Trials Bike Build/conversion?

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

  1. BikePilot

    BikePilot Long timer

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    Rizzer's home made picture looks awesome! If you can fab up something like that you'll have a bike that'd be competitive in the lower levels of modern events at the very least.

    You could definitely make that fun to ride. For specs just search online for dimensions etc of any modern trials bike - Gasgas, Ossa, Sherco, Scorpa, Montesa etc.

    I wouldn't worry too much about a cradle frame (I don't think any modern bikes use full cradle frames - I know my Montesa doesn't). The engine really isn't the big concern, just get something light. Focus on the chassis.

    You really need an 18" rear wheel as that's the only size trials tires come in and they are rather unique. A 21" front would be good too, but a proper trials front is less critical. A knobby front would work fine for most stuff.

    Regarding the two stroke engines and exhausts they are tuned to run well at very low rpm and be very controllable. They aren't especially powerful. I think my liquid cooled, 250cc two stroke makes about 17hp. IIRC it actually has a rather short stroke as compared to my CR250R motocross bike. I'm not sure why this is,but it works out ok. It may simply be to keep the engine dimensions smaller.

    The exhaust is very critical on a two stroke to how the engine runs. There's a whole literature on exhaust tuning and design - you'd want to read up before building a system I think. I don't have a quarter the knowledge to design my own expansion chamber. I will say that on a trials bike exhaust design looks to be driven mostly by packaging requirements and only secondarily by performance requirements.

    Engine performance really isn't a big deal. You need very little power. What you need is a really compact bike, light weight bike with a controllable engine and super-low gearing. A 100cc-ish two stroke would be fine. Folks do all sorts of amazing stuff on 125 and even 80cc trials bikes with no issues - just run a bit lower gearing and rev the motor more.

    I've known folks to take bikes apart and put them in their airplane luggage. Would that be illegal for your country even if you won't be registering the bike for road use? Here you can realistically do most anything you want as long as its only used off road "closed course" only. In '97 when the honda CR250R was available in Japan but not the US the editors of Dirt Rider magazine flew to Japan, bought a CR250R, took it apart in the parking lot and brought it home on the plane in their luggage. They published the whole story with tons of pictures - good fun :clap

    I have an '01 Montesa Cota 315R and it works well enough for me. They are about 1k pounds in the UK.

    lamotovita, D6 as in AMA District 6. I am moving up next event (I've only been at this for a couple of months so started out only one class up from the bottom - cleaned the whole day). I certainly couldn't clean a novice D6 section on a non-trials bike, but I couldn't even ride around the section with a whole bunch of dabs. I'd probably have to get off the bike and drag it around corners or make a 30-point turn or similar. Many sections are pretty darn tight. A few are more straight and wouldn't be hard on a big bike and one was really straight and rocky and would have probably been easier on a big bike :lol3

    Maybe your novice is more like our vintage-novice - that I could defo ride on a dirt bike as its a much less tight course - or maybe I just suck.

    Anyway, I totally agree that you can have a ton of fun doing technical, trials-ish stuff on a dirt bike.

    wet rocks
    [​IMG]

    Steep ledges (this was part of an old trials section)
    [​IMG]

    Dry rock
    [​IMG]

    Big log
    [​IMG]
    #21
  2. Sting32

    Sting32 Trials Evangelist

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    (OP):

    I think, your best bet is it get a trials bike, modify it back to whatever trail ridng you wish to aim for, best bikes for this are the pre 2000, for the gasgas models, IMHO. probably at least that era form most bikes produced then.

    Gasgas made (so very few) true trials/trail bikes where they used a top of the frame, gastank and seat for a MX type; bike married to the neck and engine parts of a frame and suspension for a trials bike. That was in 96-98 called the Pampera. I dunno how many made it to the US or canada either. there was a 329 model and 250, last ones were grey, early ones were red and photograph in used condition as PINK, lol. BTW, not that long ago, there was one forsale on gasgas.com.

    problem is, AFAIK they arent good for either use, the fork rake that is used in trials, is scary at motocross speeds, the supension and frame is lightweight, so I wouldnt enter a stadium cross that is for sure. but I still would own one, I would love to play as I drive to work, in places I probably shouldnt even be thinking of tresspassing on.

