Mongoose XR-75 MTB Weed Eater Friction Drive

Hi all!

I have posted a few times, but figured now would be a good time to formally introduce myself! I stumbled upon these forums while looking at different ways to upgrade my son's Power Wheels, oddly enough. The more I read, the more I yearned to motorize a bike! I gave up on motorcycling when my son was born, so thought a motorized bike would be a fun way to putter around the neighborhood without the dangers of traditional motorcycling (on the open road).

I really wanted to go with a 66cc China Girl, but since I currently have a cheapo Mongoose XR-75 mountain bike from Walmart whose frame does not allow for one, my plan for my first build was to get a cruiser style bike. I still want to do this, but plans for my initial build changed when I found a 30cc Ryobi for free on Craigslist. I figured the price was right, so friction build here I come!

The biggest issue for me is that I currently only have access to hand tools, jig saw, Dremel, drill, sockets, etc. I would LOVE to get bigger tools some day, table saw, drill press, scroll saw, welder (!). So this build was all done by hand in my garage with my brother-in-law and I, using hand tools, and supplies from Home Depot. Mainly 1/8" aluminum bar stock. I would have preferred steel, but too hard for me to work with using the equipment at hand. Even though I tried to make things rigid, there is still more flex in the system than I would like. But, it was my first build and I learned a lot for next time...

Since the motor I got was from a pole saw and not a string trimmer, I was thinking the clutch might hold up to the stresses better. Mainly I wanted to leave the clutch on to be able to idle without lifting the motor (or just leaving it to stall). But after seeing it in action and finding that the motor still stalls at idle if the motor is not lifted, I probably should have just not even bothered with the clutch, lol. I think I just need to tweak the idle speed to get the clutch to disengage...

So, that being said, on to the pictures!

For my drive spindle, I wanted to used a knurled bike peg. Since I could not find any locally, and was too impatient to order one, I went with a 3/4" drive socket. I made some grooves in it with my Dremel, but against a standard mountain bike tire, it just slips and slips and slips... Even with it slipping, I still got a surprising amount of torque and speed. The 3/4" socket has an outer diameter of about 1". I mounted the socket to the clutch housing by grinding the center part of the housing square to tightly press fit into the socket drive hole, then filled it with JB Weld, up to the opening where the clutch torx screw goes.





I wanted an easy way to have a bearing on the other side of the drive spindle, and someone else on the forum (sorry, I forgot who it was, but thanks!) mentioned putting a bearing inside then socket instead. So I found a bearing that fit well at a local bearing store, tapped it in with a hammer and a block of wood, and used more JB Weld to fill the gaps between the socket and also creating a lip to help hold the bearing in place, just in case.



While the main motor pivot point is a piece of aluminum that I bent in my vise to a U shape, I secured it to the frame with a U bolt. I found that it would pivot a bit when the motor was on it, so I bent up some additional support pieces and attached them above and below the U bolt, and bolted those pieces to a support bracket, just below the front of the seat.

I went with a scissor lift, just in case I needed to lift the motor off the wheel if the stock clutch did not work like I had hoped. Which is what happened, lol.



There is a little more angle on the motor than I would like, but that is basically the aluminum flexing. I need to add some more support...



For bearing support on the non-motor side, I just have a 1/4" bolt going from my bracket into the bearing hole. It does not add as much stability as I had hoped, though, and if I were to do it again I would build a fully enclosed FD system like others have done. Pretty much a support system that covers the top of the wheel and has a bearing on both sides of the roller. This system seems to work ok, though, so at least I am happy I gave it a shot!



The springs I used to add tension were not very strong, so I looped them under the swingarm for some extra pull, lol. They will get replaced with shorter springs and possible turnbuckles for tuning.

For the throttle and scissor clutch, I used bike brake levers mounted backwards. I was pleased with the quality of the levers since they were only $12 for the pair at a local bike shop.



And I knew the old, dry-rotted mountain bike tires were going to be an issue with slippage, especially with using a socket as a drive roller, so after the shakedown run, I replaced the rear with a Bell Streamliner balloon tire that I picked up from Walmart a few days ago. Apparently these wear quickly in FD applications, but we shall see how long it holds up.



My initial run was just up and down the street (with my motorcycle helmet and gloves on). It ran much better than expected! After double-nutting and locktite-ing the nuts the shook loose, I went for a longer ride, about a mile, around the neighborhood. With that small drive roller and small contact patch, I was still moving at a good clip, maybe 15-ish on the flats (I am 205 lbs). She even carried me up the hills. I cannot wait for the next ride, with the new back tire on I should get some more bite.

Now I am looking for something to put around the socket for a little more diameter and traction. I think a piece of rubber hose with a 1" ID epoxied to it, maybe? Small RC car tires?

Anyways, if you read this far, thanks!!!

So we reworked the throttle so it works a little more smoothly now, and improved the scissor clutch operation too. Along with the new rear tire, we also wrapped the roller in Gorilla tape (yes, I know it will not last long, just wanted to see how the extra bite and diameter would feel).

The improvement was enormous. The new tire and taped roller meshed great. Much more pickup and speed! Only rode for a few minutes and there is a nice tire shaped mark in the tape, lol. Definitely time to find something durable to add to my socket for the extra grip.

I was really surprised how well that little 30cc motor was able to move me. And when my brother-in-law rode it who weighs 70 lbs less than me, it flew! lol.

Thanks for the tips, guys! I went to Lowes and picked up a foot of 1" ID PVC hose (they did not have any rubber hoses that big). I cut a 2.25" section and it fits over the socket, but a tad loose. I used some epoxy that I had in the garage and coated the socket and inside of the tube.

