Yamaico's new el Cheapo Project.

[GMC]

Chmaber design has pretty much been summed up by others, long headers for bottom end and a long overall length for low revving motors, the ‘box’ is about faking long length other wise you wouldn’t be able to fit the pipe on bike. Not aware of any formula to calculate the box

It all depends how fussy you are, do you want to create something state of the art or just something that will work reasonably well.
If your only after 'reasonably well’ then take any 125 MX chamber and add 5” into the header length and another 2” into the mid section.*

* Normal disclaimer, if it works good I will take credit but if it sucks then I will remind you not to believe everything you read on the net.

[Rookie#1]

* Normal disclaimer, if it works good I will take credit but if it sucks then I will remind you not to believe everything you read on the net.

 

[yamaico]

Chmaber design has pretty much been summed up by others, long headers for bottom end and a long overall length for low revving motors, the ‘box’ is about faking long length other wise you wouldn’t be able to fit the pipe on bike. Not aware of any formula to calculate the box

It all depends how fussy you are, do you want to create something state of the art or just something that will work reasonably well.
If your only after 'reasonably well’ then take any 125 MX chamber and add 5” into the header length and another 2” into the mid section.*

* Normal disclaimer, if it works good I will take credit but if it sucks then I will remind you not to believe everything you read on the net.

Thanks Geoff,
Certainly don't need state of the art, "reasonably well" would do me fine considering I've never ridden trials

[firko]

I reckon the stock CT3 pipe might be just the ticket considering it's designed for a pretty broad application.

[JC]

I left a hint in there for ya

Thats what I tho't you meant, but....

There is no negative return wave only positive

That goes against everything published on expansion chamber design. As I understand it, the -ve one is reflected too, off the open end of the header & diffuser

the ‘box’ is about faking long length

That is a very useful bit of info relevant to trials exhaust design Geoff. Many thanks

Jennings does give a formula for natural frequency in a 'flask' w pipe inlet - effectively what most trials exhausts are

[yamaico]

I reckon the stock CT3 pipe might be just the ticket considering it's designed for a pretty broad application.

I reckon so too Mark. I rode a lot of CT's in their day, and remember them having a nice even power curve and not pipey at all. I'd like to try it with the CT pipe to at least establish a baseline so I know where I'm going, before I start making too many changes.

[tony27]

Back from holiday on gold coast & went out to garage before it got too cold, took some measurements of the modern bike & my TY250-only approx as it's parked behind the CZ & YZ465

Sherco measurements are
Front of footpeg to swingarm pivot   65mm
Swingarm length                           530mm
Front of footpeg to rear axle           440mm
Front of footpeg to front axle          900mm

TY250C twinshock approx measurements
Front of footpeg to swingarm pivot      0mm
Swingarm length                            485mm
Front of footpeg to rear axle            465mm
Front of footpeg to front axle           930mm

The TY is too light in the front (lifts too easily even with no compression) & is about to get lengthened by 50mm to bring the bike back to standard wheelbase which is possible due to the steering having been altered sometime in the 90s by the owner's son

Hope these numbers help out with getting the footpegs in the right position

[yamaico]

Finished narrowing the tank and made the front and rear mounts. Bit of good old Bondo and you'll never know.



Modified the rear brake pedal to get the rod pivot close to the swingarm pivot and it now works well. Fitted up some MX125 triples (thanks Vandy) which have more offset than the original CT triples, which will decrease the trail a bit. They will also extend the wheelbase marginally so I may pull the head angle back a little more. which will also decrease the trail. Will make the seat next and then start on the pipe

Back from holiday on gold coast & went out to garage before it got too cold, took some measurements of the modern bike & my TY250-only approx as it's parked behind the CZ & YZ465

Sherco measurements are
Front of footpeg to swingarm pivot   65mm
Swingarm length                           530mm
Front of footpeg to rear axle           440mm
Front of footpeg to front axle          900mm

TY250C twinshock approx measurements
Front of footpeg to swingarm pivot      0mm
Swingarm length                            485mm
Front of footpeg to rear axle            465mm
Front of footpeg to front axle           930mm

The TY is too light in the front (lifts too easily even with no compression) & is about to get lengthened by 50mm to bring the bike back to standard wheelbase which is possible due to the steering having been altered sometime in the 90s by the owner's son

Hope these numbers help out with getting the footpegs in the right position

Thanks very much for that Tony. i've just checked mine and the dimensions are:

Front of footpeg to swingarm pivot      55mm
Swingarm length                            460mm
Front of footpeg to rear axle            395mm
Front of footpeg to front axle           935mm

The numbers are a bit different but perhaps you (or someone) can tell me what the implications will be.
Cheers,
Pete.

