I believe therenarena couple of model ic engines use this idea where one valve is operated above the cam and the other valve below the cam with a pivoted bellcrank

That must be done for economy of manufacture I would think as performance must be compromised to make this work. Not quite sure how the Dolomite Sprint engine achieved its performance if limited by this arrangement.

Mike

With strange bell crank cam followers that pivoted above the camshaft, one running on each side of the cam lobe. Some (earlier) models had two lobes, others (later I think) had just one lobe. Famously Sammy Miller's trials bike GOV had the single lobe cam.

Edited By Hopper on 24/01/2023 09:52:00

Edited By Hopper on 24/01/2023 09:52:15

Edited By Hopper on 24/01/2023 09:53:46

The Sprint engine is limited by this arrangement and by other questionable design decisions. While it is poor compared to its theoretical potential, it does make more power than 8valve versions of the same engine. Merlin V12s use a similar design, which is probably where they borrowed it from.

That must be done for economy of manufacture I would think as performance must be compromised to make this work.

Doesn't appear to affect performance too much, as this is a method used by Honda on current bikes – taken from a description of the latest Hornet 750 :

It’s instantly apparent that the new Hornet’s engine takes a leaf from the Africa Twin’s book, sharing a similar Unicam setup – where a single overhead camshaft acts directly on the four intake valves and opens the four exhaust valves via a set of rockers – to give similar performance to a DOHC design but with fewer components and a more compact layout. Honda’s CRF450R motocross bikes have long adopted a similar setup.

Doubtless having to make fewer parts will reduce costs as well.

Nigel B.

But don't the Hondas have separate intake and exhaust lobes, on one camshaft?

Dougie, I would no doubt use my CAD drawing package to work it out on the computer. But you can also use Tim's suggestion of old school Cardboard And Drawingpins

As well as positioning the two followers the required angle apart you may well need to make allowances for the fact the bellcranks move in an arc rather than pushing directly on a pushrod. The lengths of the bellcrank arms could also be tweaked if you wanted more "lift" on one valve than the other simply by having different lengths to each arm to reduce or increase the amount of movement. Diameter of roller followers may also allow some difference in duration for the same cam profile

Sorry, got distracted, and wrote about two cams (which is actually easier to design) – but the same principle applies if one cam is used, and the different timings are achieved by varying the shapes of the followers. Remember, too, that if you use one pivot for both followers, one follower will be backwards in appearance. In other words, one will lead the cam, and the other will trail – like the shoes in an old-fashioned drum brake.

One problem you might find with the 'one pivot' design is that if you wish to change the timing, you will be limited to changing the dwell (open period) for both valves by the same amount. And you won't be able to change the valve overlap usefully without a lot of compliaction, because both followers use the same pivot.

Books have been written on valve timing, so a paragraph or two can only hint at the complications – sorry.

Cheers, Tim

Dougie, the drawing of a JAP engine on this page should give you an idea of what sort of shape to cut your cardboard.

Quite a simple pair of staggered levers with the followers on pivoted from opposite sides of the cam, not sure if you are making a single or V twin but same would apply to either and the straight arms remove the complications of bellcranks changing the direction of movement.