Hey guys. Here is what's going on.
I've got an extra S5 chassis I picked up, and I intend to make a reasonably powerful ( mid 400's) road race/ autocross car out of it. Ideally I'd like the chassis to be as stiff as possible. Seam welding seems like a good option, so i decided to bring it to the " experts" and get some opinions from those who race more than i do.

 

I'll be able to tell you exactly how stiff it makes the chassis in a couple months.
I'm building a torsional stiffness test stand for my FC. Hopefully the material will be here by the end of the week.
I'll take measurements before and after stitch welding.

 

Wow, that will be some interesting data!

Riz.

 

From a drivers point of view, my RX7 is a standard shell with an 8 point cage. It feels stiff and responsive when driving and you can feel all the various things going on with the car as you go around the track. My Cougar has been fully stitch welded and on track the car just feels like a single block of steel. Everything from the track is transmitted back to the driver.

Also, when I jack up the Cougar, everything stays working and aligned. With the RX7, I can see door gaps opening and closing about 1/2 an inch.

My advice would be to go for it as it will make a much more stable platform for the suspension and it will make the car noticeably more sensitive to the driver.

Eric

 

My stitch/seam welded FC is very stiff. I've driven a few FC's now, some caged, some not and I'd say the stitch/seam welded car feels much more stable and responsive. You lose the compliance between the front and rear suspension and gain a more predictable chassis.

-Trent

 

You can over do it very easily if you've got an overbuilt cage as well. A little chassis flex is not a bad thing, keep your stitch welding focused on high tensile points like strut towers and other structural parts that will benefit. Mild stitch welds, a nice cage and nice strut bars is more then enough to get excellent response and feed back out of an fc.

 

how about spot welds every 3 inches or so. That's basically just strengthening the chassis without compromising chassis flex.

 

Why would I want any chassis flex in a race car? Seems like the stiffer the better to me.

 

imagine trying to turn a reinforced "I"beam .... or here is a better example . ask yourself if you were building a sky scraper if you would want the building to be as rigid as possible or have some flex to absorb the wind or in this case stress in the chassis. it's the same basic principal, although there is probably a better way of explaining it.

 

A race car already has the ability to absorb stresses through it's suspension. And the beautiful part of it is, it can be adjusted to suit different conditions.
A flexible chassis is like an undamped spring connecting the front and rear suspension together. It is not adjustable or controllable and can throw off your tuning efforts.
So yes, I'll take the reinforced I-beam on wheels please.

 

The stiffer and more stable structure the better. Every constructor spends all their time trying to make a stiffer structure to bolt the suspension on to. As far as I am concerned, you can't have a too stiff unibody. Control the wheel movement with the springs, shocks and bars.

Eric

 

There sorta work out to being practical limits to how stiff the chassis needs to be to take its stiffness out of the equation... but the only downside to going "too" stiff is weight, while the downside of not having a stiff enough chassis is a bit more severe. And it's not like stitch welding the chassis is going to really add that much weight anyway.

 

Travis,

I wholly agree with you, with a chassis that has a 14 or 15000lbs per one degree of rotation, the driver can feel the effects of suspension adjustment, in my opinion it's better to go as stiff as possible. However have you ever noticed at a NASCAR race you see the pit stops with a guy and a big speed handle putting a whole turn of wedge in the car, go figure