250R Motocross Build, Your Input Wanted

[ATV On Demand]

We are using Chickenhawk's ATC250R as the base for a vintage A/2 Stroke A race project build. We have a lot of the machine figured out but are coming up short on the tire and wheel setup. We are looking to run the Dunlop KT151, 22x7-11 front and most likely an 18xsomethingx8 rear of some sort. We are also considering a 20x11-10 Fast Tracker rear. More ground clearance with the same sidewall as an 18x11-8.

Chickenhawk is concerned that the 250R doesn’t have enough ground clearance to run 18-inch rear tires without framing out, even on flat ground. He tried a set of 18x10-9 Razr MX tires and felt he was framing out on flat landings.

What size and type of rear tires do you run for motocross? Can you flat land with your setup and bottom without framing out?

[Swinger]

I would go by what chickenhawk is saying. Its his ride so he knows it best. Is there anything that can be done to the suspension yet to help this cause or just strickly focusing on tires now?

[ATV On Demand]

We will be addressing the suspension, but when the shocks reach the end of their travel, do most people with 1985/1986 250Rs feel like they feame out on flat ground? We see a lot of people running 18 inch rears.

[Swinger]

I was on a friends a yr ago that had 18s but didnt hit anything huge on it. Some small stuff and no real problems. I was about 170lbs then and would imagine rider weight has a good role in this also. I cant tell you for positive or not on my friends ride because like I said was just messing around and nothing crazy. Its in pieces now or I would have to try it out for ya.

[TatTooL23]

If u r racing then I would say for sure go with 18's. Then if u r serious upgrade the suspension so it corrects the framing out issue. Seems anyone that races seriously always has 18's. So I would go with them and then do what has to be done elsewhere to correct the issue.

[atctim]

Honestly - being a purpose built race bike, start with suspension to correct your framing out issues, then choose your tires. By simply redoing the top rear shock mount, you can retro-fit a 450R shock in there, which is far superior to the 250R shock. I think Chickenhawk could corner so much faster on 18s. I know personally he is used to 20s, and that is fine, actually better for big jumps, but for the serious MXer - corners is where you can shine!

Much as I love watching Ray air his machines out - this saying applies "jump for show, corner for go".

One additional note, 18s on 10" wheels are the very best thing for corners, however, knowing Chickenhawk's ride style - 18s on 9" wheels are a must for him - maybe even on 8" wheels!!!

[ATV On Demand]

Tim, I understand where you are coming from on the 18s allowing for faster cornering. We have built a lot of ATV projects, just never a 250R three wheeler. I would never run more then an 18 for motocross on a quad, unless it was a mud race, or perhaps a sand track with extremely deep ruts.

We are having the shock and the fork rebuilt, revalved and resprung for motocross so it should bottom out far less often. Ten inches of travel is still ten inches of travel though, so the question is do, 250Rs have any ground clearance when the suspension bottoms out with 18in rear tires? Or do you run out of ground clearance before you run out of suspension travel?

[JasonB]

If you find somewhere to buy fast trekkers in a 20x11x10 let me know. I bought a set of 20x10x10 Fast Trekkers and need a 20x11 not a 20x10. On that note, anyone want to buy my 20x10x10 fast trekkers :D


back on track; sounds like you guys have the right idea for ChickenHawks vintage racer, he was really soaring at TF in 2011!!!

[ATV On Demand]

I was sort of guessing on the tire width. I knew they made a 20x??-10.

[atctim]

I guess I am not sure of if it will "frame out" or not. We do however know that it will have 10" travel front and rear. With 18" tires, how much ground clearance does it have (measure this with zero sag). The truth of the matter is goign from 18" to 20" rears will only afford you 1" extra of ground clearance. Thinking about flat landing is one thing, but rarely is there a true 100% flat landing. so your 1" will be easily taken up by unlevel ground - if that makes any sense. Anyway - take those measurements and let us know.

I'm a rather large guy at 270pounds in MX gear - and I have never framed out my 86 ATC250R other than casing jumps. Keep in mind I don't get the air Ray does, but I still can make doubles and large table tops.

