Yamaha RT100 “Big Wheel” conversion

Jimmy Cincinnati /

So a little Project I’ve been screwing with for a few years now. Basically I bought it cheap but put it aside to focus on other stuff. Slowly I’ve been amassing random bits for it and recently found a spare afternoon to swap the stock forks out with DT50 forks and I extended the swing arm about 1.5 inches in the rear while adding additional mounts to give me plenty of options for rear seat height / suspension travel. I plan to raise the rear up another 1-2 inches. So my reasoning for doing this is... stock it’s a Yamaha Rt100 with a 18” front and 16” rear wheel. I really like this drive train because Yamaha did very little to change it and this motor was used from like 78-03ish. They are good motors and parts are cheap. The stock bike leaves the suspension wanting for sure and I’ve always felt the market lacked a lot in the “mid size” dirt bike class. Basically stuff along the line of a KX100 (19 / 16 wheel base). This bike will be slow but should fit adults nicely while not being overly tall nor heavy. Light and nimble is the aim. Oh, the front rim is 19” now thanks to the superior dt50 setup... just wish it had a disk brake. The 1st pic is with the stock rear swing arm while the 2nd is the extended swing arm. The last pic is not mine but is a pic of it stock.

It’s dirty, and some covers are not mounted on it but overall how do you like it? Do the front forks look too raked out? Feel free to bash it or give me pointers. This really is a labor of love that perhaps only I really dig.


Re: Yamaha RT100 “Big Wheel” conversion

jimmy, i got REALLLLY excited there for a second


Re: Yamaha RT100 “Big Wheel” conversion

I always wondered what you put the DT50 front end on, cause it's honestly not that great of a front end and it doesn't have a disc brake lol

Re: Yamaha RT100 “Big Wheel” conversion

this does make sense. I got some SUUUUUPER long skyjacker shocks if they'd work, i think theyre 16 inch like 420mm. theyre massively long like a good 2 inches + over even like magnum shocks. "obnoxiously long" is an appropriate descriptor

Re: Yamaha RT100 “Big Wheel” conversion

I just picked up one of these to beat around the farm! I was looking for something in that size that would be forgiving and crashable for teaching people to ride + start on one kick and ready to rip without warming up. Pretty much narrowed it down to that, mx100, ke100, etc.

It's goofy how many parts they used from other bikes, the engine is obviously a DT but even dumb other stuff like the oil tank is repurposed.

I'm missing the left tank cover and side cover, really ruins the look of the bike without it.





It was pretty badly beaten but I was able to clean it up pretty good, stripped off the black rattle can on the seat and repainted white over the original white with plastic paint.. definitely a 10 footer but it runs great and ticks all the boxes. I can run out to check on stuff around the farm, go over to the neighbor, and once we start having people out to the farm again I think it will be great for teaching people to ride.

Mine is 19/16 stock but the front forks are so bad, they don't have a rebound spring so at the top of travel they slam hard. The rake is crazy too, geometry in general is just real bad. It's not an MX by any means but it's good for what it is. Quite a bit faster than it should be too, for how crazy bad the suspension is.

Re: Yamaha RT100 “Big Wheel” conversion

Actually I should check maybe it is 18 front, I don't know that I even looked that close, I threw it together pretty fast. I'm definitely not upgrading it although I do have a extra dt50 front end kicking around...

Re: Yamaha RT100 “Big Wheel” conversion

> Potr zebie Wrote:

> -------------------------------------------------------

> jimmy, i got REALLLLY excited there for a second

> >

So did I, those things are stupid cool! :)

Re: Yamaha RT100 “Big Wheel” conversion

friends little brother had one but it was just an 80 and we'd bomb around the pits but youd get stuck in ruts on tight trails and roll over your feet

i havent seen one around here in awhile

Re: Yamaha RT100 “Big Wheel” conversion

Jimmy Cincinnati /

Sorry been meaning to get back on this thread for a few days.

Sorry its not truly a "big wheel," my inspiration for the title was like a big wheel KX85... stock those are 17/14 but they did make them in a 19/16 at times. I do dig the big wheel (fat tire) bikes but never seem to find one cheap. I had a Suzuki RV90 for a minute but sold it off in an effort to focus more on my yamahas - plus the motor on those seemed more odd ball to get parts for.

