2003 American Le Mans Series Torsion Bar Chassis

2003 American Le Mans Series
Torsion Bar Chassis

by Chris Briggs

For this years ALMS Proxy Race, I have decided to do a torsion bar chassis. The basis chassis design is a design that I borrowed from Larry Geddes. Larry has had great success with his torsion bar chassis in the Marconi Proxy races, so I thought I would give the chassis design a try.

First of all, what is a torsion bar chassis? Well, here is my understanding of how the chassis works. The torsion bar chassis is based on 2 main points, first a center pivot point running down the center of the chassis and second two torsion bars set on the either side of the center pivot point. The chassis is in fact cut into 2 sections, the front half holding your front axle carriers and your guide holder and the back housing your motor and rear axle unit.

As your car enters a corner the inside wheels want to lift off the track. This lifting takes traction off the inside rear wheel, with the torsion bar chassis the front wheels are allowed to lift but the pivot point lets both the rear tires stay flat on the track giving you better traction and acceleration our the turns. I am sure there is more to it than that but I don't have the needed knowledge to explain it properly to you. The one thing I am sure of is that this is one of the best handling chassis that I have come across in a long time. I also find that I can build my cars lighter and still get much better handling than my other chassis designs and as you can see in the above picture, the other nice thing about the chassis is it is not all that complicated to build.
You start out with your basic pan chassis and then make 3 slots down the pan and then 3 cuts to free the front and rear sections from each other, add the center rod and the torsion bars and you are done. Oh and before I forget you can make this chassis as a sidewinder, anglewinder or even an inline. For one of my cars in this years race I wanted to build a Ninco BMW V12.. I have the same chassis under my CLK and Panoz as well as all the other new cars I will be building in the future :-))

My Ninco BMW V12

Okay, lets get started....... The first thing I did was to get the wheels base set on my chassis jig....... Then all of my chassis start out as a 2" wide piece of 1/16" thick brass sheet cut to the length I need for the car I am making the chassis for...
The next step is to do all my layout lines with a scribe.... marking out the cut out for the front of the chassis where the guide will go...... I also mark out the center line of the chassis front to back...
After the layout lines are done I start by making up my rear axle carrier.... Now you can do the carriers 2 different ways...... the carriers on the top picture are pillow block carriers..... They are nice and neat but take longer to layout and get right....... For this chassis I wanted to save a little time.. So I used a L-brace to make up the rear axle carrier.... You can do the same for the front also... They do save time and are much easier for a new builder to do...

Rear axle unit with the L-brace for the axle carrier

To set up the rear axle carrier you need to cut a section of brass tubing that will fit the oilets you are going to use..... Then mount the tube, oilets and gear on the axle and place them in your jig...

You must use the gear or a gear of the same size on the axle to get the right set back for the motor..... I use brass spacers on the outside of the gear and the oilet on the other side to keep the carrier and gear centered on the chassis.... Oh and as you can see I have made a notch (cut out) for the gear to clear the chassis...
Once you have that done take a motor.....( I use a old motor with the right pitch pinion attached ) And set the motor in place so that the gears are meshing.. Then hold the motor in place and scribe around the motor to make the motor cut out...
The cut out will allow the motor to sit flush with the bottom of the chassis and help lower your CG of the chassis...
The cut out is the best way to go.... But you can just solder the motor to the chassis without making the cut out...

Hold motor in place and scribe around it to make the motor cut out

motor cut out.

Once you have the cut out for the motor done you can place your L-brace in position and in my case with the motor bracket I am using I have to cut a slot in the brace to accept the motor bracket...... Then I solder in the L-brace and the motor bracket...
Now all of my soldering is done with a propane torch.... So I always use metal gears and old motor to hold the motor bracket in place while I solder it in.
Again before you start soldering in the motor bracket make sure the gears are set up to mesh...

Finished rear axle carrier and motor bracket set up in the chassis...

At this point I am ready to cut out my front end so I can place my guide holder in position........ the cut out will very depending on how much room you have under the nose of the car...
I then mount my front axle carriers... in this case I used pillow block but to make it easier you can use a 1/4"X 1/16" thick brass stock bent into a L bracket to do the same thing...... (Like I did on the rear axle carrier)
I then make up my front guide holder and mount that to the chassis..... From here I use the center line I marked earlier and use a punch to mark out where I want to drill a series of 3/32 holes in the chassis...... The holes just make it easier and quicker to use the Dremal and cutting disk to cut out the slot needed for the center square stock I will be soldering into it...

By the way the guide holder is made of 1/2" wide stock...

For this chassis I have marked a line across the chassis where one of the cuts will be made to cut the chassis into 2 sections..... and the last 2 holes are on the motor side of the line.... And the cut out will be about 1/4" long...

center pivot slot cut out

After making the center line cut out I do the cut outs for the Torsion bars...
They are done with same way with a series of 3/32 holes drilled into the chassis up to the layout line I made for the cross cut mark..... (If you look closely you can see the layout lines running diagonally on each side of the motor cut out......)
The torsion bar slots are cut to a length of 7/8".. And are set in from the sides of the chassis 5/8" on center...
For the hard plastic bodies I am using I have found this length of slot (7/8") gives me just enough Flex from the 1/32 dia. piano wire I use for my torsion bars...
f you want a chassis that has more flex (twist) then you can make the slots longer. If you want less twist in the chassis then make them shorter...
Now that you have all the slots cut in the chassis.. The next thing to do is cut to length the square stock for the center pivot point... The square stock you want to use will fit a 1/16" dia. section of piano wire... Once the square stock is cut to length you want to solder that into the center slot..
After the square stock is soldered into place you want to make 2 small slots in the top of the stock.... about half way along the length what will be the front section and the shot rear section..
These slots will be used to put oil into the front section and to solder into the rear section..
With the center square stock soldered into place and the slots cut into the top of the stock, you now cut the chassis into 2 sections... I cut the short section that houses the center line first and then the 2 diagonal cuts next..

You now have the 3 slots cut in the chassis for the center pivot rod and the torsion bars

after the center square tub is soldered in.. You make the 3 cuts that will separate the chassis

After you have cut the chassis into the 2 sections it is time to add the 1/16" dia. piano wire to the chassis, to make up your center pivot point...
Cut the wire to length of the center tube, then put the chassis back into the jig. with all your jig wheels, gears, axles etc. attached...... This will make sure that everything stays in line...
Now the pivot rod will be soldered to the rear half of the chassis..
BUT before you do any soldering you want to make sure you put some oil on the front part of the rod...... This will help prevent the front part of the wire from accidentally getting soldered to the front half of the chassis........ YES... I did that the first time :-))
Now solder in the rear part of the pivot rod to the rear half of the chassis... When you take the chassis out of the jig the rear half should come right away form the front half..

From here put your chassis back in your jig, again with all the axles and things in place....... Again this will ensure that your chassis stays aligned right.
With the chassis back in the jig you can now cut and solder on the 1/32 dia. piano wire that will be your torsion bars...

For the most part your chassis is now finished.... You will still need to make the cut out for your right rear tire... And on mine I added a extra section of piano wire near where the right tire cut out will be for some extra strength.. And I added some brass rod to the sides of the chassis to get the chassis as wide as I could for the BMW body....
Well, that's it........ The chassis is not hard or complicated to build...... And is a really nice handling chassis..
At this time my CLK car holds the pole qualifying time for the up coming ALMS race that will be held March 1st. 2003... And my other cars with the same type chassis are close on it's heals..
If you have any questions please feel free to contact me by Email...

Chris..