    Oh, BTW, I have the honda tlr200 (Reflex) and is was more trialsy for that moment in time (86-87). the newer than that era, but older BETA ALPS would be cool as well.

    Sherco I thought (via scorpa?) was going to produce a more trials "ish" trail/enduro bike, but AFAIK never took off this last year? or still in developement? Hoping to dominate the enduro cross or something, {excuse my weak memory on this subject, I have general idea of what I may think i read here...) and i assume new prices would be near $8,000 so not the bargain you are probably hoping for.

    Try to find a reflex, if nothing else it will make a really nice girlfriend bike, or a potty bike, as I call it, in stock form, not terrible (twinshock type application) with some serious mods.

    Search for ITSA (internation twin shock association, I think it is... then you might find more conversion bikes?

    But Modern, I just think you would work your but and wallet off, then be handicapped and discouraged if any perception was to use it for trials...

    Moto Trials, that it is called now, is at a brink, where they might have to adopt an "enduro" specification class type bike, so we can convert more fellas like you. or at some point (which will never happen I bet) to de-evolutionise the bikes they use for modern trials.

    Problem is, the sport is ran and lives (die hard) due to people who eat and sleep trials. MX is the same, the pro's are what at least the 5% that grow up to compete at that level, aspire to day in day out. if they didnt, when things get hard, or weather is not perfect, nobody shows up for an event, or for that matter in trials, will bother to set one up. the guys that would like (actually think they would like) to just come and "putter around on it" always find excuses to not make it to the events, be it "football season, so sunday's out" or kid has x... or old buddies are headed to the hills/racetrack.

    I been doing trials for over 30 years, we have to get up at 5 am and make the drive (all Year round, in Kansas) to compete that sunday morning. Ive had 30 riders watch the news, on friday night with forcast for cold, hot, rain, or what have you, and even we left the house in a respectable storm, to end up with the nicest dayfor a trials at the site 177 miles south of my home. My folks, pop who is 75 and still competes, has made this our family outing all these years, for the most part. but my buddies, who have wives and kids that avoid the motorcycle stuff, dont ever make it more than 14 months, because they cant go out with the wife all night sat night, then get up and go, and meet thier own hopes of beating the rest of the families that accept that way of life, lept well last night and cleans thier clocks!

    that is dedication, spirit of competition, and love of the fun we have on a trials bike... I can practice for an hour in my 2car wide driveway, and have even more fun, with less than most suburban back yards to practice in/on all week leading up to the event!

    Anyway, if the market nich you are trying to sell to, doesnt do the above, realistically I think that might be why there isn't a bike like you are thinking of, as most anyone who knows business, don't see a real market for them!
    #22
  3. dmay

    dmay Been here awhile

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    TY350 is like a YZ/IT engine from the early eighties,the last air cooled Yamaha offroaders,unique bore and stroke,massive(too much!) flywheel

    A DT125/DT175 could be a good candidate if you can get the front end rake pulled in to about 23-25 degrees.
    #23
  4. lamotovita

    lamotovita Ageing Adventurer

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    Check this out.

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    #24
  5. rizzer

    rizzer no longer bikeless

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    BikePilot, thanks a lot for the post :)
    there'll be a lot of technical aspects I'd need to cover to plan my build. it's at least comforting to know that I won't have to prioritize for large displacements. I was looking at a Gas gas street legal bike (forgot the name) that was only a 125cc 4 stroke and it made me think I got a good chance :D
    I was particularly interested in the exhaust of the 2 strokes and the effects of it towards the power band. I mean you can barely see any expansion chamber there. I'll probably need an engineering course to understand that though heheh...
    and a 250 smoker with a 17 hp? must have a crazy amount of torque for trade-off. would love to know how it rides

    about taking engines apart. I still doubt it'll pass customs, cause even if it's not going to be for street use, there's a bullshit stigma going around in the law enforcements there that all bikes without papers are assumed to be stolen which makes the owners suspects of GTA. and the government have even made things tighter by banning engine and frame imports. but to be honest, the bureaucracy is too complicated for me to understand and will need a whole new thread just to discuss it hahaha...

