Soft top cars can be made stiff enough to handle well, and yet their rigidity comes from the floorpan which is say <12″ thick…

…how come they don’t weigh like twice as much ap a roof-ed car, which must have massively more torsional rigidity?

Soft topped cars do weigh more, and aren’t as stiff.

yep.

cars designed from the outset as soft tops suffer less. something like a golf cabrio will be heavier, handle more poorly, and suffer scuttle shake, when compared to the tin-top equivalent.
the only difference may be something like a tvr t350, where the grp lid doesn’t add much structural rigidity, when compared to the tamora. history now though.:(

RealMan – Member
Soft topped cars do weigh more, and aren’t as stiff.

I know that, my point is that the stiffness appears to be out of proportion to the weight gain.

Whether or not you have folding seats makes a huge difference as well.

BMW E46 Sedan (w/o folding seats) 18,000 Nm/deg
BMW E46 Sedan (w/folding seats) 13,000 Nm/deg
BMW E46 Wagon (w/folding seats) 14,000 Nm/deg
BMW E46 Coupe (w/folding seats) 12,500 Nm/deg
BMW E46 Convertible 10,500 Nm/deg

It’s all torques, moments, levers or whatever you want to call it.

Make a triangle out of bits of steel and keep it all within the sills, transmission tunnel etc and you’ll need a lot of steel to make it stiff.

Make it a big bigger and purpose design it to take all the loads and it can be much lighter (think TR etc, the chassis is a big influence on the layout inside.

Now make the triangles huge and part of the roof structure, it can be hugely light, think frerrari/lambo/noblel etc, they can be phenomenaly light and phenomenaly stiff. The ‘hypercars’ (FXX, MC12 etc) are very rarely open top for this reason.

Aye but why is my MX5 so stiff when it weighs < 1 ton?

It’s not.
If it weighs less than a ton it must be a mk1, and they are noticeably bendy in my experience.

Ah fair enough, not ragged many other cars TBH, felt better than a 3 series cabrio I drove tho.

Isn’t the transmission tunnel a strength member so it joins front to back and adds to the stiffness all at the same time?

must be

I read many Rover 2 series cabrios were returned to dealers in the 90s as they suffered from some sort of floating dash effect due to the ammount of body flex. Rover had some sort of if you’re not happy after 30 days bring it back sort of deal going on…

The Chinese seem to love Rovers except that they call the Roewe!

But a lot of soft tops are front wheel drive, so don’t have a transmission tunnel.

I drive a Transit chassis cab which is built onto a traditional girder chassis, even that suffers scuttle shake on rough roads.