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Thread: Race chassis

  1. #1336
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    Quote Originally Posted by Grumph View Post
    This is one of my engine customers. While I'd love to talk to Rossi, I think I got the gist of what it feels like from this.
    Yes, I see what you're up against Grumph.

  2. #1337
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    6th February 2012 - 08:54
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    I bought a tube bender

    I want to try this geometry

    Quote Originally Posted by Frits Overmars View Post
    Sure you can. Here are some racewinning dimensions.
    wheelbase 1200 mm
    fork yokes offset 0 (yes, zero) mm
    rake angle 16
    trail 90 mm
    Concentrate all masses so that the center of gravity will be directly in front of your knees.
    Waiting for the 125 nsr engine that arrives

  3. #1338
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    Some pictures from a Freetech project by Dutchman Robin Wittebrood. He didn't buy a tube bender .
    In case you're wondering why you can see right through the engine: it will get reed valves right and left.
    Robin Wittebrood 2018-05.jpg Robin Wittebrood 2018-01.jpg Robin Wittebrood 2018-02.jpg
    Robin is using very strong duct tape: Dutch tape. But he is considering welding the tube joints as well .

  4. #1339
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    Lot of work has gone into the fabrication combining engine/shock/swingarm mounts.

    I'd wonder if it isn't quite a bit heavier than Scott's GPR frames where those functions are served by tubes.
    Reason is a tool - remember where you left it..... The late, great, John Clarke

  5. #1340
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    Quote Originally Posted by Grumph View Post
    Lot of work has gone into the fabrication combining engine/shock/swingarm mounts.
    I'd wonder if it isn't quite a bit heavier than Scott's GPR frames where those functions are served by tubes.
    Maybe so, Grumph.
    I liked the straight tubes, though I think the frame could have been even simpler and lighter if the rear suspension had been designed without a link system.

  6. #1341
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    Quote Originally Posted by Frits Overmars View Post
    Maybe so, Grumph.
    I liked the straight tubes, though I think the frame could have been even simpler and lighter if the rear suspension had been designed without a link system.
    Agreed - but so often you've got to use what is available - and affordable. If he's sourced an arm, shock and linkages as an affordable package it's understandable.
    I do think that rear suspensions are too often overcomplicated. A friend has an ATK offroader which he's run for years now. He's also one of the top local suspension unit rebuilders/revalvers. He's often heard muttering about overcomplcated valving to compensate for poor linkage curves - and why can't they all be as simple as his ATK.....
    Reason is a tool - remember where you left it..... The late, great, John Clarke

  7. #1342
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    The curves I've seen published for modern-ish RR rear suspensions seem very linear and within what is possible with a conventional non-linkage suspension. Linkages seem like something you add if you need them for a specific reason such as packaging/locating the damper in a specific spot where you need the link for it to work correctly for the application. Long travel dirt bikes or big tourers where a large rising rate might be wanted seem like a good application for linkages. My guess is that on RR applications mass centralization is being given a higher priority, so in order to move the damper to a needed location they need a linkage.

    Crichton deliberately used twin rear dampers on the Roton and it seemed to work OK. I've always like the looks of the Simon Martin (NWS) bike with a single rear damper -- put it on the side away from the chain and you can have a large cross section on the swing arm:



    cheers,
    Michael

  8. #1343
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    I don't disagree with any of your conclusions Michael. The Roton though had it's own unique packaging problem I believe. According to reports at the time the Spondon single shock frame suffered from shock overheating due to the very high temps close to the engine.
    Reason is a tool - remember where you left it..... The late, great, John Clarke

  9. #1344
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    Quote Originally Posted by Grumph View Post
    Agreed - but so often you've got to use what is available - and affordable. If he's sourced an arm, shock and linkages as an affordable package it's understandable.
    I do think that rear suspensions are too often overcomplicated. A friend has an ATK offroader which he's run for years now. He's also one of the top local suspension unit rebuilders/revalvers. He's often heard muttering about overcomplcated valving to compensate for poor linkage curves - and why can't they all be as simple as his ATK.....
    Husaberg a and KTM with a few minor exceptions all are, they call it PDS. although i think this more applies to the shock nedding to have progssive damping characterstics
    to appease the Supercross and MX markets they have had to go back to linkage of them for the same feel for the riders.

    From what i understand the linkage rates on GP bikes became practically linear in the mid nineties.
    Quote Originally Posted by Frits Overmars View Post
    Maybe so, Grumph.
    Quote Originally Posted by Frits Overmars View Post
    I liked the straight tubes, though I think the frame could have been even simpler and lighter if the rear suspension had been designed without a link system.