The grip was great! And by "was", I mean until the epoxy started giving out and the tube was slowly sliding off the roller and getting eaten by the bearing support.

So I cut the remainder off and put on a new section with JB Weld, so far so good. After a couple miles, it looks like the PVC is not wearing off as much as it is compressing, so the contact patch with the tire is noticeably harder than the rest of the roller. We will see how long it lasts.



In the meantime I will be on the lookout for some rubber hose instead. I will try Pep Boys the next time I am in town. I like the idea of getting a super tight fit and heating the hose and cooling the socket!

Thanks again guys, it has been a ton of fun running around on the bike this weekend. The next order of business is getting one of those clamp-on cable stops for my clutch cable, the zip tie I am using to hold it in place is not working as well as I would like. I am looking at one of these guys: Amazon.com: ORIGIN8 Alloy Single Cable Stop: Sports & Outdoors

BTW, here is where this build all started, the pic from the freebie Craigslist ad:


And here is a video of my brother-in-law riding the bike this morning in our back alley (I usually wear my motorcycle helmet, he is a little braver than I, lol)

Ryobi 30cc Weed Eater Mongoose XR-75 - YouTube

... and this morning, the clutch self destructed! I noticed the motor bouncing a bit more than it should on my last ride, so checked it out in the garage this morning and notice the clutch bell had a LOT of play in it. I pulled it off, and the flat piece of metal on the inside of the clutch bell fell off in my hand...

It was never engaging right anyway, so time to go direct drive.

Unfortunately, since my drive roller was JB Welded to the clutch bell, time for a new roller too. I picked up a 1.25" diameter bike peg today, and will attach that to the crank tonight. I think I will not have to relocate the motor, but we will see. I will still want to run a bearing support on the non-motor side of my mount, but that will take some more doing...

Once I get things sorted, I will post pics. I knew using the stock clutch was a gamble, but at least it was a good learning experience!

First off, I want to send a huge thanks to my brother-in-law Dan who put in a ton of time on this project with me, and whose engineering ideas made it a much better build than originally planned. Thanks bro!

So, after the clutch failure, I scoured Austin for a bike shop that actually stocked BMX pegs. The fifth shop I visited had them, and they had a LOT (Empire BMX). I picked up an Odyssey Joystick peg for $15, which fit nicely over the 3/8" shaft and had an outer diameter of a little over 1 1/4". I then picked up some vinyl tubing with a 1 1/4" inner diameter to cover the peg for traction.

I put the peg in the freezer for a couple hours, then put the tubing in hot water, and then used a ton of force to get the tubing onto the peg. It fit great, but unfortunately when heated up by the tire, gradually started coming off of the peg.

Which is ok, since the added diameter took away more torque than I liked. But I am finding that the peg alone, with the weight of the motor and tension springs gets pretty good traction and provides just enough torque to accelerate up the slight hills we have around here and still gives a decent top end.

We also beefed up the bracing on the motor side since the original motor mounts were flexing.



Dan cut the springs to size so we did not have to curl them under the swingarm any more, and we also modified clutch cable to pull from both sides of the scissor lift.



So for now, she is running great. I definitely want to invest in a comfier seat, though, lol.

The next step will be to have a machine shop make me a version of the roller that is a couple inches longer and have a 1.25" flange mount bearing on each side of the roller.

Trying to come up with a name for the bike, I was tossing around "Rygoose Weecycle", but after spending some time in the saddle, it is definitely "The Rattler" LOL.

I also picked up a "double d" carb adjusting tool to adjust the mixture screws since she was running lean, and have a little more giddyup on inclines.

I am definitely impressed with the power this little mill puts out, hopefully the bearing/shaft hold out until I get around to the bearing supports.

Thanks all for your help and experience!

Thanks!

I added a brace to tie both sides together better, I will add another one on the other side of the roller later. The problem I am having with working with aluminum is flex. If I had used steel I would not have to have braces everywhere, but with the tools at my disposal that would have made things quite a bit more difficult. Things are finally pretty rigid now, and the bike feels great.

I think that I will end up replacing a few bits with steel, though, like the U bracket that everything pivots on.

Today I decided to see what kind of speed I am getting with the 1.25" bike peg, so downloaded a GPS app on my Blackberry to get speed and went for a ride.

On a slight uphill grade I was getting 17-19 MPH, and on a short flat I got up to 24 MPH! I probably could have squeezed out 26 if I had tucked down and had a longer stretch of straight road. This is with my 191 lb self (down from 205 a week and a half ago, woot).

I am very pleased with those numbers from a 30cc weed eater motor and a bike peg! On Cannonball2's recomendation, I strengthened the pull on my springs to increase downforce on the motor, and it got rid of my slipping problems without having to coat the roller. Thanks CB2!

There is a good amount of deflection on the tire, I wish I had a force gauge to measure how much. This weekend I will try to get a good picture of the peg sitting on the tire to give you an idea of how much deflection I am getting. I did not want too much force to add additional strain to the weed eater crank/bearing, so I am going to experiment with loosening up the tension on the springs until I find that tipping point.

Grats on the 49 lbs, that is AWESOME! Atkins has always worked well for me, so I hopped back on the Atkins train, along with riding my pedal powered bike (ugh). I have 10 more lbs to go to reach my target weight, so the easy weight loss is over, now I gotta work for it

I sold the bike this morning in order to raise the funds for an in-frame China Girl build. The guy who bought it seemed really nice and definitely interested in the build, so hopefully he will join us here

I had no idea on how to price it, so put it up for $150 on craigslist and got three emails from people interested in just a few hours, so was pleased with the response. I figure that takes care of about 1/3 of the costs of what I have planned for my next build, so that is a good start.