Removable backbone retro-fit

[Ophitoxaemia]

I'm rarely on this forum anymore, but I wanted to post here about a recent project.

I've autocrossed my Mark I car for over 12 years now (1400 runs and counting), and made extensive modifications to the suspension during that time. Increasing the spring rates has been part of that plan. After retrofitting the car with IRS in 2001, spring rates have gradually climbed to their current 750lb front and 1000lb rear.

However I've had a growing problem with inside right rear wheelspin, which worsened to the point the car would lift the right rear tire off the ground. Removing the rear bar and a low rebound setting on the shock did not help. The suspension does not bind.

I think my frame was bending enough to lift the tire off the ground. I called FFR to ask if I could have worn out my frame, and they did not think this was possible.

So I embarked this year on retrofitting a backbone. My backbone needs to be removable, as in my tiny garage, my only choice is to get the transmission out is through the top. And given the speed with which I go through clutches, that is fairly often.

We were able (with the incredible help of David Borden, his shop, and Dan Pellow) to get the lower welding done, with both forward diagonals welded directly to the motor mounts.

The rest of the tubing is relatively normal, but the top K section pieces are meant to be removable. My FEA analysis of the frame 12 years ago indicated that 90% of the torsional stiffness is lost without the K pieces.

However, there is some increase in bending rigidity, and so at the last autocross event, even though still missing key pieces, the car no longer lifts a tire, puts power down better, and has crisper turn-in. The ride is better on the freeway as well.

Who else has retrofitted a backbone into their early car, and what were your impressions?

[CraigS]

Glad to see this post James. I was afraid you had gotten away from your FFR and autocrossing due to life intervening. How about some pics. I would really like to see what you did as I followed Davids posts re: his MkIV chassis upgrades and did some of them to my MkII. Welcome back!

[MDR]

I would love some more details (pics would be great), I am currently "upgrading" to a 351w for my MK1, #1853.

[Wade Chamberlain]

I built a backbone for FFR2229K a few years ago out of 1" square stock. It made quite a difference in how the car works on the autocross course.

[ROADRACER83]

Ophitoxaemia: (I had to look that up to find out what the heck it means).

I'm not a stress analyst or chassis engineer, but I have had a lot of experience trying to make production based cars handle for a lot of years. (An American Sedan will twist like a Slinky without a good cage, sub-frame connectors, stess bars to the shock towers and firewall, etc.)

With help from my Boeing mechanical and stress engineer friends (cost me a lot of free beer and pizza) plus talking to some people who should know, I have a few insights that may help.

1. A ladder type frame will twist and the more load you put into it the more it will twist. Adding longitudinal reinforcements like the backbone will help some in bending but not much in torsion.

2. The more trianglulation you can add to a chassis, the more you can resist twist. Adding a roll cage with door bars, forward side bars to the dash hoop, a Petty bar, and bars going forward to the top of the shock towers will add a lot of torsional rigidity. Look at a FFR spec racer to get an idea. The FFR challenge car is about 50% stiffer in torsion than a roadster. This will add weight but it will ensure that when you make a suspension change you will not just be bending the frame. It will also help if you ever put that puppy on it's head. Since you are not racing in SCCA or NASA the tubing can be thinner wall, which still provides good resistance to twisting.

3. Softer springs and big sway bars will allow the chassis to work better than huge springs and stiff shocks when you are doing fast transitions in all four axes like autocrossing. They allow the suspension to load up in a progressive manner, instead of all the load going to the corner tire instantly and overloading it. That is true no matter if it is a front corner under braking or the rear under acceleration. Also adustable sway bars can allow you to tune the car much more easily to compensate for track conditions than swapping springs.

If you prowl around on Google for "Roll cage design" there are a few good sites that have some info on how to triangulate a cage or chassis.

Roll cage - Wikipedia, the free encyclopedia

Have a great season!

Ron

[SeanHM]

X2 on the pics. I'm very curious now.

[trevor]

the diagonals reduce the twisting of the front to rear sq tubes. Im not sure you could make it removable. If the ends were bolted it would be able to twist. It seems like the ends would have to be welded. Im sure Dave B. can figure it out.
Maybe if the ladder had a large flat plate on the front and rear. Each plate is clamped together with 4 bolts. I think that would work.

[Ophitoxaemia]

Trevor, the FEA analysis I did on the backbone was done with the connect point restricted in displacement, but unrestricted in twist. In my removable design, the removable elements are only significantly stressed in compression and extension. This should work sufficiently, because the small 1" tubing used in the design is very weak in bending, quite weak in torsion, and only strong in compression and extension. So I think I"m ok with removable elements in my design. I welcome evidence to the contrary!

Thanks for all the comments!

[Ophitoxaemia]

Here is a picture from the top.

http://thevenom.net/cobra/backbone/backbone_top.jpg

We were able to make the front cross pieces wide enough so that both the front diagonals tie directly into the bases of the motor mounts on each side. The rest of the design is very similar to that found on newer cars.

I can report that the car rides much smoother on the freeway, the door latch doesn't rattle on city streets, and it's been fast at autocross.

James

[Wayne Presley]

Hi James, glad to see you post and the box car is still running well.

[h82crash]

Trevor, I agree with James and his analysis, Diagonal bracing resists torsion by keeping the "boxes" they divide into triangles from collapsing. This done through resisting compression and extension.

I was thinking the same thing as Ron's #3 point. If the springs are too stiff they won't allow the tires to follow the contour of the road surface. I realize your journey to this point is based on changes you've made during all of your autocross experience but sometimes results to changes may be subtle until they are compounded and then we're surprised with something undesirable.

[tirod]

Over the years I've studied it, high spring rates were directly related to flexible chassis. And vice versa. Since the chassis is now stiffer, it's entirely likely that the spring rates could be dropped and more improvement seen.

Bolted connections aren't required to have play. The use of precision steel bushings bored and fitted with pins could be done, just like rods and pistons. If the joint only has .015" clearance, things tighten up quickly.

I see the point of the stiffening with a backbone. What I don't understand is why the transmission has to come out the top. Every production car I've change a clutch in drops the trans straight down and back, and they use a removable cross member. This chassis has none, just the rear trans mount, so it seems easier.

Having installed a top loader four speed more than once with it resting on my chest under a car, my perspective is taken from that view.

[Ophitoxaemia]

Transmission comes out the top because I have a very small garage, and I can't imagine how I could get anything that heavy out the bottom. From the top, I put a strap around my back and sling the tranny out.