Yamaha SR250 Twin Shock Trials Project

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Yamaha SR250 Twin Shock Trials Project

Postby gongon » Fri Sep 04, 2015 11:51 pm

Emigrated to Oz from UK last year.

Had several classic trials bikes which I sold before leaving UK

Difficult to find one here in WA within my budget.

Considering building one using Yamaha SR250 and TT250 bits as a base.

The SR250 has a compliance plate to prove it's age.

In UK can build pretty much what you like as long as the bike's parts are "similar to those available in the period"

I have in mind to build a slow revving 4 stroke single twin shock to emulate the 350 Royal Enfield I had in UK but to compete in post-classic trials ( using 1980-1983 parts)

I have the equipment and experience to do this.

Is there anything in the rules stopping me from building and entering such a bike to compete here.

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Re: Yamaha SR250 Twin Shock Trials Project

Postby David Lahey » Sat Sep 05, 2015 9:03 am

The rules in the MA MOMs are not clear on what you can and can't do as far as modifications go for Post Classic trials. They were initially written with a cut-off date (pre-Jan 1984?) intended to exclude the first TY250 mono, but later on the cut off date was made later (pre-Jan 1987?), to allow people to ride TLR250s, but there is no mention of how to view the use of parts taken from the TY250 mono. There are some post-classic bikes around that use TY250 mono front ends, and some with TY250 mono engines in twinshock frames. I've not seen anyone getting excited about things like that, but those sort of bikes are not usually seen at title events. If someone started winning state or national titles with a bike that was perceived to be a cheat bike, there might be a discussion start about bike eligibility. The only time I can remember a bit of controversy on the subject was in the mid 1990s when the Aussie titles was won by a rider on a modified Suzuki RL250. The rules for post classic at the time still had the pre-84 cut-off, and this bike had either the whole front end, or maybe just the front brake, from a (post 84) TY250 mono.
Your project sounds like good fun to me, especially if you use the SR250 motor. My dad had a red tanked SR250 in about 1983 and I remember thinking that the motor had decent flywheel effect

relax, nothing is under control

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Re: Yamaha SR250 Twin Shock Trials Project

Postby JC1 » Sat Sep 05, 2015 8:01 pm

I'm with David; it sounds like a fun project. I love this sort of ingenuity.

I had one once, it's not a bad place to start - low seat height, wheelbase 52.5", rake 27.5deg. And the frame is fairly light. Carb is a bit large tho (34mm).

Keep us updated if you go ahead.

"Men are never more likely to settle a matter rightly than when they can discuss it freely"

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Re: Yamaha SR250 Twin Shock Trials Project

Postby gongon » Mon Mar 07, 2016 1:29 am

Thanks for that.

In my mind, I'm OK as all of the bits are pre -1984

Smaller carb and maybe heavier flywheel should be easy enough as will lowering the compression.

The SR250 has an electric starter so that has to go I guess? Although may be an advantage considering my riding ability!

The TT250 has a kick start so I might experiment with the crankcase from that.

Needed a petrol tank and ended up buying a complete Suzuki TS185 for $50 (in bits of course but definitely built 1977 at the latest).

(So that's another project :-) total cost for 3 bikes $225!)

Hubs seem better on the Suzuki so may use them.

Have a slight problem now as my 67 year old Lathe I brought from UK is not yet fully commissioned (need to make some drive pins) but shouldn't take me too long to sort.

Been busy with the house but my wife has decided that I need some bike therapy (having retired and emigrated to Oz, my life is now so different so need to do something I know I enjoy) so will be getting on with it this week.

Initial progress will be fixing the lathe and dismantling the Yamaha motors (both seized) and then assessing what I've got.

Yes, I am expecting it to be a lot of fun.

There is no danger that it will end up as a national winner with someone like me on it so will always stay under the radar as a "fiddle bike"!

I watched the pre-65 scene unfold in UK (where I rode Royal Enfield and DOT and occasionally a Beamish Suzuki in "twin shock" class - wonderful times riding in the same events as my childhood heroes, Peter Gaunt, Jeff Smith, Mick Grant, Mick Andrews to name a few).

A lot of money was thrown at the big singles and they had mods such as post 65 fork internals in Norton forks, Suzuki flywheels and pistons, magnesium parts, replica frames, modern/foreign carbs etc.

You could buy a brand new Ariel HT5 replica for instance! A far cry from the original idea of "pre-65" to give riders the chance to drag out their old bikes for a bit of harmless fun - got serious again!

It meant that the old "classic" sections were then too easy so they made them harder and the bikes got modded again!

It became a nightmare for the organisers as (particularly in the pre 65 Scottish) they were reluctant to exclude the top riders (and therefore the crowd pullers) for using modern electrics and carbs etc.

They have, however "bitten the bullet" so things are changing but it's a delicate process as they don't want to spoil it competely as the "fiddle bikes" are so interesting as examples of engineering ingenuity.

They can still do mods like making flywheels etc. as that could nave been done in 1965.

That's never going to be the case for me. it will be genuinely pre 1983 and, if I ever compete, it will never win with me on board!

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