[tony27]

Looking at your figures suggests that your weight will be to rearward which generally causes the front to plow on in turns rather than bite, your pegs are almost as close to the front axle as on my TY but due to the very short swingarm are closer to the rear wheel
Most kits for TY footpegs end up with them being just behind the pivot which is a bit further back than standard
I'd be lengthening your swingarm to help with keeping the front down, I had major issues last time I rode at Kaikoura with the front being too light on climbs even though the engine was seriously sick, could turn motor over by gripping end of crankshaft with the plug still in. David Lahey has put up some very good photos etc on trialscentral & trials australia detailing how he does it to his TY175s
Looking at the figures for my Sherco shows how the engine must be closer to the front wheel as even with a longer swingarm the wheelbase is shorter. Modern bikes seem to be all about mass centralisation which is where the very low seat comes from
Standard TY wheelbase is listed at 1295mm which must be a misprint as I work mine out as 1395mm with reduced rake, always possible that I measured wrong

[JC]

Pete, brake arrangement looks good, as does tank.

Tony, I think you've definitely measured wrongly.
1395mm is typical MX wheelbase. TY250C std w'base is definitely 1295mm.
Mine is 1300 w the axle back a little on the adjusters.

Other measurements on my std TY250C are:

Swingarm length            380mm
Front of peg to rr axle     380
Front of peg to fr axle     920

Seems to me Pete's in the right ballpark

[EML]

I have been pondering GMCs post-----not the bit about the disclaimer as that's a given
He mentions that "faking long length (where have I heard that before?) otherwise you wont fit it in" (again-where have I heard that before?) but then concedes that he is "not aware of any formula"
By 'faking length' and turning it into diameter-is that not the 'formula'?
i.e. shorten header by 2'' but make it 4'' diameter instead of 3''
But then he goes on to say "add 5''s into the header and 2''s into the middle of an MX125 pipe". Not sure about you but if I added 5''s to my header I'm pretty sure it wouldn't fit anywhere even without adding 2''s to the gurth--just sayin'  
But then if you look at the modern 2 smoker pipes (2000 on) they are very short and fatten up very quickly.

[Lozza]

There is no negative return wave only positive

That goes against everything published on expansion chamber design. As I understand it, the -ve one is reflected too, off the open end of the header & diffuser

yes there is a reflection off the diffuser and header BUT it is still postive pressure,reflected from the outbound exhaust wave. Again all simple to see on a basic simulator, what you see is a depression with  a small hump or not quite as big depression.


There is a formula for everything including box mufflers(had to do them for chainsaws in stock classes). The box isn't faking length, that's what a KIPS/ATAC/AEC valve does, if that's all there was to it there would just be a big resonator 'somewhere'.

I think the pipe would be hard to get wrong and a look inside a box of a trials pipe might have some clues.

[GMC]

I have been pondering GMCs post-----not the bit about the disclaimer as that's a given
He mentions that "faking long length (where have I heard that before?) otherwise you wont fit it in" (again-where have I heard that before?) but then concedes that he is "not aware of any formula"
By 'faking length' and turning it into diameter-is that not the 'formula'?
i.e. shorten header by 2'' but make it 4'' diameter instead of 3''
But then he goes on to say "add 5''s into the header and 2''s into the middle of an MX125 pipe". Not sure about you but if I added 5''s to my header I'm pretty sure it wouldn't fit anywhere even without adding 2''s to the gurth--just sayin'  
But then if you look at the modern 2 smoker pipes (2000 on) they are very short and fatten up very quickly.

You shouldn’t think about things too much, it will only hurt, especially if you think too hard about something I wrote.

The best way to fake length is in the dark, as for pipe design you can simplify a chamber down to three parts…
Length
Diameter
Angles

Length is easy, long pipes produce low revving motors and short pipes produce high revving motors.

Choosing diameters and angles is more complicated as they sort of go hand in hand with each other but in a nutshell, sharp angles produce abrupt power characteristics and shallow angles produce mellower power characteristics.

The ‘add 2” to the belly was meant for the length not the diameter.

The box / resonator will produce a flat torquey type powerband,  and generally kill any wild top end, much the same as a long chamber. That’s why I suggested it as ‘Faking’ length. Apostrophe’s show that I use the word loosely for people to be able to grasp the concept.

Long headers produce good low end power so modern bikes don’t require this as much as they make up for it with power valves so modern pipes are designed more for top end power in comparison.

The only negative wave I have experienced is handing my wife money than waving her goodbye as she heads off to the shops.

[tony27]

Pete, brake arrangement looks good, as does tank.

Tony, I think you've definitely measured wrongly.
1395mm is typical MX wheelbase. TY250C std w'base is definitely 1295mm.
Mine is 1300 w the axle back a little on the adjusters.

Other measurements on my std TY250C are:

Swingarm length            380mm
Front of peg to rr axle     380
Front of peg to fr axle     920

Seems to me Pete's in the right ballpark

Pretty sure I got the measurement wrong, was holding & leaning over the CZ to do it. Appears to be the swingarm that is wrong

[yamaico]

Got the seatbase made and shaped some foam ready to get covered



Machined up a fuel tank cap that won't shrink



Pulled the carby apart and it was in terrible shape inside. The pilot jet was stuck solid and no amount of penetrene, combined with heat, could remove it. As happens, the screwdriver slot in the top of the jet is no longer there, so I'm off to the wanted section to see if anyone has one that they might sell me.