So having spent a lot of time talking about centres of gravity, forces, rake & trail it perhaps comes as no surprise that we might like a nice stiff structure to maintain the shape of this machine.
This is where the frame comes in. Of course it has a lot of other jobs to do but what it is mainly about is:
So, stiffness is the name of the game. Look at this:
Marvel at the thickness of those two pairs of tubes bracing the steering head and connecting it to the swinging arm mounting at the back. Notice the cross bracing between those tubes and the cast & bolted structure in the centre – it’s huge. Then look at that swinging arm – those square tubes make a very stiff structure as well.
Now look at this:
That’s a swinging arm Bantam frame. It is a bit of an unfair comparison, since this is a lightweight commuter and the modern frame is a fast, high capacity sports bike. The point is not to compare modern design with 60’s design, but rather to emphasise the stiffness that is important in a frame.
You can see the headstock, where the forks pass through, with it triangular bracing. Whilst upright this will be relatively stiff, but because the tubes and headstock are all in the same plane it will bend when cranked over – hardly a problem for a Bantam, but this deign would see the wheels out of line on a bigger bike. See the relatively large tubes that go down to the engine front mounting and along to where the seat front is mounted, then curving and dropping down to the swinging arm. This, combined with the bracing effect of the engine, stops the frame bending in the middle – the modern frame is a much stiffer beam, with those big tubes and all that cross bracing.
Notice the lighter structure that supports the seat, and the top of the suspension units. Functional, but the modern frame with its single suspension unit is much lighter, because it doesn’t have this structure, and stiffer, because the rear suspension top mounting is attached to a very stuff area and not to a flexible outrigger.
Then look at the round section (cheap & easy to make) swinging arm and its mounting at the front – a simple tube welded across the main part of the frame. Horses for courses.
For completeness, here is a D1 Bantam frame:
Not dissimilar to the D14 frame, but more rigid around the rear end since the rear wheel is more rigidly mounted – there is no weakly supported pivot for the swinging arm. This particular example is from a plunger framed Bantam.
This one is a rigid frame, from a heavyweight Ariel:
Notice the strong triangular shapes around the steering head and the rear end, and the cross bracing at the rear. This was the state of the art for the rigid frame, built in the early 1950’s as manufacturers started to experiment with suspension systems.
This is where the frame comes in. Of course it has a lot of other jobs to do but what it is mainly about is:
- Keeping the wheels inline
- Keeping the steering head in the right place
So, stiffness is the name of the game. Look at this:
Marvel at the thickness of those two pairs of tubes bracing the steering head and connecting it to the swinging arm mounting at the back. Notice the cross bracing between those tubes and the cast & bolted structure in the centre – it’s huge. Then look at that swinging arm – those square tubes make a very stiff structure as well.
Now look at this:
That’s a swinging arm Bantam frame. It is a bit of an unfair comparison, since this is a lightweight commuter and the modern frame is a fast, high capacity sports bike. The point is not to compare modern design with 60’s design, but rather to emphasise the stiffness that is important in a frame.
You can see the headstock, where the forks pass through, with it triangular bracing. Whilst upright this will be relatively stiff, but because the tubes and headstock are all in the same plane it will bend when cranked over – hardly a problem for a Bantam, but this deign would see the wheels out of line on a bigger bike. See the relatively large tubes that go down to the engine front mounting and along to where the seat front is mounted, then curving and dropping down to the swinging arm. This, combined with the bracing effect of the engine, stops the frame bending in the middle – the modern frame is a much stiffer beam, with those big tubes and all that cross bracing.
Notice the lighter structure that supports the seat, and the top of the suspension units. Functional, but the modern frame with its single suspension unit is much lighter, because it doesn’t have this structure, and stiffer, because the rear suspension top mounting is attached to a very stuff area and not to a flexible outrigger.
Then look at the round section (cheap & easy to make) swinging arm and its mounting at the front – a simple tube welded across the main part of the frame. Horses for courses.
For completeness, here is a D1 Bantam frame:
Not dissimilar to the D14 frame, but more rigid around the rear end since the rear wheel is more rigidly mounted – there is no weakly supported pivot for the swinging arm. This particular example is from a plunger framed Bantam.
This one is a rigid frame, from a heavyweight Ariel:
Notice the strong triangular shapes around the steering head and the rear end, and the cross bracing at the rear. This was the state of the art for the rigid frame, built in the early 1950’s as manufacturers started to experiment with suspension systems.
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