    That frame could have had the rear box cast in alloy (it still could)
    Frits could encourage him to whip off a few patterns off it because he glues it together with weld.
    Then he could run off as many copies real easily.


    Kinky is using a feather. Perverted is using the whole chicken

  10. #1345
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    Quote Originally Posted by Frits Overmars View Post
    Some pictures from a Freetech project by Dutchman Robin Wittebrood. He didn't buy a tube bender .
    In case you're wondering why you can see right through the engine: it will get reed valves right and left.
    Robin Wittebrood 2018-05.jpg Robin Wittebrood 2018-01.jpg Robin Wittebrood 2018-02.jpg
    Robin is using very strong duct tape: Dutch tape. But he is considering welding the tube joints as well .
    It's a good approach. Not that I would do it that way, but he comes close. It's motorcycle suspension, not rider suspension as with conventional twin shocks.

  11. #1346
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    Quote Originally Posted by Michael Moore View Post
    The curves I've seen published for modern-ish RR rear suspensions seem very linear and within what is possible with a conventional non-linkage suspension.
    That's right Michael.
    Without a link system, just by choosing the proper geometry, you can achieve a maximum of about 15% progressiveness, which is sufficient for road racing.

    Crichton delerately used twin rear dampers on the Roton and it seemed to work OK.
    When I was working on the suspension of the Garelli 125-single, which initially had the same twin-shock setup as the unbeatable 125-twins, it turned out that on the lighter single the gas pressure in the twin shocks alone already gave too much pre-tension. The twin shocks also had too much plunger rod friction.
    This was the reason to adopt the mono-shock system, internally nicknamed RON (Jan Thiel may remember it as the abbreviation of "Recht Op en Neer").

  12. #1347
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    KTM persevered with PDS for like a decade but when even their factory bikes had a link it was clear. Must have been like Yamaha leaving 5v heads behind.

    Messed around on my old 50 frame stripped out the linkage and lost a heap of weight. I'd also been inspired by the old ATKs. Played around with Springs and damping. Main problem was it twisted the frame up despite triangular bracing and stiff ally sw.

    NF4 handling was miles better. But clearly suspension was behind others like GPR over stutter bumps.
    I've been told. Dreaming`s free.
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  13. #1348
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    20 years ago I had a Kawasaki EX250 race project. I converted it to dual dampers and cut all the heavy linkage stuff off the swing arm and boxed it in with light sheet metal. I don't recall the exact number of pounds that saved, but it was lighter, and it made room under the swing arm/behind the engine for tucking the megaphones up and out of the way.







    Those 16" rims sure limited the tire choice.

    If you've got an engine with side-draft intake not having the damper right behind the engine can free up room for a large air box.

    cheers,
    Michael

  14. #1349
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    Can' help but think that class would have been so much better with say TZR250 parallel twin engine.
    I've been told. Dreaming`s free.
    Think I'll go, back to sleep.
    Everybody listen, voices in my head
    Everybody listen, do yours say, what mine says?

  15. #1350
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    Quote Originally Posted by Michael Moore View Post
    20 years ago I had a Kawasaki EX250 race project. I converted it to dual dampers and cut all the heavy linkage stuff off the swing arm and boxed it in with light sheet metal. I don't recall the exact number of pounds that saved, but it was lighter, and it made room under the swing arm/behind the engine for tucking the megaphones up and out of the way.

    Those 16" rims sure limited the tire choice.

    If you've got an engine with side-draft intake not having the damper right behind the engine can free up room for a large air box.

    cheers,
    Michael
    Quote Originally Posted by F5 Dave View Post
    Can' help but think that class would have been so much better with say TZR250 parallel twin engine.
    Bear in mind Dave that CAMS down here has a 150/2T 250/4T class. Given that the 250 production Kawasakis are now obsolete by virtue of the 300's being adopted....I hope Michael's pics inspire someone.
    Ah 16's....My GPZ500 F3 bike went onto 17's when it was transformed from a road bike. But it was raced for most of a season on 16's - Dunlop 591R's.
    Not bad - but if you hit 7000 while cranked over, it went sideways....I'd got quite a lot more HP out of it while keeping stock cams/carbs but it was all up top.
    When it was done properly with big cams and flatslides, it was actually much more pleasant to ride.
    Reason is a tool - remember where you left it..... The late, great, John Clarke

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