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700cc twostroke single in RS250

Discussion in '2-Stroke Machines' started by wmcdonal, Jul 20, 2005.

  1. wmcdonal

    wmcdonal New Member

    I am in the process of building a could of bikes for club races and ride days.
    One is a 2000 TZ250 with a CR500 motor, the other is a 2002 Honda RS250 with a Zabel 700cc two stroke single.
    I am worried about the vibration that these motors will have as they don't have a balance shaft.
    I see a few people on this list have put the CR500 into a GP chassis and was wondering if they could share some details on what it is like to ride and what they did to quell the vibrations
    :wow:
     
  2. RB

    RB Well-Known Member

    The one guy I know of spent most of a season DNFing from cracked pipes and other vibration related problems. I seem to remember him having all sorts of trouble actually making good power at max rpm.

    Use a Rotax model 257 250cc shifter kart / ATK motor. 6 sp, balance shaft, 51 hp stock. Thrives on revs, something no big bore 2s does.
     
  3. wmcdonal

    wmcdonal New Member

    To address the power issue I am altering the pipe and port timing. The operating range of the motocross pipe is not what you want for RR use.
    The cradle for the motor has rubber mounts where it connects to the frame, these are like the rubber mounts in old car shocks.
    The pipe should be ok if it is rubber mounted (I hope).
     
  4. sunshine

    sunshine Well-Known Member

    Just make sure that those rubber mounts don't distort too much under load. It would be no fun to wad a chain up after it derails at WOT....

    Sunshine
     
  5. 418

    418 Expert #59

    After tirelessly researching this issue, I came to a realization that it's just not worth putting anything bigger than a 250cc single in any RR chassis.

    Stories of shit vibrating apart run rampant.
     
  6. No Brakes

    No Brakes Well-Known Member

  7. wmcdonal

    wmcdonal New Member

    I know that most people say it will vibrate itself to bits but I don't seem to be able to find anyone who has done it.
    Perhaps the vibration was so bad they can no longer type on a keyboard ;)
    The rear of the motor is a rigid mount, this is to make sure the chain runs true.
    The rubber mounts I am using have a static load capacity of 100kg per mount and only deflect .5 to 1mm.
    The mounts are going between the engine cradled and frame.
    I hope that the cradle will absorb some of the vibration and the rubber mounts some more.
    I know that CR500's are used in desert races where they are held at WOT for long periods at high speed. I assume that they have done something to combat the vibration.
    I have put a YZ450F motor in a TZ250 but it blew up first time out and I have not had the guts to pull it apart to assess the damage.
    The big twostroke can take lots of abuse, I like that.:clap:
     
  8. tm316

    tm316 Well-Known Member

    I'm in the same conundrum right now, I've got an awesome custom fabricated chassis, putting the motor together right now. I spoke with Eric Gorr, of CR500 fame, he told me that vibration can be reduced on a Cr500 motor, but only in one particular engine power setting or engine speed. In other words you can make it relatively smooth at high engine speeds, but it will shake and vibrate at lower engine speeds. He recommended "Crank Works"/ www.crankworks.com , said they have a lot of experience with desert racers. I have tried emailing, but no reply yet, I may give them a call today. Eric said they will probably recommend trueing/balancing and welding the crank. Keep me posted on how you are coming along. I've been traveling quite a bit on business this summer, haven't had a lot of time to work on this project. Send me some pictures of your project if you have any. [email protected]
     
  9. E=MC2

    E=MC2 Well-Known Member

    there was on guy who stuck a 426 motor in an rs50, he sold it for like ~$4,600 on ebay.
     
  10. 999

    999 Well-Known Member

    Last edited: Jul 27, 2005
  11. RB

    RB Well-Known Member

    Would someone please figure out the piston speed of a CR500 motor doing 9500 rpm? I sold my copy of "Two Stroke Tuners Handbook" and don't have the formula anymore. Use English unit of measure please :p
     
  12. 999

    999 Well-Known Member

    It looks like 79mm is the stroke of most CR500 motors. So multiply that by 2 (to get mm traveled for a complete revolution).

    Now convert mm to feet (mm/25.4 equals inches) (inches divided by 12 equals feet).
    Multiply that by the rpm and you have feet per second.

    A shorter way is to take stroke (in mm) times rpm divided by 152.4

    It's about 4924 fpm, which seems awful high to me for a two stroke.
     
  13. tm316

    tm316 Well-Known Member

  14. RB

    RB Well-Known Member

    When I did it I got 4904.

    All I have to say is " No way".

