Before the internet came along and I couldn’t get my daily fix of bikes and breasts online, you’d quite often find me with my head in a magazine. If I wasn’t pulling out the centrefolds for masturbatory purposes, I was ogling over the latest and greatest superbikes; dreaming of the day I’d be old enough (and rich enough) to have one. I’d memorise power figures, 0-60 times, kerb weights and compression ratios (what a loser). But one thing in the motorcycle specs list I never gave a toss about was ‘frame type’. It didn’t seem to matter what shape the bit of metal holding it all together was. As I got older and (not very) wiser, I started to realise just how important it was. I started to understand just how much that ‘bit of metal’ could affect the way a bike feels and handles.
For years though, I still found the whole subject a bit complicated. With all the different types, how was I to tell the difference. I’m guessing I’m not the only one that didn’t know one from the next, so to clear things up here’s a brief explanation of the five main types.
This is the kind of thing that loads of bikes used to use in the olden days. It’s called a cradle frame because it, quite simply, cradles the engine. They tended to be made out of heavy steel tube, with one or a pair of tubes looping underneath the motor. They’re fairly cheap and easy to make but quite heavy at the same time. And whilst they’re stiff enough for old bikes with put-put engines making barely any horsepower, it would probably end up bent and twisted double quick if you put a modern superbike engine in one.
This motorcycle frame can, and does take superbike engines. These days, most are made from aluminium (although the BMW HP4 Race has got a carbon fibre one), and you see them on most conventional sportsbikes. It’s where you have two spars, or beams, curving round the perimeter of the engine. They go from the headstock at the front, to the swingarm and the subframe at the rear. Some use the engine as a ‘stressed member’; but even the ones that don’t are capable of dealing with everything that a 200bhp superbike can throw at it. As a bonus, you get cracking access to the top of the engine. If you’ve got a big inline four-cylinder engine in there though, they can sometimes feel a bit wide.
These are a bit like a twin spar frame, but rather than using wide beams, use steel tubes cleverly welded together. Thanks to their design utilising the strongest shape in the world, the triangle, they’re nice and stiff. They are fairly light and dead strong, so great for racing. But they are a bit of a pain in the arse to manufacture on any sort of scale. Harris and Spondon are really good at it and KTM were using one in their MotoGP bike up until fairly recently. It’s Ducati though, who have made the trellis frame famous. Not many of the major manufacturers still use them much these days; much to the dismay of Ducatisti, the world over (the latest Monster upset them by not having a trellis frame).
The monocoque is something that’s been brought over from the car world. The idea is that you have a big hollow box, with all the vehicle’s parts within it. It doesn’t really work the same way for bikes; so what you end up having is big bulky boxes with airboxes, throttle bodies, looms etc. running through them. Ducati like to call their Panigale frames ‘monocoques’, although they’re not exactly that, in the true sense of the word. In a true monocoque chassis, all of the forces are transferred through the outer shell of the frame, and all of the suspension components are attached directly to it.
This is about as simple as a motorcycle frame can get. A big, fat steel (usually steel, anyway) tube runs from the headstock, over the top of the engine to the subframe mount and the swingarm pivot. The engine is bolted to the backbone and quite simply hangs off it. They’re not very stiff, so the engine has to be fixed to it really well. In fact the engine is quite often used as a load bearing part to help with the bikes overall stiffness. You don’t see many modern bikes with these kind of frames, apart from smaller bikes without too much power.