Tubing Size??

[jay395]

Originally Posted by Talonmotorsport

The back end will break away even with 90-100hp even in the dry if you try to push the laws of physics, tyres that are too wide for the weight of car and running cold will not help this either. A Caterham runs 185/50/13 wheels and tyres for a car that weights around 600kg where as most roadsters run 195/50/15's?
I understand people wanting to run 150hp plus to have pocket money fun for super car thrills but how often will you be able to use it on the road, great fun on a track where you can open it up on the straights but when your being held up by the 4 cars and a bus?
If you want to improve the chassis in a quest for taking the power or better handling upping the chassis tubing to 2mm will add weight this will warm the tyres up more which is not a bad thing but only add marginal stiffness. If you look at the intended construction of a basic chassis it is ment to have sheet material bonded to the sides to act as added diagonal stiffness between the chassis rails, if you use GRP sides you rob the chassis of this benefit. Another thing to consider about the use of sheet material is the floor, if this is made in two coffin shapes they are free to move independent of each other. A floor that is one piece with the a cut out for the gearbox will act as the only diagonal between the near side rear corner and the off side front footwell.
Some well thought out tubular bracing added to a 1.6mm walled chassis will be lighter and just as stiff as a 2mm basic chassis, being lighter adds to the power to weight ratio and being stiffer than standard will add to any improvements made to the suspension. The only way to make the chassis stronger beyond the basic design is to add a full rollcage to bridge the large space that is the cockpit area.

i can get loads of the 25x25x2 mm box but not as easy for the 1.6 down here in sunny cornwall, i would have to order it all, would the extra 0.4mm really add much to the overall weight?

[alga]

It's very easy to calculate: a meter of 25x25x1.6 weighs:

25 mm ^2 - (25 mm - 2 * 1.6 mm)^2 * 1000 mm * 7.8 g/cm^3 / 1000 = 1168 g

For 2 mm thickness this is:

(25^2 - (25 - 2 * 2)^2) * 1000 * 7.8 / 1000 = 1435.2 g

Which is a 22% increase in weight. If we use 42 m of 25x25 and 14 m of 20x20, the increase in chassis' weight will be around 14 kg. Whereas the increase in bend stiffness of these tubes will be around 15% or so.

A much better increase in chassis torsional rigidity can be achieved by the "Aussie mods": triangulating the front frame, bridging the chassis under the gearbox where the bottom rail is cut out, triangulating corners of the engine bay.

[beardydave]

Originally Posted by Talonmotorsport

That all ways seemed silly to me cutting out part of a perfectly strong chassis design, would it not make more sense to make the chassis 50-75mm taller and leave it intact?

The problem with that is that you are then raising the engine and drive train by 50mm, and as that is the main weight in the car that will be bad for handling.

I want to have a completely flat underside to my car for aerodynamics as well as stiffness, but it's putting my tall engine 80mm out of the top of the bonnet. That is just from the gearbox which sticks out further than the sump at the moment, and will even further when i've fitted the dry sump.

[Davey]

In response to the original question it depends on whether you mean "how many BHP will make it undriveable?" or "how many BHP will break it?". Sadly the answer to both is "it depends". Undriveable is an unmeasurable concept, F1 cars are pushing 800ish BHP through rear wheels in a 600 KG chassis at 200 MPH but few of we mere mortals would be able to drive it at all.

As for how much power it will take to break it, again "it depends". How heavy will it be and rather than BHP look at the torque the chassis has to contend with. If there's lots of torque ( think of Robo's V8 beast or my own V6 Cosworth job, not massive power but serious torque) and you are using soft sticky tyres and driving it in anger on track then there is a good chance of something breaking, probably the rear diff/suspension area for a guess.

Just my humble opinion, D.

[robo]

Originally Posted by beardydave

I want to have a completely flat underside to my car for aerodynamics as well as stiffness, but it's putting my tall engine 80mm out of the top of the bonnet. .

80mm what is it a cummins truck engine?

Bob

[The V8 Files]

I'd be interested in seeing the triangulation options/pictures, has anyone done this? This might be something I should also consider.

[beardydave]

Originally Posted by robo

80mm what is it a cummins truck engine?

Bob

Saab 4-pot turbo. It's a very tall engine, and it has a big clutch for all the torque (260ft/lb as standard and 400ft/lb with a minor retune) so it needs a big bellhousing - the gearbox is a frankenstein's monster of Saab and BMW box welded together.