Body and Chassis Flex

[pls01]

I've heard of the Ace body cracking around the doors, but never heard the root cause or the accepted fix.  I've also heard that occasionally chassis welds in the rear structure can fail.
   
   I noticed a lot of movement in the door openings.  I did a thorough inspection of the frame, outriggers, differential structure and surrounding tubes.  I didn't find anything.
   
   It seems the chassis design has the most flexibility just in front of the rear bulkhead.  In this area, there are no bracing tubes, only the main chassis tubes.  This flexing may be a characteristic of the design.
   
   The Ace has only 24,000 miles and always stored indoors in a dry heated garage.  It hasn't been in the rain in 25 years, so there is no corrosion or degradation of the chassis structure.
   
   Is this normal?  Am I too accustomed to moderns?  Is it me or the Ace?

[A-Snake]

That's why leaf spring Cobras are referred to as "Flexible Flyers". The fix is not a minor one. It involves inserting tubing as internal sleeves in the chassis tubes. They are then ‘plug-welded’ along the length of the tubes.

[administrator]

I have heard it said that the chassis flex is also part of the suspension.  If you stop the chassis flexing the handling will change markedly.

[pls01]

A-Snake
   I understand the concept and technique of stiffening the tubes.  Do you know anyone in the USA that has done this?  I may do some calculations and modeling over the holiday to see how much additional wall thickness is need to significantly change the bending stiffness.
   
   I also wonder whether handling is more affected by chassis torsional stiffness than bending stiffness?

[nikbj68]

quote:Originally posted by pls01I also wonder whether handling is more affected by chassis torsional stiffness than bending stiffness?

   
   More stiffness? Why would you want to increase your annual tyre spend by having more wheels on the ground?
   
   (pic courtesy of Steve Gray@Brooklandsspeed)

[A-Snake]

quote:Originally posted by pls01
   
A-Snake
   I understand the concept and technique of stiffening the tubes.  Do you know anyone in the USA that has done this?  I may do some calculations and modeling over the holiday to see how much additional wall thickness is need to significantly change the bending stiffness.
   
   I also wonder whether handling is more affected by chassis torsional stiffness than bending stiffness?
   

   
   I know of one shop that has done more than a dozen leaf spring Cobras in the US. The original tubes are 3" OD with a .095 wall. The 'inserted' tube is 2 3/4" DOM with a .120 wall.
   Not being an engineer, I would guess the handling is more affected by the chassis torsional stiffness. [?].

[A-Snake]

quote:Originally posted by nikbj68
   
   More stiffness? Why would you want to increase your annual tyre spend by having more wheels on the ground?
   
   (pic courtesy of Steve Gray@Brooklandsspeed)
   

   Actually tire wear is balanced out by the additional downforce applied to the three tires that remain on the track [)]

[pls01]

A Snake
   What's the name and location of the shop that worked on the Cobra chassis?
   
   I'm looking at options.  Let the flex continue until maybe the body cracks.  Stiffen the chassis and avoid the cracking, but incur the cost and difficultly modification.
   
   Thanks
   Peter (BEX 375)

[bex316]

Peter,
   
   From what I've understood the body flexing (caused most probably by the chassis flexing) is more or less accepted as a niggle instead of a serious fault. The common spot where a crack can (will?) develop is where the door is nearest to the rear wheel aperture.
   My car has it on one side. The other side shows no signs (yet) but may have been repaired in the past. I haven't checked it accurately enough to be sure about that.
   Once the crack is there it doesn't seem to harm the rest of the structure too much. That's also what I have read others say in the past but some owners might have other experiences.
   I'm not sure if it's a successful modification to strengthen the body in the area where the stress crack usually appears.
   You didn't make it clear to me if it's only the possible crack(s) or maybe also the general loose/alive feel of the body that bother you. Or maybe even the road manners that might suffer from this phenomenon. Now it's difficult to make a verdict on that last statement until one has also driven a car that's modified in the way described before in this topic.
   However, you probably have an opinion about the quality of the road manners in general. Are you also looking for improvement in this department?
   As a long time owner of a few MGAs which have a quite rigid but also heavy structure I really had to get used to the Ace's general feel.
   
   Jerry

[tim isles]

Dear Peter,
   
   I've recently had to deal with this problem on my own car, BEX 313, which I am in the process of restoring. My own observations and actions are as follows - I'm pleased to report that flexing in the chassis resulting in the need to take drastic action to strengthen it was not necessary!
   
   First, there will always be very slight movement at the gap between the top of the door and the rear bodywork. This is healthy. However, in my case, holding the rear bumpers and lifting the car up and down by hand produced significant movement. This movement had been sufficient not only to crack the rear bodywork at the usual place on both sides, but in addition vertical cracks had also appeared where the rear bodywork wrapped around the cockpit rail and ran down to the door lock.
   