Born, I appreciate the offer on the shocks. 16" is probably too tall but Ill keep it in mind.

Graham, your DT100 came out nice. Too bad your missing the cover tho. These bikes are cheap to repair motor / suspension pieces but plastics are scarce and pricey. That rear fender is an odd ball and pricey to find. I believe the set on my bike is the only tank covers i have or else id send you some. I do however have like 2.5 of these bikes in pieces so i have other misc bits if your looking for something. You could also consider just getting the older MX/DT 100 gas tank and putting that on there.

A few years back my buddy bought a ragged out 80' Yamaha DT100 from AMA for like $200. It ran and we beat it on the trail for years. That stupid bike to this day still runs. Im very impressed by these motors... nothing to race on but snappy enough to have fun with.

Potr zebie, I have a street legal GT80 that is very snappy. Ive determined Yamaha is my favorite enduro / trail bikes... they are cheap, plentiful, and actually pretty peppy generally. I thinned out my fleet of bikes to focus on my QT50's, GT80's, and RT/DT100's.

Re: Yamaha RT100 “Big Wheel” conversion

"Potr zebie, I have a street legal GT80 that is very snappy. Ive determined Yamaha is my favorite enduro / trail bikes... they are cheap, plentiful, and actually pretty peppy generally. I thinned out my fleet of bikes to focus on my QT50's, GT80's, and RT/DT100's."

a shitty used non-running yellow MX80 was my very first bike, 5th grade i think

older next door neighbor kid, Mark, helped me get it running (got gas in my eye for the very first time the same day it came back to life)

anyway, someone had already broken and changed the front sprocket before i had gotten it AND WELDED THE REPLACEMENT BACK ON

we rode it around for awhile then broke the very same sprocket and couldnt weld and had no money

so we put gas on it, and ghost rode it down our front hill

then my dad brought it to the dump

Re: Yamaha RT100 “Big Wheel” conversion

The only stuff I'm still missing is this dumb little cover, and apparently a little spring washer dingus that goes in there around the throttle tube?


I'm also missing the silencer core and exhaust heatshield, no idea why someone would even take the heat shield off...

It's been fun project, just kinda fixed everything the most straightforward and cheap way possible, rather than overthinking it.

I would have had it done in like 2 days but partzilla fucked me over, took 16 days to get a wrist pin bearing, then when it showed up they sent me a damn screw, then they wouldn't refund their expensive ass $10 shipping charge. Last time I ever order from those fuckers. Seems like every time I order from them it's some bullshit, Babbitt's is only a little more $ and way better to deal with on OEM stuff.

Bummer about the side covers, I have been pulling my hair out, I had the same thought on the tank but the seat mounts super goofy and I've had dirties with the seat not mounted to the tank right and it's annoying as hell. I'll just keep searching.

Re: Yamaha RT100 “Big Wheel” conversion

Been riding this thing a lot, man what a great little bike. If someone had put me on a bike like this when I was about 12 I literally wouldn't have gotten off until it ran out of gas.

I like that you can just let it idle in first gear and it's basically going walking speed, it's going to be really nice to teach people to ride.

I was wondering if the 4 stroking at like, 6k rpm is normal? Porting looked pretty good but it's almost like it hits a rev limiter around 6k, four strokes like crazy and load/throttle don't make much difference. Maybe I'm just used to bikes that rev higher but this seems like it falls down way before it should. Maybe a plugged exhaust? Unless it actually has some kind of crazy rev limiter or someone put in a wacky jet

Re: Yamaha RT100 “Big Wheel” conversion

Jimmy Cincinnati /

could it be the CDI? I'm a crap tuner so I don't want to speculate too much. people do love their home modifications, so make sure someone didn't drill holes in the air box or something stupid.

also, I didn't mean to ignore your parts list. I honestly haven't touched my bike since I posted this. i plan to look for your stuff when get free time to mess with that stuff again. kids really eat up time.

Re: Yamaha RT100 “Big Wheel” conversion

Yeah it's got 2 holes drilled in the air box but my experience with the DT bikes, they aren't as restricted as a lot of them (ke series comes to mind) to where they get really weird with leaks upstream of the carb.

Yeah man don't worry about the parts, I'm texting from the hospital right now with a new baby as of this morning. 3 boys now, gonna have to get a few more little bikes, haha

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