Trials Bike Build/conversion?

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

  1. motomofo

    motomofo Been here awhile

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    There was a guy who showed up to a few state series events here in TX a few years back who used a Montesa 315R chassis, converted it to twin shocks, machined his own brake drums and shoehorned a Yamaha Blaster motor into it. It was an interesting machine, and the coolest part was he claimed that he could easily bolt all of the original parts back up and have a regular, stock 315R again.
    MATTY, jonnyc21 and Brewtus like this.
  2. motobene

    motobene Motoing for 49 years Supporter

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    Cool! Liking the Blaster option better already.

    Yup, the mods are usually oriented to dust, noise, and mayhem crowd. The Blaster looks like it is just basic 2 stroke, no power valve or hopped up BS.

    I think the `84 and `85 Kawasaki KDX 200 motor would also be great, as it's faily compact, has a decent clutch, and not all top end, but there are fewer of those around compared to the Blaster. In `86 they did the power valve BS. I had two 1985 KDX 200s. I found them to be incredible machines and I ended up preferring them for cross country racing than my amped up YZs.
  3. MATTY

    MATTY BORDER RAIDER

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    How are these blasters off when it comes down to the primary gearing? i imagine they could be tall in this respect being ex ATV motors, are they high geared? and if so will this be surmountable with only say a oversize rear sprocket or can lower gearing be found that fits within in the motor from other yamahas perhaps. ?
  4. Brewtus

    Brewtus Buffoonery, Inc.

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    That is what I meant by "quadbilly" gearing. Most likely it comes with a high primary ratio, and widely-spaced gear ratios for what 'Bene called the "noise and dust" crowd. Maybe it's time to compare the ratios between the TY and the Blaster transmissions..... :hmmmmm
  5. Achro

    Achro Adventurer

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    Blasters do seem like they have potential, but I can't find any for less than 1k on local craigslist. You guys have any better luck?

    Maybe I'll watch for some police auctions... I'm sure there are some wheelie boys out there on blasters.
  6. motobene

    motobene Motoing for 49 years Supporter

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    Brewtus & MATTY, agree! That and the quality of the clutch, the amount of bottom-end power, and how much crank/flywheel inertia there is are critical aspects of fitting trials.

    I'd assume the transmission ratios are typical, but number of gear teeth there would be nice to know too. Knowing the primary, transmission, final drive, and rear tire diameters, you can calculate overall ratios to compare trials bikes to Blasters.

    I'm doubting there is a high primary ratio (larger small gear and smaller large gear) in those Blaster cases. I suspect more run-of-the-mill number of teeth on both gears. Observe the small diameter rear sprocket to go with the small diameter rear tires? Perhaps a typical primary like you'd find in the TY175, or close enough to just tweak the final drive to get there.

    Can someone give is a count on TY175 primary gears? The reduction equals the number of big-gear teeth divided by the number of little-gear teeth.

    Example: 60/22=2.73
  7. motobene

    motobene Motoing for 49 years Supporter

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    I've got two redneck specials in my area (Oklahoma) on Craigslist, roughly $300 to $600 OBO range. The only thing that would have to be good enough is the engine, in crank and transmission and hopefully top end too. I could pop for a non-trashed later-model Blaster, use the engine only then sell the rest back into the donuts-and-dust crowd.

    If I ever do my own air-cooled twin shock, or ACM, it's going to have to be top notch and work extremely well, even if it looks cobby. Making my own frame and swing arm alone will suck major time, and I'm not going to wast it on rehab of some clapped out rag of a motor.

    I chased down feedback the cheap whole top-end kits, some with head included. Maybe not all, but on some, like the Niche kits, the cylinder liner metal is just way-soft junk with voids, and the piston is crap too. Very low life expectancy.

    A fellow is probably better off boring an OE cylinder and fitting Wisco forged piston. Better yet just a hone and a new Yamaha stock cast piston, which is probably lighter than the forged piston and less likely to cause added vibration. But such considerations are moot if one scores a newer Blaster not yet abused.
    Brewtus likes this.
  8. Achro

    Achro Adventurer

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  9. motobene

    motobene Motoing for 49 years Supporter

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    I mentioned it, but it is worth considering more fully. That is, the difference between a custom bike based on air cooled, drum brakes, twin shocks, and air cooled, disc brakes, and monoshock.

    Yeah, I think drum brakes and twin shocks would make your custom bike a little more amazing if you place well, but if it's about the experience of building your own bike and riding it, why not do it in the ACM format?
  10. motobene

    motobene Motoing for 49 years Supporter

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    Thanks, that's perfect! That is not a geared-up primary. Pretty ordinary stuff that 3.2 reduction.

    Haven't found the TY Specs yet, but the monoshock DT175 motorcycle has the same primary ratio as the Blaster, so the Blaster is typical compared to motorcycles.

    DT175
    Primary 3.23
    Final 49/15=3.27

    1st: 3.35
    2nd: 2.04
    3rd: 1.44
    4th: 1.15
    5th: 1.01

    The site I got this info from was full of errors, so I don't necessarily trust the numbers.

    The DT's transmission is geared down a bit overall compared to the Blaster, and the Blaster has an overdrive 6th. The Blaster is well within the range of final drive gearing change to work with trials, and given the 6th gear you could still cook along on the loop!
  11. Brewtus

    Brewtus Buffoonery, Inc.

