Yamaico's new el Cheapo Project.

[EML]

Any particular reason the kick-stand is on the right?
Is it a 'trials' thing, a southern European thing or are all the trees at a trial taken by a broken down Bully? 

[vandy010]

havn't you got a job Vaughan?
sidestand is on the right because the left side gets "busy" with the chain and possible tensioner.
it's a clearance thing dude.

[EML]

I'm waiting for another VNXer to come in a buy/beg a bit of pipe that they don't want me to cut/bend/weld ("I can do that myself")

[firko]

Thanks very much for your kind offer Mark, but as Brent says they have an odd rake offset to the steering head. Also, I think the TY250 forks are 34mm, whereas I have 30mm forks, with brand new freebie tubes. I think I'll go with John and Vandy and get the MX125 triples. Much appreciated though.

No worries Pete....I think they'll be a good experiment for dirt track, perhaps on the Hindall tracker if I use the billet clamps on the Trackmaster XS650.

I had another read of Bells book in bed last night and unfortunately JC and everyone is right...he doesn't get into trials systems much, if at all. While reading Bells book I had an inkling from long ago of reading something by Gordon Blair (or maybe it was Jennings) on trials exhaust but I've just spent over an hour Googling for it with no result.

Having said that, JC has unearthed a few trials pipes cut in half (courtesy of trials guru David Lahey) and they have a straight perforated pipe inside. The lawn mower muffler is looking better all the time.

If that's the case, perhaps my TL muffler might not be as silly as it originally seems. It's here for the taking. I'd be tempted to cut my Maico "expansion" box to see what's in there but I'm out of gas to weld it back together .
Here's a PDF file of Dr Blairs book. It probably doesn't solve Pete's immediate problem but it's an interesting read for technophiles.
http://www.media.rmutt.ac.th/media/e-book/Engineer/Automotive/The%20Basic%20Design%20of%20Two%20Stroke%20Engines.pdf

[Lozza]

Ditto my hand made Maico trials bike. There doesn't seem to be a lot of science in the exhaust design but the bike performs really well in trials situations so the original builder must have done some form of exhaust research. I had a bit of a look through A. Graham Bells 'Performance Tuning in Theory and Practice' bible and there's not a mention of trials exhaust design.

Modern trials pipes are calculated to the mm, just because it looks simple doesn't mean it is. They are hard to get wrong , a long parralel header will help them pull off the bottom well.

[yamaico]

Be prepared to move the frontpegs after your first ride, they look a bit further back than ideal. If they're too far back the front sledges when turning

That's the angle of the camera shot making it appear like that Tony. The footrests are actually only 15mm further back from the front axle than TY175 pegs and, because my swingarm is a little longer, are further away from the rear axle than the TY. Still, if they are too far back its's very simple to move them forward.

[tony27]

I suspected that camera angle may have had something to do with it, best part about using plate mounted pegs is ease of adjustment if needed.
Standard TY175s are too light in the front end & benefit from lengthening the swingarm
I could take some measurements of position relative to swingarm pivot on my sherco & TY250c when I get back to NZ from the gold coast in the weekend if that helps out to compare

[yamaico]

I suspected that camera angle may have had something to do with it, best part about using plate mounted pegs is ease of adjustment if needed.
I could take some measurements of position relative to swingarm pivot on my sherco & ty250c when I get back to NZ from the gold coast in the weekend if that helps out

That'd be great, thanks.

[JC]

Modern trials pipes are calculated to the mm, just because it looks simple doesn't mean it is. They are hard to get wrong , a long parralel header will help them pull off the bottom well.

Lozza, tell us more. The long parallel header is a given, tho some more modern designs appear to have a slight taper for about half its length.

But we want to know about the design of the so-called "mid-box" specifically for trials. We all have a decent grasp of expansion chamber design for MX, enduro etc, but none of us can find anything specific to trials bikes design.

If you apply the usual expansion chamber design criteria, adapted for broad powerband & low-end power, you get a design much much longer than the typical trials exhausts. A couple of manufacturers went that route circa '80 but it didn't last.

[Lozza]

it's easy they 'smear' the return wave (it's not sound either more later on that) and make a low amplitude pulse.
If the taper has been incorporated into the header that's needed due to the better transfer geometry of modern engines.
An exhaust pulse has 3 parts. Pressure, sound and particle, pressure wave moves fastest, then sound and last the particle. Practicaly they are hard to separate in a pipe, easy with transducers

[vmx42]

it's easy they 'smear' the return wave (it's not sound either more later on that) and make a low amplitude pulse.
If the taper has been incorporated into the header that's needed due to the better transfer geometry of modern engines.
An exhaust pulse has 3 parts. Pressure, sound and particle, pressure wave moves fastest, then sound and last the particle. Practicaly they are hard to separate in a pipe, easy with transducers

So he should be using transducers not a chamber?   

[JC]

it's easy they 'smear' the return wave (it's not sound either more later on that) and make a low amplitude pulse.

Which one?  The -ve return wave or +ve one?

it's not sound either more later on that

    duh, yes of course; pressure wave. Seniors moment!

[Freakshow]

I like the idea or wooping and hollering.  I think i need the drugs EML is on.  lets play cowboy and indians

[Lozza]

it's easy they 'smear' the return wave (it's not sound either more later on that) and make a low amplitude pulse.
If the taper has been incorporated into the header that's needed due to the better transfer geometry of modern engines.
An exhaust pulse has 3 parts. Pressure, sound and particle, pressure wave moves fastest, then sound and last the particle. Practicaly they are hard to separate in a pipe, easy with transducers

So he should be using transducers not a chamber?   

Just where you put it makes all the difference Jeff

[Lozza]

it's easy they 'smear' the return wave (it's not sound either more later on that) and make a low amplitude pulse.

Which one?  The -ve return wave or +ve one?

it's not sound either more later on that

    duh, yes of course; pressure wave. Seniors moment!

I left a hint in there for ya There is no negative return wave only positive.