    Sting, yeah the truth is trials has little to no market in Indonesia and it isn't covered (or even recognized) by the national motoring association. there is a little trials event that a cigarette company runs but it's purely for entertainment purposes.
    http://www.youtube.com/watch?v=VO8D7uybrmE
    and I've only recently found out that there are little bike clubs strictly for trials. other than that, the major consensus don't even know what a trial bike is. they'll be amused by it not having a seat, but other than that there's no meaningful market interest. so yeah, there's basically no motive for a trial producer to sell out there :/
    I'll be happy enough just to have a home-made trial bike to have fun with :)

    Dmay, realistically speaking, and RX engine would be easier to maintain as DT's spares are no longer widely available. but I'll look for a comparison of the two. btw,the pic of the bike I posted earlier was from an RX. It can take an air-cooled YZ125 head without needing to mod.

    Lamotovita, whoa is that a S90? wicked cool! :D

    btw fellas, since I already have forks from a KTM525, will that be good for a trials bike? I mean the size/length and all...it is a bit bulky though
    #25
  6. Pete-NZ

    Pete-NZ Been here awhile

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    The white yammie in back..
    looks like a late 70s vintage TY50 or 80...
    #26
  7. PSchrauber

    PSchrauber Been here awhile

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    The custom made T.K.R.J. looks great, this bike is really cool!
    The frame geometry is up to date, to the motor I have not seen
    this engine here or heard anything about the RX model.

    The threat questioner should tell us probably something about his manufactoring skills too, then it would be easier to post a good hint.
    (I have a lot of respect about the improvisation skills the folks have in asia).

    Starting trals with an modern enduro like bike is IMHO not a so good idea,
    the frame geometry and engine power output is too different. If you have many skills and some machinery it is possible to create a trials machine of
    an modern enduro engine:
    [​IMG]

    But even the frame work needs a lot of machinery work to get a good result:
    [​IMG]

    Then you have to make a lot of changes to the gearbox and the
    power delivery too.

    To rebuild an older engine is IMHO easier, also to build or modify a standard tube frame, like this Stornello engine of the beginning 70's
    with a home made frame:

    [​IMG]

    I would try to get one of the already mentioned Yamaha DT 125cc from the 70's to 80's models that may be even available in your home country
    and take that model as a start. Any trial specifiec parts you can take from a TY 125 or 175cc model. These bikes or parts to this bike model are easy to get in the UK and not very expensive.

    The 125cc TY:
    [​IMG]

    Later on you can rebuild this bike with a 175 or 200cc cylinder/head/piston and do some small other changes too, (chopping of
    parts that you don't use for trialriding, ...)

    The result can be something like this:
    [​IMG]

    Very competive even nowadays!
    #27
  8. BikePilot

    BikePilot Long timer

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    Your KTM forks could work - they are really overkill and the biggest issue I expect you might have is that they will be a bit long and large in diameter (which might limit turning radius). They are certainly strong enough though. all current trials bikes use fairly skinny conventional forks to maximize turning radius. If you use the KTM forks consider finding the softest springs you can and run very light weight oil (maybe 2.5wt) at a very low level. You might even try them with a spring only in one leg and damping only in the other leg.

    The torque on a modern trials bike isn't all that impressive really. They scoot ok off the bottom, but I doubt they really make a whole lot more power from low rpms than say a 250cc two stroke MX bike and they might actually make less (the gearing is so dramatically different its hard to tell - 4th on the trials bike is probably something like 1st on my CR). Main difference from a MX bike is that the trials bike runs really clean and smooth from very low rpms where the MX bike would be unhappy, blubbery and prone to loading up and fouling. The trials motor is ultra-controllable with smooth, clean, linear response. Its all about control, when you have super-low gearing and a 160lbs bike, you don't need much power at all. Also the flywheel is very heavy so if you need a quick burst of power you just get the flywheel spinning (rev the motor) than take advantage of some of that stored energy in the flywheel. If you watch a video of a pro doing a big splatter, especially on a 125cc bike, you'll note that the rider revvs the motor out very high with the clutch in before doing anything. When you've got a heavy flywheel spinning quickly you can dump an enormous amount of power into the rear wheel for a quick burst.
    #28
  9. BikePilot

