Basket Cases

[JC1]

Hi Everyone,

After going through some more :Bultaco: parts, I found another head with 3 holes. Here are some comparison photos:

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Both heads appear the same except for the number 105 stamped in the underside of the second beadblasted head, maybe it means something? The centre hole looks to me like it was machined at the same time as the others with a similar recess even though they have different depths. They fit neatly on to a 325 cylinder with 83.4 mm bore.

?

Greg, both those heads are from Mk5 M87 and Mk6 M104 350 Pursangs which had 83.2mm bore and 60mm stroke for 326cc, same as early 326 Sherpa Ts and Alpinas. They have the narrow stud spacing (for the 4 long studs) of all the 125/175/250 Buls (prior to the Mk 11 250s) and the early 326 Alpinas and Sherpa Ts (M99 and M92).

On the Mk7 360 Pursang (M121) they went to 83.7 bore and 64mm stroke for 352cc and also moved the studs out wider as they did on Alpinas and Sherpas, but I'm not sure on which of those models they changed. I think it was the M116 Alpina.

On the Mk 8/9/10/11 etc Pursangs and similar era Fronteras they went to 85 bore for 363cc and moved the studs out further still.

So your heads will fit the MK5 & 6 Pursangs, and early 326 Sherpas and Alpinas (obviously with fins trimmed), but not the later ones.

[Bully fanatic]

Yes that M193 is Micks John. He has 2 of them, 2 M192s and a M194. The M105 is a MK 6 250 Astro. I have raced a M106 which is a MK 6 360 Astro.

[David Lahey]

There's nothing I love more than testing different setups on my trials bikes looking for the ideal.
Many years ago I fitted Racetech gold valves to the TY250 forks on my Majesty and it took a lot of experimentation before I was happy with the fork action.
In the last couple of years I've been quite promiscuous (ridden many different twinshock trials bikes). The more I rode the other bikes back-to-back with the Majesty, the lower my satisfaction level with the Majesty fork action went.
I have been quite happy with the action of the modified forks on the KT so decided to try the same setup on the Majesty forks. The KT has aftermarket TY250 aluminium damper rods and TY250 Magical springs. Compared to standard TY250 damper rods, the aftermarket damper rods have smaller damping holes and are a bit longer.
So for the fork testing on the Majesty, I first fitted a set of the TY250 aluminium damper rods and ran with the shortened B&J racing springs that had been used in the Majesty TY forks with the Gold Valves. In Motion suggest to use 10W oil with those damper rods so I did that. The action was very nice indeed.
Then I tried Magical TY250 springs with the same 10W oil. Also very nice action but not quite as much bottoming resistance as with the B&J springs.
Then I tried the Magical TY250 springs with 15W oil. Still very nice action and slightly more bottoming resistance when dropping down a step big but not as supple on small stuff as with the 10W oil.
That's where I'm at, trying to decide which oil and springs to use with the new damper rods.

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While I had the springs out, I measured them up and worked out their rates:
Standard TY250 spring (dual rate spring) initial rate 0.26 final rate 0.42. Spring travel to reach the higher rate 137mm.
Magical TY250 springs (dual rate springs) initial rate 0.36 final rate 0.54. Spring travel to reach the higher rate 134mm.
B&J Racing TY250 springs shortened by about 8mm (single rate springs) 0.39

If anyone wants to do their own calcs on the springs here are the relevant dimensions:
Standard TY250 springs OD 24.95mm - wire 3.4mm - wide spaced coils 35 - total coils 57 coil - gaps in close coil 2.4mm
Magical TY250 springs OD 24.6mm - wire 3.6mm - wide spaced coils 37 - total coils 56 - gaps in close coil 2.4mm
B&J Racing springs OD 24.8mm - wire 3.75mm - coils 60

[David Lahey]

The MAR motor on my bike workbench was getting in the way so I've done a video before storing it out of the way.
This video is to show the very high geared kickstart.
Most two-stroke kickstart bikes have kickstart gearing chosen so that a healthy down stroke of the kickstart lever turns the motor far enough to move the piston past top dead centre and well towards the next compression stroke. On these old motors, one downstroke of the kickstart rotates the motor through two complete revolutions.
There are advantages to this concept. The ignition spark strength is helped by the higher cranking speed and the motor has more goes at firing per kick.
The negative is that it takes more work from the rider. In the case of , to reduce the force required by the riders leg, they made the kickstart lever very long. That means it moves through a big arc, which can take a bit to master.
Luckily it's on the correct side (like a ) hey Bullylover?
Anyway here's the youtube video demonstrating the unusual kickstart gearing.
https://youtu.be/u3694t69OpU

[David Lahey]