   I scratched my head for a long time as I too could find nothing broken in the rear superstructure, but it was clear to see that there had been 'work' carried out in two areas. First, at some time in the car's life repair work had taken place in the area of the rear shock absorber mounts as well as to the horizontal tube to which they are mounted. This is a known weak point on Aces that are hard used. In my car's case a poor repair had been carried out which allowed the tube to flex. A new tube, this time of slightly larger inside diameter, was correctly fitted. This , and this has virtually eliminated any movement at the door gap.
   
   My car had also been 'modified' in the area of the rear chassis outriggers, where the cockpit rail tubing runs down to attach to the outrigger. The original outriggers had been hollowed out to take a large diameter exhaust pipe on both sides, and consequently had lost most of their strength. New outriggers were made, again in slightly thicker gauge steel, and this has stopped any movement at this point.
   
   To complete the repairs the damaged areas of rear bodywork just behind and above each door were cut out and new pieces of aluminium were let in. The car looks as good as new, and the flex is almost undetectable - if there were none at all I'd be worried!
   
   To conclude I must thank Nigel Winchester of Winchester Motorsport for taking on the work, which was much beyond my skills. He did a superb job.
   
   I hope these experiences may offer some possible solutions.
   
   Tim Isles

[pls01]

Tim,
   Thank you very much.  The information is extremely helpful.  It enabled me to catch the problem before the body cracked.
   
   BEX 375 also has movement at the door gaps.  I first noticed it on the road and then lifting the bumper also produced movement.  The body has not cracked yet, but it is just a matter of time.
   
   The rear chassis out riggers are in good condition.  Actually, they are doubled in thickness and boxed forward and bottom to support a roll bar mounting point.
   
   I was looking in the wrong places for cracks in the rear of the chassis.  It looked like a chassis bending problem so the diagonals and longitudinal tubes were suspect.  I didn’t know about the weak point on the lateral tube for the shock mounts and wasn’t looking there.  It is cracked and the body must be working up and down.
   
   I would like to fix this with most of the panels in place.  Here are some options:
   1. Open the end off the lateral tube and insert and another tube.  The repair tube is about 7/8” outside diameter and ¾” inside diameter.  As well as welding the cracks, this would increase the strength and stiffness about 60%
   2. Open the end off the lateral tube and insert a solid bar.  This would increase the strength and stiffness about 140%.  It also makes closing the ends easier since the bar can be beveled on the ends for a weld.  The increase in weight is negligible.
   3. Box the area around the cracks with 1” x 1/8” plates fore, aft and top.  This increases the strength and stiffness about 250%.  All the welds would be down and horizontal, a great advantage in welding.
   4. Cut the vertical tubes at the outer ends.  Cut the top ends of the shock mounts and slide a larger tube over the existing tube.  This increases the strength and stiffness about 140%.
   
   I am leaning to options 2 or 3.  Option 1 is as much work as option 2 with less result.  Option 4 is a lot of work and potential for error with no better result.
   
   The entire tube could be replaced by removing more panels in the boot.  The structure would require bracing before removing the tube since it will sag and be difficult to relocate.
   
   Has anyone done a repair short of replacing the entire tube?  How was it accomplished?
   
   Thanks again Tim.  The information was most helpful
   Peter Stanwicks
   
   

[pls01]

Tim,
   After some investigation, I could not find anyone who has actually sleaved the main chassis tubes on an Ace or a Cobra.  It seems this repair is folklore or legend.
   
   On the other hand, the body flexing is a result of a broken horizontal tube above the differential just as you described.  While one wouldn't thing a traverse tube would add longitudinal stiffness, it does by transferring load to the diagonal tube shown in the upper part of the photo.
   
   
   
   In the end, I decided not to modify/fix the broken tube, but cut it out entirely and install a new 1" tube with 0.25" wall.  It's about 3 times stronger in bending and 4 times stronger in shear.
   
   Thanks again
   Peter

[bex316]

Peter,
   
   Maybe the repair (sleeving) of the main chassis tubes is not entirely folklore or legend or at least not anymore and we have the images to prove it as they say.
   Just watch the YouTube video of BEX308 being sleeved by the restoration company. It almost seems the mechanic doing the job wishes he hadn't started it or at least not by hand.
   http://www.youtube.com/watch?v=1wjuLlx20Tc
   Good to know in your case it was a more "minor" cause. Again I enjoyed reading your very informative article in ACtion.
   
   Jerry

[pls01]

Jerry,
   Thanks for the YouTube link.  The sleeving apparently isn't folklore, but when bodging something together, I wouldn't want to be filmed.
   
   I didn't write it up at the time, but the video shows the practical problem of sleeving.  In order for the sleeve to work, it must be a very close fit to the main tube.  However, the main tube is not perfectly cylindrical.  In many places it is distorted by welding minor tubes, brackets, outriggers, etc.  The main tube may also not be straight.  A close fitting sleeve will get partly installed and then jam.
   
