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Old 02-23-2011, 12:44 PM   #1
jar944 OP
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KTM RFS/SXF Hybrid

Ok so this isnt the first hybrid KTM RFS motor in a 250-450/505 SXF frame(seel links below) and likely will not be the last, but I felt like posting it anyway.

my 2nd hybrid *may* be the first to use a 250sxf frame

other hybrid build threads
http://www.ktmtalk.com/showthread.php?t=375749
http://www.advrider.com/forums/showthread.php?t=620478
http://www.ktmtalk.com/showthread.php?t=408919
http://www.dirttricks.com/rfs_in_new_frame.htm
ride test
http://www.enduro360.com/2010/09/27/...tm-rfs-hybrid/

This was the first thing I noticed, once I cut the lower engine mounts of the frame (to sit the engine inside.) There was much less plumming clearance than I expected.


dirt tricks got around this by machining a 90 degree adaptor for the t connector (as can be seen below)



The main issue with using the full height RFS motors (450 exc/mxc, 520/525) is the exhaust clearance around the yoke area. I chose to cut and weld in sections of a sacrificial 250sxf engine cradle.









Yoke finished, lower mounts in:






More to come.

jar944 screwed with this post 02-26-2014 at 08:30 AM
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Old 02-23-2011, 12:51 PM   #2
BRYANDENNY71
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Why?
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Old 02-23-2011, 12:59 PM   #3
jar944 OP
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Quote:
Originally Posted by BRYANDENNY71 View Post
Why?
1. RFS bikes handle like a truck. (Terrible)
2. SXF bikes handle very very well
3. Not even a worked 505 can match the HP and Tq of a 600-613cc rfs
4. SXF (RF4) motors dont live long
5. RFS motors do live long.
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Old 02-23-2011, 01:05 PM   #4
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Quote:
Originally Posted by jar944 View Post
1. RFS bikes handle like a truck. (Terrible)
2. SXF bikes handle very very well
3. Not even a worked 505 can match the HP and Tq of a 600-613cc rfs
4. SXF (RF4) motors dont live long
5. RFS motors do live long.


Watching this one closely.

Did you fix the frame somehow for welding or were you not concerned with distortion?
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Old 02-23-2011, 01:09 PM   #5
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Originally Posted by LukasM View Post


Watching this one closely.

Did you fix the frame somehow for welding or were you not concerned with distortion?
I kept the engine bolted in as much as possible during the welding/fitment.
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Old 02-23-2011, 01:38 PM   #6
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lookin pretty good. Have you figured out what makes the new chassis better? I'd like to see the frames side by side.
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Old 04-05-2011, 10:27 AM   #7
BRYANDENNY71
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Quote:
Originally Posted by jar944 View Post
1. RFS bikes handle like a truck. (Terrible)
2. SXF bikes handle very very well
3. Not even a worked 505 can match the HP and Tq of a 600-613cc rfs
4. SXF (RF4) motors dont live long
5. RFS motors do live long.
I understand all of perks of the RFS motor......but the RFS bikes dont handle that bad.

& 600-613cc RFS????? You are boring & stroking the thing too???

My '06 EXC handles pretty damn good. Good enough for me & I'm an ametuer/intermediate class desert/GP racer.

Although I do wish I had the SX suspension on it because then my suspension guy could work major wonders (he's KTM's factory supercross suspension mechanic).

Are you really good enough to benefit from all of this?

With the amount of time & money you've put into it you coulda bought a spare RF4 motor. haha.
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Old 04-05-2011, 11:52 AM   #8
jar944 OP
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Quote:
Originally Posted by BRYANDENNY71 View Post
I understand all of perks of the RFS motor......but the RFS bikes dont handle that bad.
They do not handle nearly as well as the SXF bikes, additionally the pre 05 framed bikes are even worse.

Quote:
Originally Posted by BRYANDENNY71
& 600-613cc RFS????? You are boring & stroking the thing too???
Yes 99mm (+4mm) bore and 78mm (+6mm) stroke

Quote:
Originally Posted by BRYANDENNY71
My '06 EXC handles pretty damn good. Good enough for me & I'm an ametuer/intermediate class desert/GP racer.

Although I do wish I had the SX suspension on it because then my suspension guy could work major wonders (he's KTM's factory supercross suspension mechanic).

Are you really good enough to benefit from all of this?
Yes, If from nothing other than the susp upgrade alone. significantly stiffer frame makes a noticeable difference as well.

Quote:
Originally Posted by BRYANDENNY71
With the amount of time & money you've put into it you coulda bought a spare RF4 motor. haha.
I'm generally good on spares...

Though I have less in the compete chassis than what I paid for my spare RF4 motor.

Also this bikes primary use is hillclimbs (Reasoning behind the 600cc motor) not my race bike, I have a 450sxf and yz250 for that.
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Old 04-05-2011, 01:35 PM   #9
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If its just getting used for hillclimbs....then why all the trouble for a little better suspension & a little more rigid frame?

You dont really need all of the perks if its just for hillclimbs.

You're putting make-up on your butthole. hahaha.

