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Discussion Starter #1
A few weeks ago I stumbled across a guy on Craigslist with a totaled Grom. His frame was garbage and I wasn't looking for another Grom so I bought the front end (wheel, tire, front brake, forks, triple tree, front brake lever, bars) and the rear swing arm and rear wheel.

Grom parts.jpg

I'm going to use these parts and some other junk parts and add a $100 200cc pull-start lawn engine to make a mini bike for the Gambler 100 Mini Bike Rally next Spring. Sorry, I realize that might annoy some folks here to use salvage Grom parts for a frankenstein thrasher build but these are actually pretty decent parts for what I'm doing. And they were cheap!

Next step was to see what kind of frame I could drop the front forks into. All Balls Racing has a pretty good search feature to see what frames are compatible with your forks (https://www.allballsracing.com/forkconversion). Believe it or not, the Grom steering stem fits most of the old Honda CB frames. I found a guy with a 1972 Honda CB175 frame and I cut that in half. Then I put the Grom forks into that.

CB175.jpg

Today I welded the CB175 frame into a rusted and busted old mini bike frame.

The Grom rear swingarm doesn't fit into the mini bike frame yet unless I attach it to the very back (and that will make my wheelbase over a foot longer than the stock Grom wheelbase). I want to keep this thing small and nimble so I'll do some more cutting and welding on the minibike frame tomorrow.

More to come if anyone is interested.
 

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Discussion Starter #4
Here's a shot with the chopped Honda frame welded to the old and nasty mini bike frame. I had to partially cut and bend the rear end to make room for the swingarm. I'll weld it back together later and add some more support.

The shock is probably overkill but it is adjustable and at least it won't bottom out on big drops like the stock Grom rear shock. Plus, the price was right (free).

frame.jpg

I'm planning to chop the swingarm supports out of the old Honda frame and weld those onto the mini bike frame so I have a strong connection there. (It's just sitting there temporarily for now so I could get a rough idea of my wheelbase.)
 

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Discussion Starter #5
I want to see this build. And now I want to go to that rally, but it's 2,500 miles away!
Here's a video for you:

We did it last year and it was a blast! 22 laps of 4.5 miles each. Not a whole lot of rules other than you have to ride a bike with a pull-start motor (like a lawn mower). There were about 120 bikes that started the race but only about 30 actually finished. Our team managed to finish but we were several hours behind the winning teams.
 

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Discussion Starter #9 (Edited)
Very cool build. I'm surprised you didn't just buy a Grom frame which is cheap. This is way more adventurous for sure! But in the end you'll have a custom bike.
I thought about just buying a Grom frame but the stock Grom frame doesn't have a ton of ground clearance for deep ruts offroad. And then I would still have to make a big flat spot to mount a horizontal shaft-drive motor.

I was also nervous that if I showed up with a Grom, it would be "too nice" and they wouldn't let me race. This race is a rolling collection of old, home-built mini bikes and shitty Chinese mini-bikes you can buy at Wallmart for $400. Most of these bikes don't even have headset bearings! Their front forks are attached to the frame with only a fat carriage bolt, some fender washers at top and bottom, and a lock washer and a nut. The smart riders put red lock-tite on everything but stuff still rattles apart.

4LUAY7W67M74X2Y44373GXZAVQ.jpg

R3UMBIXDLGARFS5X7HMUZUYEIY.jpg

Images taken from this article about the event: https://www.motorcyclecruiser.com/dogfight-in-high-desert-at-oregon-gambler-mini-bike-enduro/
 

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Discussion Starter #11
Next I attached the Grom swingarm and chopped the back off the mini bike frame.

I used some wire to hold the parts in place for now to get a better look. I’m not sure if this is where everything will stay but I think it might work.

If I do go this route, I need to figure out how to support that rear shock and then make some mounting brackets for a seat. I have a seat from a Ducati Scrambler that isn’t being used and it would be funny to use that.
 

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Next I attached the Grom swingarm and chopped the back off the mini bike frame.

I used some wire to hold the parts in place for now to get a better look. I’m not sure if this is where everything will stay but I think it might work.

If I do go this route, I need to figure out how to support that rear shock and then make some mounting brackets for a seat. I have a seat from a Ducati Scrambler that isn’t being used and it would be funny to use that.

I'm really enjoying watching this project and your resourcefulness. I'm hoping we get to see the results at the race. One question, will the skinny grom type tires work in that sand and rocks or are you better off with something way wider?
 

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Discussion Starter #13
One question, will the skinny grom type tires work in that sand and rocks or are you better off with something way wider?
I'll find a slightly wider tire with more off-road tread if I can find one.

Last year my tires were much wider but I had no rear suspension and only a semi-functional rear drum brake. I think I'd rather have decent brakes and full suspension. That's ultimately what pushed me in this direction: I want the best mini bike I can build for the lowest possible price.
 

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Discussion Starter #14
I was also thinking it would be funny to put duct tape and zip ties over my welds. Make it look like tape and zip ties are the only things holding the frame pieces together. :big smile:
 

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Discussion Starter #15
Today I have a rolling frame!

I pushed it up a steeper hill nearby and rolled home. Dropped off some curbs and the rear shock works pretty well. Only a little sag in the rear shock when I sit on the bike but it flexes on drops.

The shock mount is only tacked in place for now so I can cut it back off and modify the geometry if I need to. The front fork angle is supposed to be 25-degrees and it’s a little too steep currently. The problem is if I set the front fork to the correct angle, I loose ground clearance in back. Just in case it’s not obvious, I’ve never built an entire frame before so I’m leaning as I go.

Still need to mount some foot pegs and figure out the rear brake. And finish the seat mount. And so many other things..,
 

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Discussion Starter #17 (Edited)
Here is how it looks with a seat on it. Yep, it looks goofy but it's comfortable.
rolling frame2.jpg

The next thing I've been thinking about is how to mount my rear brake.
IMG_0682.jpg

I don't have one of the stock side pieces that holds the rear brake, the foot pegs, and the exhaust in the right places. Not sure what this thing is called but here's a photo so you know what I'm trying to recreate out of scrap metal. (Image from a Grom owned by a guy up the street.)
IMG_0681.JPG

The first thing I need is a shaft for the brake to pivot around smoothly without being sloppy. I took my brake lever to Home Depot to see if I could find something that would work. The closest thing I could find is this metric bolt that is just a little too large.
IMG_0683-2.jpg

The only part I really need is that smooth shank and it needs to be just a few millimeters smaller in diameter. I cut the top off and turned it on a grinder to get it the right size.
IMG_0686-2.jpg
 

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Discussion Starter #18 (Edited)
Perfect fit!
IMG_0687-2.jpg

The side with the threads in that pic above will be cut down and welded to the frame. I still need to make sure the brake lever doesn't slide off so I welded a slightly smaller diameter bolt to my new shaft.
IMG_0690-2.jpg

It's not pretty but it should work. Still need to cut the threads off and weld this thing to the frame.

Edit: pics weren't showing up
 

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Discussion Starter #19 (Edited)
Here' the approximate location where this thing will be mounted.
IMG_0689-2.jpg

My brother is mailing me a set of used dirt bikes pegs but I haven't received them yet. I'm going to mount the foot pegs first and make sure they feel comfortable for standing up to steer. Once I know where the foot pegs will go, I can weld this new brake lever shaft in place.
 
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