What Are The Rules For Twinshocks

[still trying]

Hi Smokey,

Like you I'm an engineer and decided to have a go at building a bike. As I didn't have access to large amounts of cash or high tech materials and machining systems needed to compete with modern machinery, opted to build a "special" that fits into the Twin shock catagory. as there is less need for these.

As I see it Twin shock class bikes should;

Have an air cooled motor

Have drum breaks

Have the rear shocks mounted directly to the frame and swing arm with little or no progression.

The limits I set were that the; motor, forks, and breaks had to come from the 70's bike, (in my case a KT250). Also the twin rear shocks had to be directly mounted to the frame and swing arm with no built in progression. That is, that the distance from the swing arm pivot to the center line of the shock doesn't change much, (about 3-4 mm in my case). This keeps the leverage on the shock constant, therefor no progression.

I have stuck to period available materials, and not use plastic petrol tank or carbon fiber for airbox, guards etc.

For those that buy an existing bike, the amount of knowledge and development available is huge, (including lightweight frames !!). How many people have changed the front forks or carbs to more modern units on their bikes?

 

You will have some advantage with hind sight but the development is all you. You will also get to choose how compromises are sorted (ground clearance Vs C of G, and seat height for example) I am going to have a big task sorting out the rear spring rates and dampening, which will last a couple of seasons probably. Also there is the risk taken with the steering geometry, which could make or break the whole project, (nervous about steep drops in my case. If the C of G is low enough I'll get away with it, maybe).

I hope that the locals are fairly open minded and reasonable for you. Good luck and enjoy the build.

Cheers Roger

 

 

[still trying]

Oops had a stuffup with my computer and thought I'd lost the draft and re did it, choose the one you like best, sorry 

[smokey125]

collyolly,

I think we are getting our wires crossed (the joys of the written word!), my plan was to take something fairly mundane and with a little bit of lateral thought turn it into something that keeps to the spirit of the class, is a little bit different and fun. This is the same approach I’m using on my current race bike build where I’m aiming to keep build cost down to about £1000.

Unfortunately I think the decline in young riders particularly at a club like Surrey Schoolboys goes way beyond the lack of cheap bikes. Personal opinion if anything is going to turn the fortunes of the sport round it will be improved electric bikes. Even then I don’t think it will ever get back to what it was like in the 70’s. Unfortunately I think the same applies to all other areas of two wheeled motorsport.

still trying,

That is exactly the type of bike I was talking about. Something that can be built with garden shed technology. Where you have to exercise your brain to solve problems not your wallet! I'm hoping people here will be as receptive to the idea as you have found where you are.

lineaway,

I'm not likely to ever do that, I never was much of a trials rider! Closest I will get was being given a lesson on how to ride by Sammy Miller years back. It showed technology is no match for skill!

[smokey125]

still trying,

Just looked at the pictures of your build, very impressive. I've got to admit I'd not been thinking that radically but that may be my view tainted by years of classic racing politics!

[john b]

Smokey 125

I have a Whitehawk 175 for sale.......

much cheaper, cost effective and competitive ...

just a thought

cheers

John

[still trying]

Hi smokey,

I've just hunted out your bike, very cool!!

It looks like it will be a lot of fun to ride, love the tank,

Did you make it yourself?,

And what is it made out of?

Your bike gave me a thought,

Perhaps we should all start riding pit bike trials.(groan not another class to cater for)

I've just been to a brilliant trial, where the organisers set out two trials basically. A normal one with 10 typical graded sections and 5 separate sections with only red and white markers in and out a small dry ditch and in, out and around trees in gentle sloping terrain for beginners and kids to ride. This allows the beginners space to ride without having to worry about dodging the rest of us.

The kids sections I could ride  in third and without walking. Bang on for those just starting, easy to read and not frightening. Anyway I'll stop dribbling now, Cool bike

[andy m]

http://www.trialscentral.com/forums/topic/983-whats-this-then/

 

http://www.bsaotter.com/the_mini_otter_project..html

[smokey125]

still trying,

Unfortunately this isn’t one that we built. My dad brought this from someone in the same industrial units that my dad’s business was in. He had built it for his son to start trials riding. When he outgrew it my dad brought it off him, and I started trials on it. We then sold it onto a friend of ours who also started trials on it.

From memory it was based on an S90 sport frame with the back section cut off and a new sub-frame fitted to give better angles for the rear units and to allow longer units.

The tank and seat unit (all in one) designed to look like a TLR from that time was in fact a dummy cover. The man that made the bike ran a business that made vacuum formed plastic parts, things like street light covers. So that was how he made the cover. The actual tank was a much smaller item that hid under the cover.

It had a C50 engine bored out to 75cc. The reason for this was the C50 had a 4 speed box. Well some of them did and that was the only engine of this type to have a 4 speed box. The guy that built it thought the extra gear was more useful than the extra power.

As a bike it was a lot of fun and it got several people stared in trials. The biggest problems with it were that the steering angle was wrong and it wasn’t good on tight turns and it was underpowered. I was 12 in that picture and that was my first event. It was also my brother’s first event he was 9 and riding a Whitehawk TY80. The land consisted mainly of two big gully’s it didn’t matter how much I wound the little Honda up on the downhills I couldn’t get it up the biggest of the climbs. Tried every gear I had but couldn’t get to the top. After we had finished I tried them on the Whitehawk and went straight up.

I would love to get that bike back. If nothing else it would be a great run around bike.

John b,

Thanks for the offer but not at the moment. The reason for asking the question initially was to see if the idea was worth pursuing and if so what of the various bits I have would be worth keeping for it. I'm likely to be relocating in the next 3-12 months so I'm getting rid of stuff and the moment rather than collecting but don't want to get rid of anything I would then replace once I've moved, equally I don't want to move stuff I'm not going to use.

Having done a lot more searching over the last few day the most interesting comments I keep seeing is how cheep people seem to think the bikes (twinshock and classic) are yet they are already well into the range of being more costly than an older modern bike. With genuine bikes already being modified beyond all recognition to gain a competitive advantage seemingly at very high cost I wonder how long it will be until it ends up like road racing with classic bikes being significantly more expensive than modern ones! This isn't meant to be an inflammatory comment but more an observation.

[rtm]

This thread got my goat up to start with but having looked at some of the pit bike engine budget machines I am well behind it. Brilliant stuff and in the spirit of the sport.