Trials Bike Build/conversion?

Discussion in 'Trials' started by rizzer, Jun 23, 2011.

  1. motobene

    motobene Motoing for 49 years Supporter

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    I'd keep the balancer in because the cases wouldn't get any smaller, and why buzz more when revving out? A balancer is worth the little bit of added weight.

    The typical Chinese Honda-copy engine is single overhead cam, not push rod. If there was a simple push-rod 4-stroke single engine that was minimalist, compact, light, and had a great clutch and the right gear ratios, I'd be all over it. Four stroke simplifies the exhaust, so packaging for a custom frame is easier. But all those small four strokes tend to be on the chunky side. I've never weighed one. Anyone weighed a Honda 200 engine?

    And when it comes to air cooled, I'd rather have a 2 stroke. The air-cooled 4 strokes tend to run SO hot, and super heated engine oil negatively affects clutch performance and precludes use of gear-clutch-specific oils in models that share the oil with the clutch - which is the case with pretty much every smaller-size engine.
  2. motobene

    motobene Motoing for 49 years Supporter

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    If by 'old' you mean an old twin shocker, I'd end up changing SO much, as so many things have evolved since then.

    If by 'old' you mean something more modern but used, yeah. That's give me many parts I'd need. to wrap a custom frame around a Blaster engine, or some other cool engine.

    I've really liked the Hodakas for their shrunken ball-lock gearbox, but 90 to 125cc is too small. And the Hodaka clutch, which is on the crank shaft, isn't a better clutch. Their 250 has a 'normal' clutch the 250 but is so rare I doubt I'd ever find one.
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  3. motobene

    motobene Motoing for 49 years Supporter

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    Cool. Sprocket Specialists will make any rear sprocket you want (until the Chinese put them out of business too :-0). Front sprockets would be available so that's no issue.
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  4. motobene

    motobene Motoing for 49 years Supporter

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    Aha, perfect!

    Something like 52/12 or 50/11 would tighten up the feel lower-gears spread.

    The tune I'd want is bottom-mid emphasis and using crank/flywheel inertia for clutch dumps at high rpm, typical to trials. Could lower the cylinder a smidge, which would up lower ports height relative to the piston and up the compression. There has to be enough on bottom and mid, and from what I can see of Blaster no-squid vids there is enough. Some added flywheel moment of inertia would round things out nicely.
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  5. Brewtus

    Brewtus Buffoonery, Inc.

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    I gotta admit to trolling sales sites for derelict Blasters over the past couple of days in addition to trolling for derelict GasGas 50s. It's not like I don't have other bikes to build or anything..... :bluduh
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  6. Achro

    Achro Adventurer

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    Motobene, seems like we've had a lot of the same ideas. I think a twinshocker would be more realistic for a home build. It still allows for the geometry to create some progressive rates with decent ground clearance. Good enough for me.

    I've also looked for alternatives to the blaster engine and honestly can't find anything as simple, light, with workable gear ratios, or with as many parts available (they were produced for 15 years!!). Anyway, next step is to figure out how the blaster's exhaust would work in a trials frame... Here are some pics for inspiration.

    [​IMG]
    [​IMG] [​IMG]
  7. motobene

    motobene Motoing for 49 years Supporter

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    Cool pix, Achro! On the green thing a machined head. I once machined a head for a DT1. It was a lot of work! I stupidly let it go when I sold the bike. It would have made great shelf art for four decades!

    I'd not sweat the exhaust too much as for imitating the same length versus cone diameter. Could even be a straight header into a center muffler.

    The Blaster's stock exhaust has a lot of volume, which is good, and most of it is way up front, which is good for packaging except for around the steering head. Most up front leaves room to do whatever in the middle and back of a trials machine, other than where to hang a the silencer. Maybe that stock exhaust can be reworked to be more compact? Or parts of it used?

    Regardless, due to air cooled and lower speeds, I'd probably wrap the exhaust in ceramic tape to knock down radiation and leg burns.
  8. Luke

    Luke GPoET&P

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    So if you want serious engine mods, I just put my first real ride on a new motor today.
    The exhaust port is 3mm lower than stock, transfer ports are roughly original. And the crank was replaced with one with stuffer blocks for higher crankcase compression. It has the same pipe and similarly chambered head to my old motor.

    The changes were totally worth the effort. A much better spread of torque- for trails it meant a lot less less shifting. It did drop top speed from 50 to 40.

