Britbike forum Forums Projects started by Britbike forum members Members bike projects Dirt/Beach vintage triple J.A.P drag racer

Hi all hope this os in the right place it's been years since I used a forum...

I'm building this triple engined racer for 1/8 mile beach and field sprints

Plan on boring out the 3 singles from 600 to 733cc each then supercharging with an Merc AMG blower

At the moment it site 4-1/4" ground clearence with 8" wide under rails

I am a little concerned this might be a touch low for the beach, I have seen people with 3.5" clearence race very well on the sand but this will be longer and heavier

I'm running a 140/90-18" rear tyre and will run either a 21" or 23" (xl500) wheel on the front

Any advice on frame geometry and structural integrity would be welcomed

[img]https://photos.app.goo.gl/J6jp3J866RdFosv46[/img]


I'll post more pics in this thread just want to check my linking to photos works first

Cheers

Interesting project. Are you planning on having the motors fire together or maybe close together to give better traction? If they were ,say, 30 degrees apart then the power would be delivered in a series of long pulses. And of course the beauty is that to adjust the angle between firing strokes all you have to do is remove the primary chain and replace it in a different position.

I can't Google it up, but my defective memory tells me that there was a twin J.A.P. engined bike at Bonneville in the mid 60s in which both engines fired at the same time. I think that the rider was Don Chapin.

Thanks I'll have a Google see if I can find more info on the double jap bike

I was planning of firing them together to start with as this will be least stress on the joining chains and easier to regulate boost, then I'll try moving the sprockets a tooth at a time like you say to.see how they work spread out 30° 120° and even maybe 240° firing


I also have the issue of the single supercharger supplying 3 engines one after the other so boost may not be even
I'll try to compensate for this by having a decent size plenum space

As the last thing you need on sand is a front brake consider a speedway/grasstrack front wheel
Firing all 3 engines together will be brutal on the supercharger drive, just one engine with 28psi boost on the Weslake I used to race would trash a 30mm HTD belt in 1/4 mile, I switched to twin V belts and could get about 5 runs before they shredded.
Firing the engines evenly spaced will improve matters as there will be less pressure fluctuation in the manifold/plenum, cylinder filling will also be more equal between engines

Thanks Andy that's interesting hadn't even though of the blower belt wear, I run a single V on my little amr300 blower feeding my 600 JAP single so might look at a double version of that set up

I'm still bending in the frame tubes and shaping the axle plates at the moment this I sthe latest pic (it doesn't look aligned at the axle plates are where the tube wants to be)

https://photos.app.goo.gl/RGqfw5g1wxVqcZcFA

Also I believe we are linked on FB

https://photos.app.goo.gl/SqiLfLjeYjYiKEkG8

Finished the axle plates all the tube is bent and notched

My homemade bender has been great working 1-1/4" tube bends to repeatable precision

Just have to trim and tidy up the excess tube at the rear and will tack it up for a sit on

Above is the link to the album will keep updating it

Just to clarify my point, a 600 JAP SV engine has 261 degrees of inlet valve opening and therefore 459 degrees closed
While the valve is closed pressure will build up in the inlet manifold. increasing the load on the drive, the higher the pressure the higher the load. when the valve opens the pressure drops and so does the load. This causes vibration and snatching in the drive.
If you time your engines with equal firing intervals of 240 degrees, there will always be an inlet valve open so there will be a lot more even pressure in the manifold. The mixture will always be travelling along the manifold instead of stop start you will get with engines timed together