Hi all:

I'm in the final stages of my IRS chassis. I'm really just cleaning everything up and checking over my work. There are a couple things I need to finish.

I realize the book tunnel is contributes little to the overall stiffness. I'm using an IRS, though, and I think mine will be much more highly stressed. Instead of the 3/4" tube for the tunnel--like the book calls for--I used 1". I want to put some diagnal bracing in to 1.) limit damage should the driveshaft snap and 2.) for additional stiffness. Initially, I planned on using the 3/4". Hoever my chassis is geting a bit portly @ 125 lbs. If possible, I could go to 1/2" 16 ga. However, if will add no stiffness or protection, then it is a no go.

Anyone have any suggestions as to the material to use?

--Thanks, Chris

As you say, the tunnel adds little to the stiffness of the chassis- look for Cymtrics posts on the subject of increasing chassis stiffness. Apparently a welded in sheet is better than what is in the book and is lighter too.

As far as the prop snapping- well... I'm just going to cross my fingers!

James

You used no diagnal bracing then? Perhaps I worry too much!

Chris,
My intention is to buy a brand new propshaft from AutoProp UK (http://www.autopropuk.co.uk) which has been professionally built from brand new parts to my specification. I have been quoted £96+vat for a propshaft for connecting a Rover V8 to a Sierra diff about 750mm long.

Isn't it better to immunise than to cure the disease???

Besides, if the shaft does break then on my chassis it shouldn't be able to go very far because my floor goes right underneath the tunnel as well and will support the broken shaft and hopefully stop it from contacting the ground. I can't see any reason why a shaft of this length would break in the middle and thus having a diagonal in the middle of the tunnel would be of little consequence.

I hope this puts your mind at rest.
Cheers,
Craig.

Of course I will do everything I can to prevent a walkabout. Even so, I would rather not be disemboweled by a propshaft. I'm also hoping to add a bit of ridgitity, though I admit it may be in vain.

How often do props fail?

two hoops like the drag race boys do round the ends keeps it in place should it go walkies>>>>>>

Hi Chris

I braced my tunnel to match the sides.
I plan to infill with triangular padded trim
so that the chassis is still visible, being
stainless. A "bits & bobs" tray will form
the top, maybe with a hinged lid.
This braces the dif, something I felt was
nessesary with an IRS layout.
Have a look at my photo archive.

The cars a +4 so I am able to have a wide
tunnel, need it for the gearbox anyway,
to give me and he or she who dares,
elbow room when driving.
Didn't plan any of this, it's just turned out
this way.


Paul G

It all depends on the design of the rear bit.

On a live axle set up very little is required of the tunnel unless the lateral axle links feed loads into the tunnel region as on a Satchel link. Check a Caterham. All the rear suspension links go to the chassis sides and the tunnel is very light. Only three 3/8 tubes support the cables and wiring. If you use a welded in floor, a fully panelled and welded tunnel and use 1 inch tubes for the back of the tunnel I reckon you can still get away with no additional bracing for a live or Deon set up with any reasonable link arrangement.

Regarding double wishbones I'd put some triangulation in around the rear suspension box. (I'm assuming that you've done something similar to the westfield design) The top and bottom of the box, i.e. above and below the diff, need a brace of some sort. Obviously you can't fit a diagonal through an axle and you need to get the diff in some how but a V brace might fit on the back of the box. Again, as for a live axle car, if your tunnel is welded up into a big hollow tube and you have a welded in floor you should be OK.

If you doubt the strength of steel panels look at the original Elan chassis. It is a backbone of sheet 1.2mm steel welded into a 10 inches by 6 inches rectangular tube. All the loads are fed along this tube.

On the topic of the caterham chassis, i believe comments may be about an older design. The chassis i have seen in recent years splits the tunnel into almost square bays of triangulation , towards the forward 60% or so at least.

Buying a new prop does NOT mean your axle or engine cannot seize for example, causing a prop shaft to get freindly with your lower limbs. Unlikely yes but for the sake of a bit of tube???

As cymtriks suggests welded steel panelling is an option. i went for braces and removable skins for practicallity sakes.

Surely the switch from 3/4" to 1" was not done in vain. The FEA thats was done has some very convincing points, but I too am using the removeable pannels.

The read suspension 'box' has bracing all over it. I'm having trouble getting lower bracing in the box and allowing the diff to be removeable. I may try to use removeable ali honeycomb to brace the bottom. The honeycomb is not really in the cards for the tunnels sides though.

So what are the thoughts on the 3/4" diagnal bracing on the tunnel??

Stressy,
It's only the diagonal tube in the middle section of the tunnel which I didn't fit. I still have two complete loops of 20mm x 2mm square section at each end of the propshaft which should keep the ends of the prop in check even in the very unlikely event of a breakage.

I think if the engine seizes then the most likely thing to happen is that the rear wheels will just lock up. If the axle seizes I'd expect it to just stall the engine. The propshaft I am buying is rated to over 400BHP and I'll only have about 170BHP and a car weighing about 750Kg's. I think my propshaft will be just about the strongest thing in it!!

Cheers,
Craig.

Hi craig,

I have actually got my gearbox mounted so that the gear change is in what you might describe as the middle section of the tunnel. Therefore in my case the front joint of the prop is level with my knees!

I know of somebody who had a diff seize resulting in shearing loose the fasteners on the prop, leaving it slapping around in the tunnel, thies is why i have c section seatbelt mounts come prop gaurds at the hips.