Choosing a frame for a hybrid

[kd5sfq]

I'm thinking about building a CRF250R or CRF250X hybrid for my son to race. Before rushing out to buy everything, I would like some input from everyone about what choice is best for the project.

In our shop we have on hand a 300ex, a 400ex, a Z400 and a 250R frame. Am I corect in thinking that all but the 300ex are loosely based on the same geometry?

So given the choices what would you choose and why?

[Plante400]

400ex or 250.. those are the best ideas. if u have ridden both. what were u more comfortable on.

[K_Fulk]

z 400 or 250r would be easier since they already have radiator mounts.

250r would probly be better for somone smaller. Lower cener of gravity too.

[punker69q]

How tall he is? If he is small, maybe a 300ex frame would be better for him, but it's not the best geometry to build a race quad. Maybe you should make him try different bikes to see on which he is the most confortable.

[deathman53]

I don't crf 250,450 motors will fit in 250r's, the motors are too tall.

[K_Fulk]

You could reform the tank to fit around the head cover if there too tall.

[BLACKeR]

you can fit a YZ 426 in an R frame, not the new 450's but i would think the 250's would probly be short enough if the 426 is...the R frame is the best geometry, just be sure to gusset it its also in stock form going to be one of the weaker. if its gusseted it should be fine.

[Smoker]

300ex frames are wicked heavy. I would base it around a 250R frame, being a low slung quad it would probably be the best fit for your boy. Yamaha and Honda really need to make quads based on these motors, it would be a great replacement for the blaster class.

[hardkoratvmxr]

use the 250r

[nacs400ex]

Originally posted by deathman53
I don't crf 250,450 motors will fit in 250r's, the motors are too tall.

Well they make 250x conversions and 350X conversions for 250r's. Im pretty sure the crf250 motor will fit. Some modifications would have to be done to fit the crf450r motor in. Mainly with the gastank.

[FMFTRX250r]

why are you going to do this hybrid? there is no class for your son to race in....basically your building a Mojave style machine which isnt race at the MX nationals at all....its way too small for 250 classes and its liquid cooled so it cant be used in 300 and under classes....if i was you id build a 300RX and call it a day....its been done a hundred times and parts are available and the people out there to do the frame for you....just my .02 cents..

[Smoker]

If he plans on racing nationals, yes, a disadvantage, but if you want to build the kid a kick @ss quad to learn on and race local, go for it, your kid will very happy. I think eventually Yamaha and Honda need to develop these size machines and make a national class to replace the 300ex, Blaster class.


Scroll down for graphs, couldn't get link to work properly (http://www.geocities.com/gtthunder.geo/testbed_set.html)

I realize you are using a CRF motor, here is a comparison between the two:

Comparison (http://www.thumperfaq.com/mxa_4stroke_shootout.htm)