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    TY175B -

    Primary - 3.895
    1st - 3.091
    2nd - 2.462
    3rd - 1.875
    4th - 1.421
    5th - 1.0
    6th - .769

    FYI. :D
  12. motobene

    motobene Motoing for 49 years Supporter

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    Hmm. That is a lower-geared primary on the TY. What a typical final drive on the TY175?

    Blaster in 1st gear:
    3.230 X 3.091 X final =

    TY in 1st gear:
    3.895 X 3.091 X final =

    You can play with the rear sprocket teeth to see how they compare. 48/11 = 4.46, for example.
  13. Brewtus

    Brewtus Buffoonery, Inc.

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    According to the 175B manual, final drive is 13/51, 3.923 ratio.
  14. Luke

    Luke GPoET&P

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    The blaster transmission is pretty good for a trail bike. With 13/48 sprockets idle is around 2 mph and top speed is around 50. The 1st-3rd gear gaps are bigger than I'd like. Whether they're tolerable depends on the tune of the motor. For trials it might make sense to lower the sprocket gearing and ride mostly in 2nd-4th.

    I suspect the cheap bore kits are for dune riders. They can blow up anything in a weekend, so might as well buy cheap replacements. Not meant as an insult, the whole point of riding dunes is to go WFO for hours. Yamaha has 8 oversizes available for pistons, they planned on a lot of rebuilding. I spent a little more on a rebore and a wiesco, it's still good at 250 hours.

    Vibration is extremely low regardless of the piston type because of the counterbalancer. I haven't tried a big bore or stroker kit, but the bolt on ones all degrade the port timing anyway.
  15. North ride

    North ride Long timer

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    Wouldn't it be better too start with a old trials bike. Or is the availability of parts make that not work ?
  16. Brewtus

    Brewtus Buffoonery, Inc.

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    I was just throwing around the idea of the Blasted engine in a modded (of course) TY175 base to take advantage of newer components and an OE electronic ignition system. Never said it would be better than the 40-year old TY175 design, just different. I'm sure it would work just fine for most Vintage competitors who would want to pilot a Plaster-powered TY builder. "Better" is a matter of opinion. :D
  17. MATTY

    MATTY BORDER RAIDER

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    Ty gearing is lower but a bigger than typical rear sprocket is not all bad plenty were like this on the old british trials bikes Sprites dots greeves etc back in the day.
    Not sure on layout on the blaster but could the balancer shaft be omitted or is that impossible. ?
    I like two wstrokes but the Honda CG and china clones of the CG singles, are cheap and work great if a little heavy 125 150 or 200ccs simple push rod single what is not to like its kind of a super cheap any mortals budget tiger cub style power plant option and as much as i favor two strokes i think these little singles are a good option if a little heavy.
  18. Achro

    Achro Adventurer

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    Hey @motobene , have you put any more thought towards an ACM design? For my basic fab skills, I think it would be hard to make the linkage for the rear shock. I can't weld aluminum, so I thought of poaching the swingarm, linkage, and shock from another bike.

    At that point, it feels wrong to also use a heavy steel tube frame, but that is the most accessible option. That way there is still the option to control the mounts for the swingarm and rear shock. With steel, it is also more feasible to machine and fit a neck that would use the cups, stem, and bearings from a good front end.

    That would be my plan. Find a donor frame with a good front end, swingarm, and rear shock, then work in a donor air-cooled two stroke with a steel frame.
  19. motobene

    motobene Motoing for 49 years Supporter

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    Achro, if I did monoshock and did not have a linkless shock (they are different than linkage-based shocks), I'd use something from an existing machine, as making my own would be more work that it's worth. I suppose I could start with a Beta Evo and poach that swing arm and Paioli shock. But dang, zero parts are available for that shock if it is - or goes - bad.

    The twin-shock format isn't that bad. There are shocks available that have a LOT of travel and with soft springs you could get rear-end performance that would approach monoshock performance. Use of flexible joints in the eyes would allow angling the shocks in (top mounts closer together) to reduce the some-twin-shocks problem of the rear portion being too wide.

    The old twinshockers tended to be inferior for reasons other than simply two versus one shock, or lack of linkage. Super short swing arms, for example. I'd not make that mistake.

    A problem with twin shocks, however, is the tops of the shocks and their mounts takes up space for more modern muffler center bodies. Low exhaust volume was a problem with older bikes, which is one reason they were ring-ding and tended to be loud.

    But hey, if the point is to make my own frame, and I borrow from some monoshock the swing arm and linkage, the issue of rear suspension is resolved.
    Achro likes this.
  20. motobene

    motobene Motoing for 49 years Supporter

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    I saw a Blaster counterbalancer on eBay and was like, "Nah, a counterbalancer in a smaller 2 stroke?" Different Blaster model? But you've confirmed it.

    A counterbalancer is good. The space one takes up could make the engine a little larger than ideal. Or does it? The Blaster crankcases don't look like they are bigger than the typical TY/DT/some YZ/early IT. Can you drop a Blaster engine into a TY frame? The presence of a counterbalancer would make me think no, but maybe some of the mounts still line up so it's just a question of mods?

    The Blaster head has a place for a stay in the head casting, but they don't machine it out and use it in the ATV frame. We would use it in a trials application, so it's good that the casting can accommodate one. How nice of them!