This posting is to show what Gold Valves look like compared with what's inside standard mid-1970s KAYABA 34mm forks.
The insides of TY250 and KT250 forks are very similar and both are made by Japanese suspension company KAYABA. The inside diameter of these forks (where the damping pistons run and the springs go) is 25mm.
Other Japanese dirt bike forks from the 1970s are very similar inside.
Race Tech make these Gold Valves primarily for MX and Trail bikes that use the fork damping technology of the time, developed by Ceriani in the 1960s and copied by other manufacturers. The Ceriani design has one fixed rate for compression damping and one fixed rate for extension damping. The Gold Valve concept is to provide the ability to tune the damping of the forks and to provide compression damping that is rate-sensitive. Rate sensitive means that if the forks are compressing very fast, there is less compression damping than when the forks are being compressed slowly. Landing from a jump causes slow compression. Stutter bumps cause fast compression. The rebound damping rate is fixed which is a standard thing on suspension because fork extension damping does not have to deal with things like landings from jumps.
The little spring on top of the gold valve pushes on a plate valve that restricts the flow of fork oil as the forks compress. The faster the forks compress, the further open that plate valve moves. There are different rate springs you can use and the spring preload is adjustable too. This spring is the lightest of the springs supplied with the gold valve because it's a trials bike and you don't want much compression damping compared with an MX bike. Even with the lightest spring, I had the spring preload wound back almost to minimum.
Here's a series of photos to show the various differences between a standard damper rod and a standard damper rod that has been modified to fit the Gold Valve.
The important differences are that the modified damper rod has a lot more holes in the bottom end, has had the top machined off and has a big hole through the middle of the top end.

[Greg Harding]

Hi Everyone,

David, is it fair to say that gold valves increase unsprung weight? The standard twinshock springs are a little on the short side and I find that they need a fair bit of preload (tubes on top of springs) for my weight. So with gold valves, the preload spacers would be less and the springs simply ride higher up?

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While we are on , I just picked these two glamorous TY 175s up:

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Anyone got any theories as to why they sometimes come in pairs?
Up for swaps if anyone is chasing parts?

[David Lahey]

Hi Everyone,

David, is it fair to say that gold valves increase unsprung weight? The standard twinshock springs are a little on the short side and I find that they need a fair bit of preload (tubes on top of springs) for my weight. So with gold valves, the preload spacers would be less and the springs simply ride higher up?

Screenshot_20220928-062400_Chrome.jpg

Those are great questions Greg.
As usual the answers are more complicated than yes or no.
Q1
The Gold valves weigh something so by fitting them on top of the damper rods, they do add unsprung weight. The however bit is that there is a fair amount of damper rod metal removed to get it ready to allow a Gold Valve to work. I think to answer this properly I should weigh both arrangements.
To complicate things even more, some people replace the original steel damper rods with aftermarket aluminium damper rods when they install Gold Valves. I'm certain that this option would end up with less unsprung weight overall than standard, even without weighing it.
Q2
Yes if you use standard TY250 springs on top of the gold valves then the top end of the spring will sit higher. However, standard TY250 springs would be hopeless in combination with Gold Valves. Higher rate springs are needed to make up for the reduced compression damping that is a feature of Gold Valves. Increasing the preload on standard TY250 springs will increase their working spring rate once installed because they are dual rate springs but there's another however coming. Standard TY250 springs have only just enough travel (the sum of the gaps between the coils) available for the standard fork travel and standard preload spacer length. I remember doing an experiment years ago and found that if you add more than about 9mm additional preload to standard TY250 forks, the springs coil-bind before the anti-bottoming cushion can do its job. One benefit of the aftermarket springs is that they have much more spring travel available than the TY250 springs.
It might be interesting to weigh Magical TY250 springs to compare them with standard TY250 springs.

[David Lahey]

The new aftermarket TY250 fork springs from France turned up.
While the listing
https://www.francetrialclassic.com/en/f ... ki-kt.html
didn't mention the brand name "Magical", the springs in the listing photo looked exactly like the Magical TY250 springs I already had in a couple of bikes so I figured that they probably were.
The springs that arrived share all the important dimensions of theTY250 Magicals so they should be exactly what I wanted. They look a bit different because they are not zinc plated. I don't know if they are made by Magical or are exact copies of Magical springs. I'll be giving them a try soon.
Especially for Greg, I weighed one of the aftermarket TY250 springs from France and a standard TY250 spring.
Aftermarket "Magical" spring 295 g
Standard TY250 spring 280 g

[David Lahey]

Does anyone have input for which way up progressive fork springs should be fitted and why?

[Jools]

I thought it might be worth mentioning 'Mike's XS' damping conversions. I put a set in my RZ yamaha, and they transformed the handling. Much improved. It's a cheap conversion too. Not saying they are better than other brands, but certainly worth looking into.
(I have no connection with them apart from being a satisfied customer).