    BikePilot Long timer

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    #29
  10. rizzer

    rizzer no longer bikeless

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    Pete, thanks for the info. I didn't know they existed in our market :/

    PScrauber, yeah, I don't think I can afford to go through that length of total frame rebuild as your pics showed. would prolly opt for a more chop approach than fabrication. your TY example is a much more convenient option for me. btw, man that yellow TY is pretty! :D

    also, the RX-135 (AKA RX-King) in its standar form:
    [​IMG]
    there's also a Z version:
    [​IMG]

    the king version is infamous for being getaway vehicles for criminals. and being a bulletproof engine with low maintenance costs, it was also widely used for bike taxis. also, the gearing on the king is lower than the RXZ. another plus of choosing this engine is that 2 stroke value has gone down in recent years so it should be pretty cheap.

    Bikepilot, thanks for the link! :D that's gonna help me get through the sleepless nights heheh
    I agree about the suspension though, I also think it could be overkill to use the KTM fork. would make more sense to get a smaller/skinnier conventional fork from the KLX150 for example.
    I read somewhere that trials forks are "springy", and you said it should be soft. does this springy means more preload than damping? and would soft means more tendency to bottom out kinda soft?
    #30
  11. BikePilot

    BikePilot Long timer

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    On the forks yes, the trials forks are very soft and bouncy. They will bottom quite easily, but remember trials is generally very low speed so you don't ordinarily need much bottoming resistance. Same for the rear suspension. There is certainly some damping, but they are more springy than say the stock forks on the KTM or a typical dirt or street motorcycle. I'm not sure that its more preload really, but it might be. Mostly it feels like there is less damping (rebound and compression) than normal.

    The RX135 looks like a fine starting point. Strip all the bodywork off, put on longer shocks and forks, fabricate a high-mount exhaust and fit trials tires and I'm sure you'll be able to have lots of fun on it.

    For reference as far as bottoming, I was riding a section (for practice) a couple of weeks ago that had a drop off a rock to flat dirt that was probably about chest-high I'd guess. This was enough that both ends of the suspension bottomed out on the landing (no doubt in part due to my poor form). I'd guess that the suspension on the trials bike is a little softer than the stock suspension on a kawasaki KX80 motocross bike (kid's bike) in terms of spring rate and has quite a lot less rebound damping.
    #31
  12. PSchrauber

    PSchrauber Been here awhile

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    Very cool bikes these RXZ's, real workhorses!

    To your fork question, this will be complicated because trials bikes have a real wide turning or steering angle for turning as thight as possible.

    To get this done without loosing stability in balance you have the followings specs:

    offset between fork and headset, (axis to axis): 35mm
    offset between wheelaxle and fork, (axis to axis): 25mm

    Much more importand is the steering angle and the backslash!
    The backlash depends on:
    - the angle of the fork
    - the distance between axis fork and axle of the wheel
    - the distance between fork and steering stem

    all three together give the backlash

    For a trial machine it's somewhere between 70 - 75mm

    BTW:
    There is another problem if you would use a long enduro fork,
    you will have a lot more fork travel which will change the backlash too!

    For better explenation here some pic's:

    Measurements of a modern Trials bike:

    [​IMG]

    Radstand = Wheelbase should be between: 1300 - 1350mm
    Lenkkopfwinkel = Heasdset angle around: 73°
    Nachlauf = Backlash around 70 - 75mm

    Some explenations in pic's:

    The spring travel will shorten the wheelbase
    [​IMG]

    The backlash will be mostly changed by the distance between fork axis to wheel axle and by the fork angle:

    [​IMG]

    And what happens with the backlash distance when the suspension gets loaded:
    [​IMG]

    Trials bikes need a huge amonut of offset to get a wide turning angle,
    so you can make very narrow turns, this hdesign hasen't change,
    (also it is now easier possible to turn by lifting the front or the backend,
    (Just to note which is only possible if you are not riding in a high slope up-
    or downhill in sandy terrain, there you can only steer not hop around.))

    By the way the tripleclamp offset on most trials bikes is NOT parallel to the steering axis.

    If you look up the picture # 2 or Skizze # 2 you see why, through the contraction of the fork the backlash would change dramatical, which would make steering of the bike instable, '(remember you can do very close turns with a trials bike).