   This mechanic is probably stuck unless he has a bigger hammer.  One method that might work: pull the sleeve into place.  Cut off both ends of the main tube and run a very heavy chain through it.  Run the chain through the sleave and fasten to a plate at the end of the sleeve.  At the front of the chassis, install a hollow hydraulic cylinder (Enerpac makes these) and pull the chain and sleeve tube.
   
   It may work, but if the sleeve get really jammed the chassis could buckle.  Another potential video opportunity!
   
   Thanks again. I enjoyed the video.
   Peter

[mike harrison]

I have some experience of Ace chassis flexing/body cracks and, perhaps, a cure for this phenomenon.
   
   When restoring my Ace some years ago, I was aware of the weakness and breakage of the transverse tube at the point of the rear shock absorber upper mounting, so I simply reamed out the existing tube and then sleeved it over its full length with thick walled tubing - no problems with body cracking since. Any “pop” rivets will need to be replaced by longer ones, as the existing ones will be sheared off during the sleeving operation and the new ones will need to be longer to compensate for the combined wall thicknesses of the two tubes.
   
   Regarding Ace main chassis flexing, this is inherent, normal and not normally a problem. Indeed, all standard Aces will have become "set" in their ways over the years with the minor permanent bending of the main 3" chassis tubes. Sometimes, there is an additional bend in the area of the engine mounting brackets, too.
   
   I had the chassis on my car straightened out on a hydraulic press during the rebuild to redress this, but the set recurred within a few months of the car being put back on the road (probably because I’m no lightweight, myself!) - this was evidenced by the gaps closing up around the doors, as well as the bend being discernible along the length of the chassis – check yours, I would suggest. To eradicate this, I decided to sleeve each tube with a 2.75" O/D 16 swg steel tube. Within this sleeving tube, and over it's entire length, I welded a 10swg steel web vertically at about 6" pitches on upper and lower sides. I made the sleeving tube about 3ft longer than the chassis tubes so that it could be hammered fully home without fouling the rear bodywork. A steel drift is needed to fit over the end of the sleeving tube, otherwise the end of the sleeving tube will crumple under the repeated sledgehammer blows. I prepared the chassis by removing the fuel tank and also the rear caps sealing the main tubes. To remove any corrosion and welding occlusions, I cleaned out the internals of the main tubes by using oil/grease to lubricate a "mop" made of carborundum balls attached to a long shaft/tube which was turned by a slow speed electric drill. A honing type action was employed.
   The greased sleeving tubes were then driven into each of the main chassis tubes in turn, keeping the welded web vertical by means of corrective torque being applied using a pair of Stilsons, or similar, during the hammering process. The sledgehammer force needed progressively increased as the sleeving tube was driven in. Despite the theoretical 0.080" clearance between the two tubes, it is not an easy task to drive one tube inside the other! I did work out the weight penalty of this mod, but I can't recall it now - but it wasn't that much, because I used a light gauge for the sleeving tube. I went down the sledgehammer route reluctantly, because drawing the sleeving tube through the chassis using stout chain or stout steel cable doesn’t work – I tried it.
   
   I also increased the torsional stiffness of the chassis during the rebuild by using aircraft type alloys for the cockpit flooring, transmission tunnel and footwells. There is also an extra crossmember between the main chassis tubes to accomodate overdrive gearbox - I left as much of the existing and redundant crossmember in place to improve torsional stiffness. Also, I made a cradle (inverted coathanger shaped) from MS channel section  to bridge the chassis just behind the gearbox bellhousing. This cradle is bolted and picks up on holes in the existing lugs (on the chassis on my car, at least) and is detachable for gearbox removal.
   
   The above information was the subject of an article written by me and published in Action around 1993/4 or so.
   
   When restoring my Aceca, I was compelled to use this sleeving technique again (plus collaring in certain locations) because of serious corrosion to the 3" tubes caused by the very poor storage conditions over 25 years whilst the car was left derelict. I also sleeved the transverse shock absorber mounting tube as previously described. Normally, sleeving the 3” tubes on an Aceca is unnecessary because of the thicker gauge material employed, but my car was exceptional because of the extensive corrosion throughout the chassis.
   
   In my opinion, both cars handle much better than their standard counterparts as a result of this work. The ride is slightly harder because of the reduced longitudal and torsional bending/springing compared to unmodified cars, but I prefer it! The sleeving of the 3” chassis tubes was also carried by myself on a friend’s Ace, with equally successful results.
   
   Some ACOC members may baulk at this technique, because it significantly reduces the flexible characteristics of the chassis, but I swear by it’s beneficial effect!
   
   Good handling is also maintained if the rear spring is prevented from skewing/slewing out of alignment with the chassis (which in turn skews/slews both rear wheels) – but that’s another story!