Trials Bike Build/conversion?

[BikePilot]

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.

[PSchrauber]

Originally Posted by rizzer

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!

also, the RX-135 (AKA RX-King) in its standar form:

there's also a Z version:


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! 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?

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:



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


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



And what happens with the backlash distance when the suspension gets loaded:


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.



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.

[rizzer]

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 :/

[motojunky]

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.

[nsu max]

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.
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.

[rizzer]

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

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 I can only start in 4-5 months at the soonest :(

[nsu max]

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.

[dakdakdave]

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

[rizzer]

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 :)

[Pete-NZ]

Originally Posted by dakdakdave

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

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..

[Twin-shocker]

If you are not likely to be competing in serious competition, and are simply looking for something more suited to trials type riding, then get hold of a bike with a motor most suited to trials (probably 200cc 4T), which can be easily modified.

Then find a lighter/smaller frame intended for lower capacity machine that your motor will fit into (twin-shock will be much easier to do), remove all lugs, brackets, mountings that are not needed, get a 21 inch front wheel, and 18 rear laced onto lightweight moped hubs, and fit the most suitable front and rear suspension you can find.

All thats needed then is to fit the motor into the chassis, alter the steering angle to around 66 degrees, and sort out final details like the best footrest position, and a tank which fits and isnt too big and bulky.

If you have tube bending/machining facilities and are a good fabricator, then you can easily build a modern perimeter type frame along the lines of a Gas Gas from scratch, but the value of this is questionable unless you have a motor specifically designed for trials to go into the frame.

[rizzer]

Originally Posted by Twin-shocker

If you are not likely to be competing in serious competition, and are simply looking for something more suited to trials type riding, then get hold of a bike with a motor most suited to trials (probably 200cc 4T), which can be easily modified.

Then find a lighter/smaller frame intended for lower capacity machine that your motor will fit into (twin-shock will be much easier to do), remove all lugs, brackets, mountings that are not needed, get a 21 inch front wheel, and 18 rear laced onto lightweight moped hubs, and fit the most suitable front and rear suspension you can find.

All thats needed then is to fit the motor into the chassis, alter the steering angle to around 66 degrees, and sort out final details like the best footrest position, and a tank which fits and isnt too big and bulky.

If you have tube bending/machining facilities and are a good fabricator, then you can easily build a modern perimeter type frame along the lines of a Gas Gas from scratch, but the value of this is questionable unless you have a motor specifically designed for trials to go into the frame.

yea I'll be heading that direction...hopefully sooner than later :)
here's the yamaha lc135 I mentioned earlier in the thread.
stock form:

and this is a drag bike conversion (a reference to what the bare engine and frame looks like)


it looks like the engine is pretty compact. and the frame will prolly only need some chopping and protection under the engine. the forks will need a longer travel fork which I already have (the one in the pic isn't stock). what I don't know yet is if the stock swing arm will take an 18 inch wheel or not. if not, then that'll be more homework :/
From what I've read and heard, the engine (135cc stock) can take new cylinders from another yamaha (150cc stock) and can be bored up to 175cc. should be enough for a "fun" bike

but now that I have more options, I can't decide which one I'd go for...vintage-styled two shock? 4t? 2t? modern-styled?

[Twin-shocker]

That looks totally unsuitable to me, the motor looks big bulky and heavy, and is water cooled which for a play bike will simply mean added weight and less reliability. I would suggest looking at a simple air-cooled bike, as you are never going to make a proper trials bike from a road machine, its best to get something as light and compact as you can..........Chinese Honda copy stuff isnt bad, and can generally be found very cheaply.

[rizzer]

Originally Posted by Twin-shocker

That looks totally unsuitable to me, the motor looks big bulky and heavy, and is water cooled which for a play bike will simply mean added weight and less reliability. I would suggest looking at a simple air-cooled bike, as you are never going to make a proper trials bike from a road machine, its best to get something as light and compact as you can..........Chinese Honda copy stuff isnt bad, and can generally be found very cheaply.

the engine is actually pretty small. those are 17inch rims with slim rims, they're only about 1.40 wide so it makes the engine stand out.

but I think you're right about the additional water weight. might have to look for small GL series honda engines instead

[rizzer]

seems like this build "plan" will go onto further delays...my scrambler build is sucking in more funds than I thought