I started with a slightly used bike and proceeded to custom taylor it for my 71/2yo son.
Things I did: Soften the suspension by adding a Fastace shock with 550lbs/in spring. Up front I completely disassembled the forks that still had the original oil and very little of it. The internals were pretty warn so after a good cleaning I used the heaviest fork oil I could find 20wt. I also removed one of the springs and used a 1"preload spacer on the other. I replaced the bars with a more kid friendly bend Then topped it off with new levers and grips. New tires MX32 rear and MX52 front along with a new chain. I also replaced the flimsy stock foot pegs and mount (TRC pegs don't fit the SSR mount). This was all done after a cleaning, valve adjustment, spark plug, and oil change.
This bikes suspension is set up for a 160-170lb adults as delivered. With one spring it is about perfect for my 70lb son who is just learning to ride. I also added about 20cc more oil to the leg w/o the spring so the air spring stiffens as the fork is compressed. The coil springs only job is to hold the bike up in the suspension stroke under the combined weight of the bike and rider they don't add strength or stability to the fork design.
I had hoped to get my son out on it this weekend, but I ran into a couple of snags while transplanting the CRF50 engine. First the stock throttle cable had to much freeplay to work with the tiny CRF carb. I have a MotionPro cable/carb cap on order that should solve this. Second the SSR over foot peg rear brake pedal makes contact with the kickstart shaft on the 50 engine. So a under foot peg pedal should do the trick there. I also got rid of the "bicycle" steering stem bearings and placed a set of tapered bearings in place.
Finally got my son out for his first ride on the bike. The rear suspension is still a bit stiff for him but as he gets a bit bigger and faster it should be pretty good. The forks are actually pretty close and seemed to work well. He rode tons of laps at Perris raceway's mini track and had a blast. Thanks to Vince at Tbolt for the help along the way.
I have found a Fox mountain bike spring that fits the Fastace shock.
It's a 350lbs/in so it should be a lot closer for my son. He was only moving the rear suspension a couple inches at most with the 550lb spring.
i dropped a fork spring off at Racetech to see if they have anything similar in a lighter spring rate. For now I'll just remove one of the preload spacers to soften it a bit more.
After mounting the 350lbs/in spring and having my son try it out I think we need just a tad stiffer rate. I ordered a 400lbs/in and I'll exchange the 350 spring. Hopefully this info will help other dads that want to go the pit bike route for their kids bikes. I mainly did it because we already had a CRF50 and my son just needed a bigger bike not more power.
My son has ridden his bike quite a few times since he got it and it has been great for him. He is still a bit timid around other riders, but he's having a blast and that's all that matters to me. The suspension mods have been great thus far for him. He's just barely getting the wheels off the ground on occasion lol. I just did a topend rebuild and tried to get him a bit more power with a TB race cam (stk head). Looks like the cam alone does not add any power though (see my other thread on that). Luckily I got the cam used for cheap so no sweat. His little brother (3) is almost ready to take the 50 engine back so I did not want to do a big bore kit. I am planning to buy a 90cc Pirahna semi engine for this SSR chassis. My son just isn't ready to master a manual clutch yet so the stock SSR110 manual engine will just sit a while longer.
After talking to my son he wanted to try the 110 manual engine that came with his bike instead of getting a larger semiauto. I was a bit worried about more than doubling the power and going to a manual clutch at the same time. I made a small sleeve for the carb to act as a restictor. It essetially reduced the carb size by 2mm. After I test rode it I was happy with the reduction in power. He has done great with learning the manual clutch mostly due to 1st gear being so low on these engines it's almost impossible to stall.
Update on the "one spring forks" They worked pretty good for a 70lb nube and they didnt fall apart or cause mahem. When I got the bike the fork in ternals were in poor shape due to being run with no oil. When I took them apart there was only shrapnel and some metalic sludge. I have since replaced them with a new set of SSR 110 forks thinking they would have piston rings on the damper rods and with oil make some sort of damping. I was wrong no piston rings and large gaps = no damping with "normal" 5-20wt fork oil. I resorted to 75wt gear oil lol. Of note the new forks came with radically differnt springs than the originals. They are way softer and great for a kids weight. I am working on making a set of rings for the damper rod pistons hopeing to get them to work with actual fork oil.