    For a flywheel weight, I have an 11oz band which is as much as I could fit under the cover. The cover is ABS; so two covers, a saw, and some glue and you could fit as much as you want in there.
  9. motobene

    motobene Motoing for 49 years Supporter

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    Luke, that's cool!

    I have some questions. The answers will help me and others actually fit this increasingly desirable engine to a custom trials bike. It's always more interesting to me when mods go in a non dust-and-mayhem direction, as that shows restraint and maturity, especially in an ATV setting! Is your modded engine application in an ATV or a custom trials bike?

    "The exhaust port is 3mm lower than stock, transfer ports are roughly original."

    Is that accomplished by NOT using a piston with an exhaust port 'lowering' cutout. The following is from a sub $20 Chinese piston kit. Is the OE piston the same way. If so that is a VERY good thing as one can use a more normal piston choice to effectively 'lower' the exhaust port. I'm liking the flexibility of this engine more and more!

    Blaster Piston With Exhaust Timing Lowering.JPG

    (Note the 65.94 on top).

    I found this Wiseco piston, and this may be how you did it. The critical dimension is between the red arrows, but given the squish spec is the squish spec regardless of the piston, and that is set by the wrist-pin-to-crown dimension between the red arrows, and the position of the piston rings on the piston may mislead regarding crown height...:

    Wisco Blaster Piston.JPG
    The only other way to lower and exhaust port and keep transfer ports the same in a sleeved design is to re sleeve and not grind the top of the exhaust port up to the original height, unless I missed something?

    I did find this video, and see no cutaway in the piston the rider melted. Are both Wiseco pistons?



    "I have an 11oz band which is as much as I could fit under the cover."

    Is the band a steel ring you heated up then froze the flywheel to drop the band over for shrink fit to the flywheel? There's enough space to do that with the stock cover? Again, I'm liking this engine more and more!
  10. motobene

    motobene Motoing for 49 years Supporter

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    For those considering the Blaster engine for a custom trials bike, I found a couple of videos that show the engine internals in detail. The counterbalancer is in a bulge behind and below the cylinder, so it's presence may not affect the engine mounts and thus possibly could be same as the AT through DT plus TY and some IT and YZ bikes.

    Bottom Guts:


    It looks like we could do a pretty clean oil injector delete, and cut everything related to it off the primary cover. That would narrow the engine in the front, but perhaps make the bulge for the clutch more of a vulnerable wall. Welding a triangle or two on the front of the clutch bulge would protect the case from hits enough while still slimming things down.

    The biggest negative I see about this engine is the width of the engine on the clutch side. Compared to a typical trials engine, the Blaster is wider for several reasons:

    - The clutch is smaller diameter 5-springs format, requiring it to be longer for torque capacity

    - The clutch has springs in the primary gear where it meshes with the clutch basket for rotational shock absorption (see 22:33)

    - The room required for the gears for the counterbalancer steps the primary gears and clutch out (see 23:24)

    Modifications to remove the counterbalancer, toss the bearings and gears, then move the primary gears inward to suck the clutch in, then modify the primary cover would be very extensive and likely not possible short of EDM magic given the shafts and gears are hardened. Losing the counterbalance and supporting hardware would also reduce engine inertia, requiring more added flywheel weight to compensate for the loss.

    So wide and heavier it is! But how heavy? I wonder if this engine would be as heavy as a 200 Honda 4 stroke or copy of the same?

    Note the use of helical primary gears. Yamaha used helical-cut gears from the `60s. These are harder to machine but superior to straight-cut gears regarding noise. No chatter when the clutch is pulled in.

    Top Guts:
  11. motobene

    motobene Motoing for 49 years Supporter

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    Again, the 1984/85 KDX engine would be a good candidate. Narrower and probably lighter than the Blaster engine, but there isn't as extensive a parts base:

  12. MATTY

    MATTY BORDER RAIDER

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    Proportionally and overall i think i prefer the KDX engine to the blaster, over here they look cheaper too the blasters are in complete ATVs and seem expensive so putting them out of my reach.
    The fourstroke hondas / clone engines, some are push rods cg 125 copy lifan for one do them i have \a few here two bikes and in an atv , the OHC ones are Zongchen and look sort of YAMAHA XT/SR 125 clones . Personally i like the push rod motors the rumour mill has it the china CGs are not just as tough as the genuine Honda CGs push rod engine, but my kids tried to kill a 150cc ATV with a push rod clone , and in 7 years it gave no issues in that ATV only cheap nasty wiring let it down more or less from the OFF, but a half day making up a good loom from wire and some conections from a scrap ford Mondeo loom had it reliable to this day i like them and think they would cope well in a trials bike and got decent clutch too.
  13. Luke

    Luke GPoET&P

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    It's in a trail bike, a '99 YZ125. I made a build thread.