All in all I actually like the concept. Thats why I'm so leary of getting rid of my '06 in lieu of an '11 or '12. I'm thinking of just investing in the SX suspension, getting it dialed & just riding it til it dies. Then rebuild it.
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Old 04-22-2011, 07:00 AM   #10
laramie LC4
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Quote:
Originally Posted by jar944 View Post
Yes 99mm (+4mm) bore and 78mm (+6mm) stroke

who is doing the work?

i can think of a few guys that i would highly recommend, and 1 that i would stay the fuck away from. the list of players isnt very big when talking about an engine like that....

good luck, looks awesome.

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Old 06-03-2011, 11:32 AM   #11
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Quote:
Originally Posted by jar944 View Post
1. RFS bikes handle like a truck. (Terrible)
2. SXF bikes handle very very well
3. Not even a worked 505 can match the HP and Tq of a 600-613cc rfs
4. SXF (RF4) motors dont live long
5. RFS motors do live long.
I'm all for these hybrid bikes as I really like the RFS motors but I would like to point something out because I has just become aware of this.

Your point #4 is invalid for the actual frame you are using. The engine in the 250SXF (or XCF and XCFW) is not an RF4 motor but an RC4. The RF4 (450 and 505) do have issues but for the most part the RC4 motors are some of the best, if not THE best motor KTM has ever made IMHO. Unlike most of the other 250F timebombs, the RC4 motor has an uncharacteristically lax maintenance interval and it is not unusual for racers to go beyond 100 hours on both piston and rings and over 200 hours on valves. Its only Achilles heel is the questionable OEM cam chain tensioner that usually gets replaced with a Dirt Tricks or Dave Hopkins unit. These are great little motors.

Anyway, I'm not criticizing the build or anything, just didn't want people to lump the RC4 motor in the same basket as the (more) problematic RF4 and XC4 motors.

Carry on.




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Old 06-04-2011, 06:45 AM   #12
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Quote:
Originally Posted by GR0NK View Post
I'm all for these hybrid bikes as I really like the RFS motors but I would like to point something out because I has just become aware of this.

Your point #4 is invalid for the actual frame you are using. The engine in the 250SXF (or XCF and XCFW) is not an RF4 motor but an RC4. The RF4 (450 and 505) do have issues but for the most part the RC4 motors are some of the best, if not THE best motor KTM has ever made IMHO. Unlike most of the other 250F timebombs, the RC4 motor has an uncharacteristically lax maintenance interval and it is not unusual for racers to go beyond 100 hours on both piston and rings and over 200 hours on valves. Its only Achilles heel is the questionable OEM cam chain tensioner that usually gets replaced with a Dirt Tricks or Dave Hopkins unit. These are great little motors.

Anyway, I'm not criticizing the build or anything, just didn't want people to lump the RC4 motor in the same basket as the (more) problematic RF4 and XC4 motors.

Carry on.




Sean
I completely agree with you, but I don't think I said the RC4 motors were bad? The original list was specifically for the RF4.

The only reason I'm using a 250 frame (well 2 reasons) is that that are considerably cheaper and this one was (is) plated already. I have another 450sxf frame but it is titled as (off road only) and this build was specifically to be a dual sport

Any of the new style frames should work (provided you add in a new cradle.) So someone could pick up a $150 150sx frame and do the same think if they wanted

also my last post should have read 250xcf frame (you can see the spring mount for the kickstand in the pic)
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Old 06-04-2011, 07:50 AM   #13
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Old 06-06-2011, 11:46 AM   #14
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Quote:
Originally Posted by jar944 View Post
I completely agree with you, but I don't think I said the RC4 motors were bad? The original list was specifically for the RF4.

The only reason I'm using a 250 frame (well 2 reasons) is that that are considerably cheaper and this one was (is) plated already. I have another 450sxf frame but it is titled as (off road only) and this build was specifically to be a dual sport

Any of the new style frames should work (provided you add in a new cradle.) So someone could pick up a $150 150sx frame and do the same think if they wanted

also my last post should have read 250xcf frame (you can see the spring mount for the kickstand in the pic)
Yeah it was just the way the info was presented made it seem like "all" those KTM SX bikes have problem engines.

I just picked up an 07 250 XCFW for my wife and she finished 2nd just yesterday in the women's class, first time on the bike!
I finished 4rth Vet Master on the very same bike in back-to-back races after switching to it because I had "issues" with my bike. Motor aside, these frames handle night and day better than the RFS bikes. Yesterday's track was half MX and half tight woods. The XCFW is about 90-95% what my 200XC can do in the woods but (for me) it was much better on the high-speed outdoor MX style track than the 200XC, and that was with my wife's suspension on the bike. Realizing that the motor plays a big part in the handling, the new generation frames can climb out of ruts much better (like a two-stroke) and somehow do not exhibit the headshaking that some RFS bikes are afflicted with. I never once though it needed a steering dampener other than when knocking trees with the bars but when you're doing that anybody on any bike could benefit from a dampener.

Like yours, our XCFW can be plated for street but short of complete catastrophic engine failure I don't think I could yank out the RC4 and replace it with an RFS. I am building a short-stroke RFS race motor so I may just need to find another frame to put it in.



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Old 02-23-2011, 06:14 PM   #15
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Why?
If you have the skill and desire, why not?

Looks cool, good luck!
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