Four stroke options

[Tex]

I hear that there might be an interesting 4 stroke hybrid at the Beaufort Vinduro tomorrow... 

Tex

[evo550]

1984 ATK 560

[James Lee]

OK OK so I had planned on building a XT550 but a SP370 became available to me aswell as a RM125N rolling chassis.What would be my best choice to fit the SP engine into the RM frame or I was thinking of using the engine cradle from the SP and maybe grafting the RM subframe on and RM front end and swing arm?Thoughts,anyone here done similar?I have no experience with geometry/spring rates etc but will learn as I go but prefer to hear from anyone with experience in this field.

[TT5 Matt]

someone on here from africa did it by making a new engine cradle and the front down tube had to be cut and moved forward. on s.aussie gumtree theres a sp370 motor in a pe175 floater frame for $1200 right now,maybe get the seller to send you some closer pics of the conversion.the dr400 motor is a better choice with more head finning and a stronger gear box along with cdi sparks or just get the whole bike and add rm suspension under it and save the head fork.

[LWC82PE]

Have you read the article in VMX 2 issues ago on 'oldfarts' SP based bike? 1 limiting factor with the stock frame (and most other suzuki 'trail' bikes) is the position of the front sprocket in relationship to the swingarm pivot. It really depends on how wild you want to go with your set up as to what you end up doing though. Eg use another frame, or tone things down a bit and dont aim for 10 or 11" travel each end and instead use stock fame and settle for a little less travel but focus on ensuring what travel you have is the best quality it can be so no elcheapo shocks for example. The Profab swingarms specially built for the stock SP frame  are a little over an inch longer than stock. WB and FMF claimed up to about 9" of travel with these but i did the calculations and they were really stretching it there. It would be really, really picky with shock length/stroke to get what they were quoting. 8" is more realistic. I know someone who races a stocked framed one with 78 RM250C2 fork and he even has those slid up through the triples and front fender scrubs the tyre so technically they are too long. So choose your forks carefully or else you will need to have 4 or 5 inches sticking up above the triples and you can only do that with offet bar type too. He is in the process of fitting a DR400/500 swingarm i believe which is about another 1/2 longer than the Profabs specially made for the stock frame. Theres a few magazine articles out there on modded stock bikes and there's definitely some hard limits as to what you can do with stock frame withut major mods because there's only so much you can angle the swingarm/chain and this affects length/travel forks you can fit to keep the steering angle in the zone where you want it without cutting the frame and changing it.

[evo550]

When it comes to doing a four stroke for racing I'm of the belief that it's much easier to fit the MX models gear to the four stroke chassis then a four stroke motor to a MX chassis.

[Krikey]

When it comes to doing a four stroke for racing I'm of the belief that it's much easier to fit the MX models gear to the four stroke chassis then a four stroke motor to a MX chassis.

Easier no doubt but not as good a result. The MX frame will always be better for MX than a trail bike frame in my humble opinion

[Oldfart]

James ...sorting out swingarm pivot point in
Relation to rear axle and  front sprocket alignment
Is your starting point ....
The Dr 400 frame is a great donour , and will accept
Most Rm parts without too many mods
A  Rm 25o / 400  1980 frame is good also ...but you
Need to drop the cradle and extend the front down tube
As the Dr 400 motor is a tad too tall.
This brings us back to the above ...pivot point cenario

[LWC82PE]

This brings us back to the above ...pivot point cenario

Amusing, that it gets you back to where you started at with the stock frame. I really think it comes down to what you want the bike for and are you chasing trophies, which may require that little extra suspension travel. If not, i would just stick with/settle with the travel limits you are going have with the pivot point-sprocket issue, or just order one of these frame kits from GMC.

[mick25]

Leith and stew have hit this spot on ,
I raced a dr500 for five years just need to limit the travel 8 -9 inches to keep away from the above probs they just have such a tall gear box / front cog height .

Theres a guy James I know that was selling his Ktm 504 rotax engine 1983 model bike ,I think
It was on gum tree a while back I don't think he sold it ? He did want a bit for it around 6k but they are rear and its up there as being the top four beast of this era .
There's No work to be done to it , it won every race he put it in but he is a gun rider aswell

I would buy it if I had the funds .

[evo550]

When it comes to doing a four stroke for racing I'm of the belief that it's much easier to fit the MX models gear to the four stroke chassis then a four stroke motor to a MX chassis.

Easier no doubt but not as good a result. The MX frame will always be better for MX than a trail bike frame in my humble opinion

Only while it is fitted with the original motor, take out the light weight, compact two stroke and fit a tall, huge lump of lead and the whole attitude of the bike changes...and not for the better.

[LWC82PE]

That's a good point. I want to see someone buy one of GMC's frames and build one like the above photo.

There is a good article in Dirt Bike by Rick Seiman where they modded a 370.
Found the one i was thinking of here

http://forum.ozvmx.com/index.php?topic=943.75

[Oldfart]

The back of that bike looks wierd, plus the air filter looks like an after thought .....

[LWC82PE]

I agree with the filter, needs an air box. Rear fender looks a bit short.

[Oldfart]

My thoughts are the same as yours ...
Read article  ....last paragraph  sums it up well