    Point-by-point:

    The exhaust port was lowered by milling 3.5mm off the bottom of the cylinder. A base gasket was added to raise everything back up 0.5mm. Then the transfer ports were cut up 3mm to raise them back up to their original height. The piston was then proud of the cylinder at TDC so I had to cut a cylinder head with a recessed chamber to fit it.

    The cutaway pistons are no good for this. The cutout effectively increases the exhaust port height, lowering torque. So I haven't tried them. I've read that they're very useful for experimenting; put a modified piston in to simulate a raised port and if performance gets worse put the stock piston back in. If it gets better, raise the port a little more. It saves throwing away over-cut cylinders.

    All the pistons I've measured had the same pin-crown distance. The stock blaster heads have no squish band to speak of, so I set the clearance I wanted when I cut the head. I haven't had a piston fail, so don't know if my heads need to be matched to their piston or not.

    The FWW is a band, press fit over the stock flywheel. There is 4 or 5mm of clearance between the flywheel and case, so I used up 3 and a bit of it. The plastic cover needs to be ground out a bit to fit. It's still nothing compared to the flywheel of a trials bike. If you're willing to clearance the case and make it out of brass it should be easy to get a full pound in there.
  14. Luke

    Luke GPoET&P

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    I weighed the Blaster motor at 50 pounds, I haven't weighed an XR200 motor but online claims are that it is 60ish. After lifting both I'd say that's about right.
  15. motobene

    motobene Motoing for 49 years Supporter

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    This is a goldmine of info, thanks. 50 pounds. I wonder what my Sherco engine assembly weighs?

    I read your thread, Luke, and need a big award for total lack of bashfulness in modding things! You don't get hung up on appearances and just dive into mods with whatever tool you have! Making your own pipes and fuel tank and machining a head with a dinky little lathe... impressive!

    You should make a custom Blaster-based trials bike Luke! Trials is a different mentality and different-machine world. Much more minimalist and space challenged. But you are already there on the type of power required.

    I hope the OP isn't thinking there has been a hijack of this thread toward one engine :-0 This is just one little compartment I like to think around in, and the likelihood of this ever happening is low. To many projects sit in front and not enough life. I already have an awesome ACM bike, my Fantic, so IF I do anything it should be a twin shock and just for the challenge of making my own frame and other bits... like a fuel tank!
  16. motobene

    motobene Motoing for 49 years Supporter

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    Here is a good choice for twin-type shocks from the UK:

    https://www.rockshocks.co.uk/



    Very simple design! So simple it blows this suspension guy's mind. No nitrogen at all, not even in emulsion form. In fact, their idea of bleeding a shock is to loosen a screw and fully compress the shock until oil comes out, then tighten the screw. That means not only do they not have any nitrogen, it's just air and at NEGATIVE pressure when fit with springs. You'd think they'd be cavitating like crazy.

    I could see fitting a Schrader and running some nitrogen in emulsion, and choosing a softer spring. But they've been at this a long time so they probably know something about their shocks I don't. Quality stuff and as a result, not cheap.

    http://www.thehellteam.com/rockshocks.html
  17. Luke

    Luke GPoET&P

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    Thanks!

    I did give a trials bike some thought. I would likely go vintage with a blaster build, mostly because I already have a modern bike. It'd be down there on priorities though. I don't ride so much trials that I need another bike.
  18. motobene

    motobene Motoing for 49 years Supporter

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    Without someone else to ride with you'd be the lone alien:hack Could be infectious, though.

    Trials gives you a very different way of looking at terrain. If you have a lot of rough terrain available (and it's not buried a jungle), you don't have to migrate far and wide to get satisfaction while blindly blowing past all the good stuff :-)
  19. Luke

    Luke GPoET&P

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    I should have been more clear... I already have one modern trials bike, I don't get tired of it so don't need a second, slightly different, one.
  20. Luke

    Luke GPoET&P

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    Vaguely on topic... To make a 2 stroke motor work for trials, copy a trials motor.

    So copy the port timings and the pipe tune, and put a small carburetor on it. I bet a lot of old MX bikes could be made to work.