feetupfun

Photos of TY175, TY250 and KT250 illustrating fork tube angular offset http://www.trials.com.au/content/index.php?option=com_wrapper&view=wrapper&Itemid=42 In case that link does not work for you, the posting is in the Technical Help forum of trials.com.au

It is bedtime now but I will endeavour to take some photos tomorrow evening

Gav the ideal upper shockie mount location will depend on what the extended and compressed lengths are for the shockies you are planning to fit, and the leverage ratio you are intending to achieve - so there no one correct location that is ideal for every rider and every shockie. There are limitations for rear axle travel with standard TY250 frames/swingarms, but these can be overcome if the brake pedal pivot mount is moved or if the rear subframe is bent upwards or cut off, or the swingarm length increased. As far as the shock mount design goes, I will take a close up photo tonight to illustrate the way I did one bike. I can't seem to upload photos to this forum tonight so will send you the photos via the email address you posted at the top email sent

maybe you are measuring something different to what we are talking about? I can take a photo and post it up if required to remove any confusion. It is the fork tubes that are set non-parallel to the steering axis

I think you will find the OD of the standard sleeve prevents going above 70mm bore (it sounded to me like you were asking about boring the standard sleeve bigger). Bigger bores than 70mm are possible but only with a bigger OD resleeve and the maximum OD of the sleeve is limited by the location of the cylinder studs. My TY175 is 72.5mm (bigger sleeve fitted) and the sleeve is very thin. If money is no object then 72.5mm bore is possible but a 70mm bore would be much less labour cost.

The rear Bridgestone trials tyres I have seen are not competition trials tyres but do have a block pattern like a real trials tyre. They have a rounded profile, cross plies and have stiff sidewalls. Conpetition trials tyres have a square profile, radial plies and flexible sidewalls. Bridgestone trials tyres would probably be good on a bike that was never going to leave the bitumen. Bridgestone trials tyres are very much like the rear tyre that would have come standard on the TY250. Since then there has been 35 years of competition trials tyre development.

It has a centrifugal clutch and there is a manual hydraulic override operated by the LH handlebar lever that acts against the centrifugal engagement mechanism. The manual clutch feels and acts just like a normal bike clutch, but it can only drive above a certain motor RPM. It is a pretty clever set-up if you don't want to have be kickstarting your kid's bike all day (it stops the kid from stalling the motor), but the kid can use the manual clutch just like a normal clutch if they want to.

Ben you have just discovered another reason not to fit a Pirelli MT43 on a 348. This is a common problem with tubeless tyres. I have seen a work-around for this problem - four tyre clamps equispaced around the rim. Another person kept the pressure above 8psi to keep the tyre on the rim.

Maybe Montesa designers were concerned that the taper might be damaged if the sprocket rotated relative to the shaft as the retaining nut was being tightened.

Sorry to disagree with previous poster but 360mm shocks have same travel as 340mm shocks Also disagree re advisability of sliding fork tubes down to top clamp - it makes the steering feel worse Recommend 340mm shocks and standard position for fork tubes

For riding dry rocks it is the age of the tyre that is important, not how much tread depth is left. Most of my tyres go hard (and useless) long before they wear down much. If your tyres are worn but still soft, they are fine for dry rock riding. If you ride wet slimy rocks, the only tyre that works is a tyre with sharp edges. You haven't said the condition of the shockies but I would say if the shocks still move freely and have damping, you will get a bigger improvement with new tyres.

The round barrel 250 (M27,M49,M80) motor is quite soft at extreme low RPM but makes reasonable power everywhere above extreme low RPM The later 250 (square barrel 238cc) makes good power (for a 238) rignt down to extreme low RPM but is not as strong at the upper end as the round barrel. In trials use the later motor is easier to live with in modern twinshock events, while the round barrel motor is a very nice motor for more open sections and trail riding.

Our AVGAS has an SG that is quite different to car petrol from the bowser (AVGAS is less dense) and this can cause problems with carburetion when switching from one fuel type to another, particularly with bikes that have the carby leaned forwards a long way, like on Betas. AVGAS is fantastically stable fuel but is not permitted in sanctioned competitions. Luckily most service stations where I live still sell ethanol-free 98RON unleaded which works quite well in trials bikes.