    For compansating these effect the clamps hold the the fork in an angle that is around 1,5 kicked out of the degree of the steering stem. This eleminates the bad effects of a changed backslash.

    Some discussion about the angle differencies between fork and steering stem you find here:
    Trials Central not parallel steering stem and forks


    Here the part List of a Montesa, there you see:
    - the offset.
    - the difference between the upper and the lower clamp,
    (not much only around one degree)
    - the offset between front axis and fork axis.

    [​IMG]

    I hope this was not too complicated, - and please keep in mind that english is not my mothertongue -, but it describes much of the huge
    differences between a enduro and a trials bike and why it is IMHO
    not very easy to rebuild a enduro from "scratch".

    As already mentioned: try to get as many main parts like the front end,
    the engine, exhaust and wheels, and ship them to your homland using a bike that you bought in the UK. The rest you sell via Flea-Bay to get some money back.
    #32
  13. rizzer

    rizzer no longer bikeless

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    Bikepilot, awesome thanks for the info! will definitely keep that in mind when setting up my suspension.

    PSchauber, wow I didn't know that the clamps weren't parallel :/ yeah it makes sense by looking at the pics though. I'll be reading up on the link as well to understand this a bit further. but yeah, that'd be pretty hard to fabricate in a home garage

    been hunting around for some parts and can't seem to find them in "affordable" price. this project can potentially turn expensive :/
    #33
  14. motojunky

    motojunky Professional Idiot

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    I've seen a couple of the bikes produced by http://ntacycles.com/ and have been very impressed. Might be some food for thought available at their website.
    #34
  15. nsu max

    nsu max Been here awhile

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    I converted a 1968'ish Jawa 250 California to a vintage trials/trail bike the huge rear sprocket realy is'nt needed but being a full figured guy at 300 lbs. it helps the only thing I would change and I may do it yet is make a chain guide, skid plate and riase the exhaust or angle the mufflers up. Since I had most of the parts laying around the whole project cost less than $100.00 U.S. The bike itself was a rusted, incomplete junker headed to the scrap heap that my brother took pity on and brought over and gave to me many years ago. [​IMG][​IMG]
    As the bike sits now it's really not a bad mount for the begainer I competed a few times on it and have one DNF at an event that was'nt suited to vintage machines at all and a couple of mid pack finishes.
    #35
  16. rizzer

    rizzer no longer bikeless

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    Motojunky, many thanks for the link! they look awesome...+ a pic gallery of the builds :thumb

    Nsu Max, wish I had a brother like yours...nice build man! good to hear the bike gets around in events as well...did you refer to textbook trials geometry or just went for gut feeling on the build? I mean like applying unparallel triples? etc.

    Geode, nice find mate! I guess I couldn't find it in searches cause I was looking up the word "build" instead of conversion. been reading through that thread and it's mostly on twinshocks. I'd love to gather as much info on both twins and monos before deciding though. I do see there is a link for a CRF/XR trials conversion towards the end of the thread...cool build.

    the main difference about my build and theirs is that they've actually started :D I can only start in 4-5 months at the soonest :(
    #36
  17. nsu max

    nsu max Been here awhile

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    Thanks Rizzer, I went the gut feeling aproach with the Jawa, I figured if it was'nt any good for trials I'd find something else to use it for.
    #37
  18. dakdakdave

    dakdakdave an older,wiser noob!

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    Zehijiang motorcycle. 230cc pushrod. Chinese "motorcross".

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    #38
  19. rizzer

    rizzer no longer bikeless

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    dakdakdave, sorry for the late reply. was kept busy with my move back home to Indonesia

    nice one mate...how does it handle? I mean compared to a trials bike

    also I can see you still have the foot pegs in its original position as opposed to it being further back like in trials bikes. does that have any significant affects in handling?


    I'm nearing the end of my build, but it isn't a trials bike. I figured I needed an all-rounder for the commute and occasional trail riding. The trials bike will definitely be next :)
    #39
  20. Pete-NZ

    Pete-NZ Been here awhile

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    Thats dam kool...
    try turning the forks around...
    trailing axle... Will
    shorten the wheel base / steepen the rake / reduce the trail..
    get it more trials bike geomitrey..
    #40