One more thing as kind of an FYI - keep a close eye on the bikes backbone (directly under the gas tank) - that is a weak spot on 250Rs and I've seen numerous backbones break there from big air. This should be a frequent inspection point for Ray's safety!

[ATV On Demand]

Right on Tim. Me and Rob have been chatting and he's dialed to take some measurements tonight. He has some low profile 18x10x9 Razr MX tires he is going to install tonight. They are most likely a bit shorter then an 18x??X8. If it appears to be sufficient with those installed, he should be good to go with a 18 on an 8 inch rim.

Rob will definitely keep an eye on the backbone. Having his suspension setup should help that thing live a lot longer.

[rray62]

I tried those razr mx 18's a few weeks back on my track just to get ready for Earlywine. I noticed that on my table, and a few of the smaller jumps that I was purposely overshooting, that the frame was digging into the ground. I put the 20's back on and I couldn't get it to frame out like the 18's. Keep in mind that the tires have just short of 50% tread left. Before we decide on a set of tires, we were just brainstorming about the ground clearance height and the difference between the 18 and 20. I just wanted to make sure before I go try something stupid that I wouldn't pile up because of the height problem. We weren't sure if anyone came across this before or not.

[atctim]

Ray, Just shoot the engine skidplate with some PAM or other non-stick cooking spray - that way when you land it won't matter - it'll be like Clark Griswold on his saucer sled!!!!!

[rray62]

Ray, Just shoot the engine skidplate with some PAM or other non-stick cooking spray - that way when you land it won't matter - it'll be like Clark Griswold on his saucer sled!!!!!

Dude that is great!!!!!!!! You're my new hero, that is top notch engineering :lol:

[ATV On Demand]

Okay, I am taking your guys advice and we will run the 18s.

Recently we tested the new GBC Bomb Squad motocross tires on the site. I remember thinking how well they would work on a three wheeler during our testing at higher pressures, 9-10 pounds, where they slid very well.

We are getting a couple of grooved sets from GBC for testing on quads, so I think we are going to take our almost brand new set of ungrooved 18x10-8s and try them on Robs ATC.

I guess we will all find out how well they wrok in a couple of months. Here is a link to the test video if you are interested. http://atvondemand.com/news/?p=3782

[Mosh]

Right on Tim. Me and Rob have been chatting and he's dialed to take some measurements tonight. He has some low profile 18x10x9 Razr MX tires he is going to install tonight. They are most likely a bit shorter then an 18x??X8. If it appears to be sufficient with those installed, he should be good to go with a 18 on an 8 inch rim.

Rob will definitely keep an eye on the backbone. Having his suspension setup should help that thing live a lot longer.
I Run MXR 6 Holeshots 18x8 on a 250R..
I dunno about the Razr's but the MXR's are very clingy/sticky..It is near impossible to slide them in the corners on most surfaces. I work around it. I would rather have bite coming out of the gate and corners anyway. I have flatlanded out of 6-8 feet before and never dug the frame in..Came close though.Got a pic somewhere of that hehe..
I have never had any clearance issues as far as ruts, but I prepare for the hi spots while riding.

[poohbee1]

Have him try some turf savers or turf tamers 18x9x8 or some hoosier 18x9x10 the old tri tracs . I promise he will like them better than most quad tires . The tri tracs are my favorite i tried fast trekkers and they dont work very well but they look good.

[ATV On Demand]

Mosh, the first time I tried a set of ITP MXR 6 tires on my old motocross Banshee, I couldn't hardly break the rear tires loose. I wouldn't think they would be ideal for cornering. The reasons we are considering these is because they have a harder rubber compound them most of today’s modern MX tires. They broke loose easily when we ran higher tire pressures. At 9-10 pounds, the rear tires slid too easily to make good time on a four-wheeler. When we lowered the pressure to 7 pounds they hooked up a lot better in the corners. With that said, they may work quite well on a three-wheeler at 9-10 pounds. Small air pressure changes seemed to make a big difference on how well they hooked up.

I would like to see if they make a good three-wheeler tire. Might as well take advantage of the opportunity to test something new.

Pooh, if they don’t work out, we were considering testing the Duro HF 245 we have seen mentioned elsewhere, on this site I think. We thought they would work great on dry hard tracks.