If the triples are straight you can't get enough steering lock without the tubes hitting the frame

regarding achieving the ideal offset. yes it can be done with parallel offset or axle ahead of fork tube centreline, but the lowest mass moment of inertia without having the axle way forward of the line of the tubes comes with a combination of angular offset at clamps and minimising the absolute offset at the clamps.

I thought I said that already

There are a few reasons for the forks being not parallel with the steering head on many bikes (not just twinshocks) It is a way to provide the ideal steering trail without having to have the axle set forwards of the centreline of the fork tube axes (manufacturer may be limited to one fork leg design) It allows for the steering geometry to be closer to ideal through the suspension travel of the forks It reduces the mass moment of inertia of the front end compared with having the same forks but fitted into parallel triple clamps. This is due to the tubes being closer to the steering head axis (for a manufacturer stuck with using in-line axle forks) I can't see it making any difference to steering geometry at full lock

Somewhere between 5 minutes and 30 years depending on how well the bike is maintained, how it is ridden and how frequently it is ridden No a pressure test will not tell you if the piston is worn and neither will it indicate the ring condition unless they are so worn it will barely run. Piston, ring and cylinder wear can be physically measured with the top end apart. Someone with mechanical aptitude and experience with your model of bike will probably be able to judge how worn the rings are when kicking it over and riding it. Similar with the level of noise generated by the piston when hot and cold.

I recommend 340mm and leaving the fork tubes 20mm above the top clamp. Dropping the tubes down makes the steering feel terrible even with 360mm shocks (the 360mm shocks and tubes dropped to level with top clamp means that the standard steering angle is retained but the trail is different to original). It also increases the flexing of the forks. It's personal though and I do know people who reckon that the increased ground clearance is worth the loss in steering quality.

Mike I will work out what it would cost to buy the same product from Germany because I have not tried before but I suspect it will be more expensive from Germany because: 1 Germany is not near Australia 2 Postage from the USA to Australia is generally much cheaper than postage from the EU 3 I don't pay customs duty or VAT or GST from either the EU or the USA David

the tytrials unit works well and was easy to fit. The only difference I noticed was that there seems to be less variation between the strength of individual power pulses at extreme low RPM. I have not tried the others.

Magura assemblies from resellers in the US for about $70 a pair Suzuki RM80X assemblies for about $50 each from Suzuki shops Both are quite crash resistant. I use the Suzuki levers on my 1970s jap trials bikes because they look right, and AMALs on my Spanish trials bikes. If I had a 1980s trials bike I would use the Maguras.

Full synthetic gives the lesser amount of smoke, lubricates best under extreme conditions (pressure and temperature), causes less carbon build-up in the combustion zone, tends to create less ring gumming issues Semi synthetic is not quite as good as synthetic in all those areas, but protects the steel items inside the motor (conrod bearings especially) from corrosion better than synthetic while the bike is not being ridden.

The right height is whatever you think works best. Moving them up or down changes the steering angle, steering trail, wheelbase, ground clearance and how much they flex around. The standard height from the factory is 20mm protrusion above the top clamp.

Ideal gearing is a difficult subject because it is a personal taste thing. When I started riding my 348, I thought the standard gearing on it was too high, and fitted a rear sprocket with 39 teeth (33 is standard) but the motor was not running very well (AMAL problems). Once I got the motor running well, I found that the bike felt too slow in first gear in sections and second was too fast. I then put a standard rear sprocket on and it is wonderful to ride. The front sprocket was standard size all along. If you buy a Pirelli and use it till it wears out, you will be regretting your decision for a long time. They take an amazingly long time to wear out but are quite stiff in the sidewalls and are scary on wet rocks. Michelin or IRC tube-type rears are really the only decent rear tyres for that rim. For performance the IRC is better but is more sensitive to tyre pressure than the Michelin. Michelin used to sell a very good tube-type rear but the current Michelin tube-type is not as good.