow speed turnaround without taking a football field sized area to do it.

I also agree that 4.85 is way off. Like he said, depending our your setup somewhere between 7:1 to 10:1 will likely be best. Also agree that a top speed of 35 is a good range. That will put you right there with the fastest cars at the competition.

Also, if you are serious about cutting weight you will stay away from UTV tires. Wheels/tires are a place that I see lots of people packing a lot of unneccessary weight.

USF runs a 6-speed out of something but I can't remember what it is from. I think you are going the right direction with the CVT. People bitch about manuals being more efficient but the more competitions I go to the more it confirms that I don't want one. USF and maybe one or two other schools are the only teams that I have seen that are competent in driving their manual cars to the point that they are actually taking advantage of that extra power. Also, if you read what USF has said about their manual, it isn't like they just through it in and it worked. Like any other good team, it took a lot of experimenting and tuning to get it all set up right and working like it does.

LiveWire
06-30-2009, 12:38 PM
Cool thanks for passing the info on. . .gearing calculations aren't something that I've had to do a lot of before. . .

. . .thanks for the info on the Baja ratios as well!!!

I know that not having a diff last year severely affected our turning radius, you could tell just by pushing the buggy around. . .it was really easy to push until you turned the wheels and then it was all we could do to keep our momentum up with manpower.

Did you have Ackermann?