    Tuluie is widely regarded as a genius but everything I have seen him do has been fatally flawed. Flaws that could easily have been forecast while in the design stage. The Suzuki LT500 quadzilla motor has a counterbalancer. You still have the problem of excess piston growth that any 500cc single 2s is going to have under extended WFO running. One way to combat this is to port the back of the piston just below the ring so that the rear boost port is fed from just under the piston crown.

    On his snowmobile powered bike it would have been very simple to design in a counterbalance system since he already had to add a tranny and clutch. The vibration actually scared most everyone who rode it.

    You cannot rid a non-counterbalanced single of vibration by balancing the crank for a certain rpm range. If you use the flyweights on the crank to offset piston forces then you have a fore and aft couple created by the flyweights. If you use the flyweights to offset the fore and aft forces then you have an up and down couple created by the piston. The bigger the motor the worse the problems . Even small motors can be a problem. The early RS125's didn't have counterbalancers and were notorious for cracking pipes.
     
  15. zane

    zane Active Member

    not sure why you guys say you cant ride the big-bore bikes because of vibration.
    my STREET LEGAL CR500 was as nice as any 4-stroke ive ever rode at 60mph. bike was SMOOTH and i loved the full suspension travel since it was so nice a ride.
    hell ive ridden stock CR500s and they where not that bad. i would imagine the stiffer alum. frame should/could soak up the vibration very nicely.
    im sure ya could also figure out a nice rubber mount for the engine if you really wanted to do it up right. :cool:
     

    Attached Files:

  16. gpracer15

    gpracer15 Built to Ride

    cr500

    I put a Cr500 motor into a Tz250 chassis. I found a guy in NC that custom makes pipes but have misplaced that info. I aslo talked to CrankWorks and the assured me they could balance the motor for Road Race conditions. Also you need to have the wide ratio tranny found in 87 and 88 models or the desert kit.

    I had planned on letting Crank Works balance the motor and adding the countershaft weight as well as putting on a lighting could and making and race/street bike for fun.


    I sold the project before completion
     
  17. bert_chenin

    bert_chenin Member

    Back in 2001, I built a similar proto. A hybrid with a Honda Hawk frame and a KX500 engine.

    Vibrations were a nightmare.

    A typical track test would be:

    30mn. to get the dam' thing to start. Because of frame-motor interference, the bike had to get push-started. These single MX engines have so much compression that it is next to impossible to get the engine to crank over. Even with a buddy pushing it was next to impossible to start.

    5mn. riding. The bike would self destruct right-away. Every nuts or bolts would come off, even with safety wires. If they did not come off then they would break. When I replaced them with grade-5 bolts, then the parts themselves would break.

    The expansion chamber would crack from every possible seam. I probably spent more money of brazing rods than I did on gas. The frame cracked and so on and on and on.....

    A friend used to come with me and could not stop laughing. He was watching in disbelief all the bits and pieces flying-off the bike. He would waive at me, but I would not stop as I knew that it would not start again and/or that he would not push it again for half an hour.

    Even the rear shock (nice Penske) came to pieces. I got lucky not to crash that as the rear tire got covered in oil.

    In any case 5mn running time was way too long already. The vibrations in the hands and pegs were unbearable. My hands would get completly numb.

    A good idea on paper but with significant engineering challenges. Not impossible to overcome but not easy.

    If I had a TZ250 frame, I'd put a......TZ motor in it. A well set-up TZ can be devastating on the track.

    If I had an RS250 frame, I'd put a 4-stroke single (with balance shaft). The RS motor is dog slow.

    Good luck. I am sure there is a solutuion to these vibration problems. I just go tired of it.
     
  18. Alex_V

    Alex_V Dump the diesel

    :D Now that was funny. And why did you have to do that to a perfectly good hawk.
     
  19. bert_chenin

    bert_chenin Member

    Alex,

    Well....., it really looked like a good idea at the time. It was very powerful and very light. Too bad the surgery had very bad side effects.

    It's a shame really to have made a Frankenstein out of that nice Hawk. It served me well the season before as I won the CMRA Middleweight Sportsman in 2000 with that good ol' hawk. Some people just have no respect.

    I have a few pix at home. I'll post them sometimes in the next few days.
     
  20. ex419

    ex419 Well-Known Member

    I had a kx500 supermotard... Out of the three races I entered the motor only held together for two. The first (fresh, all stock) motor went in practice at Barber. The crank came apart, ruined the piston and cylinder, and forced everything out the bottom of the cases. The second died at Nashville, but it didn't hurt the wallet as bad. Finally at Talladegah it made it through a race. The power was grrreeeaaat.... it was just expensive. With a single piston moving that fast, for that long, they just don't last.
     

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