1940 Chevrolet Cabriolet question

This is a question asked before but never got a clear answer so trying again in hopes new 1940 owners or historians here can?

The 1940 Chevrolet in general was a 1 year design but the Convertible was totally unique. Prior convertible models used simple side rail reinforcements but in 1940 they decided to use a form of X frame yet not quit. It had a second side rail stacked under the normal one up to the rear tire. The X was a massive version of the older YK design but also had a heavy solid rectangular plate almost like a skid. This was a one car-one year only design and wonder WHY? Was it an experimental design because it is not mentioned in brochures, service manuals, lubrication charts, nor even parts books like it never existed but of course it did. The following year they changed over to a more conventional X design for the convertible.

Any ideas why this was?

Here is a rough stitched composite of the under side of a 1940 Cabriolet and its frame in question.

Eric

It looks like cheap structural strength to me. A cheap fix to a problem they had that year and fixed the next with the new chassis. Maybe the model was rushed into production and perhaps the chassis was not really designed for convertible use.

The front members are wide and heavy on each side of the gearbox. The rear members of the X are narrow and taller. There is a (lateral) stiffness change and there would be a stress concentration where the rear X members join with the cross member, resulting in cracks. This way, they reduce the stress by spreading the load further back into the rear members, laterally into the cross member and forward. The will greatly improve lateral and sagging stiffness of the chassis. You might also find the rear and front members don't quite line up across the cross member.

Does the car sag? Are the door openings equal top and bottom?

It looks like cheap structural strength to me. A cheap fix to a problem they had that year and fixed the next with the new chassis. Maybe the model was rushed into production and perhaps the chassis was not really designed for convertible use.

The front members are wide and heavy on each side of the gearbox. The rear members of the X are narrow and taller. There is a (lateral) stiffness change and there would be a stress concentration where the rear X members join with the cross member, resulting in cracks. This way, they reduce the stress by spreading the load further back into the rear members, laterally into the cross member and forward. The will greatly improve lateral and sagging stiffness of the chassis. You might also find the rear and front members don't quite line up across the cross member.

Does the car sag? Are the door openings equal top and bottom?

Spinney... the front legs of the X you are seeing has splash guards on the bottom side. Here is a topside view of a frame being used or a street rod but the frame was not modified when it came to the X - which you can see isn't a true X but more like a 'H'.

And another picture of a car showing the second layer side rail.

I don't have a good picture of the 1941 but here is a diagram and a picture of a later model year of the same style but can see that the next immediate year it went to a more traditional x-brace system.

The front and rear inner members do not line up. There is no transfer of force between them. It must go into and along the cross member, which will apply a twisting motion to it. The plate provides for transfer of forces across the cross member between the front and rear inner members. It will also provide some lateral restraint, reducing the tendency for the long rear members to twist and buckle under compression (e.g. on a jack under the middle of the side chassis rails). The top plate just behind the cross member is above the universal. It could be to assist with prevention of buckling when the vehicle is on its wheels (top flange under compression), but also may be a safety guard above the universal in case it disintegrates and something goes upwards. The large cutout in the left rear member will also be stabilized a little by this top plate.

You see in the '41 chassis the X members line up across the middle, allowing direct transfer of forces.

The front and rear inner members do not line up. There is no transfer of force between them. It must go into and along the cross member, which will apply a twisting motion to it. The plate provides for transfer of forces across the cross member between the front and rear inner members. It will also provide some lateral restraint, reducing the tendency for the long rear members to twist and buckle under compression (e.g. on a jack under the middle of the side chassis rails; I would image they would always be in tension when the vehicle is on its wheels).

You see in the '41 chassis the X members line up across the middle, allowing direct transfer of forces.

It looks to be an afterthought design but what is puzzling is why this design when there were other GM cars already using traditional X designs on convertibles with no issue as well as other makes for 8-years, and why tool this heavy monstrosity for 1 year use only for this car then not even list it in any published catalog as existing under it?

Here is the 1940 Pontiac (often thought of as a Chevy sister):

Even Cadillac doesn't have an odd frame but traditional:

Sounds like a rushed last minute fix-up. Maybe they had no intention of making a convertible and someone decided they wanted one at the last minute? Maybe they got caught out by other makers making changes and rushed something out to keep a presence in the market place. "Sixty Years of Chevrolet" by George H. Dammann (revised ed. 1972) says "The in-out-in-again convertible returned this year, but as a totally different concept than before. No longer was it a 3-passenger car with rumble seat. It now was a modern full 6-passenger vehicle with trunk, and with all of the passengers enclosed when the top was raised." So it looks like a bit of flip-flop management with the engineering dept running to change direction and keep up. The new chassis was not ready so they cobbled the old one together for the heavier car - and no doubt learnt a lot in the process. They were attempting to move from the "old folks car" idiom into a style for the youthful, according to Dammann.

Did the other chassis you mention have different wheel bases and perhaps treads? So they continued with the old chassis design and beefed it up a bit. How many survive? It looks like they made 11,820 of them. They were the heaviest Chevs that year.

Have you seen any factory service bulletins? Maybe there is something there about problems keeping them on the road. What about company employee magazine or newspaper? Studebaker had one at different times, GM surely did?

Sounds like a rushed last minute fix-up. Maybe they had no intention of making a convertible and someone decided they wanted one at the last minute? Maybe they got caught out by other makers making changes and rushed something out to keep a presence in the market place. "Sixty Years of Chevrolet" by George H. Dammann (revised ed. 1972) says "The in-out-in-again convertible returned this year, but as a totally different concept than before. No longer was it a 3-passenger car with rumble seat. It now was a modern full 6-passenger vehicle with trunk, and with all of the passengers enclosed when the top was raised." So it looks like a bit of flip-flop management with the engineering dept running to change direction and keep up. The new chassis was not ready so they cobbled the old one together for the heavier car - and no doubt learnt a lot in the process. They were attempting to move from the "old folks car" idiom into a style for the youthful, according to Dammann.

Did the other chassis you mention have different wheel bases and perhaps treads? So they continued with the old chassis design and beefed it up a bit. How many survive? It looks like they made 11,820 of them. They were the heaviest Chevs that year.

Have you seen any factory service bulletins? Maybe there is something there about problems keeping them on the road. What about company employee magazine or newspaper? Studebaker had one at different times, GM surely did?

There is a Chevy site that has a lot of service related information but the bulletins are spotty and there was one that pictured the chassis related to body shims and others more on the top mechanism. Nothing to say why there was such an odd change to the frame design from the 1938 Chevy Convertible (which was void of an X but had strengthened side rails) versus the 1941 that was conventional except for the heavy cross brace running perpendicular trough the X. Was there any mention about the 1941 convertible that may give us a hint?

The convertible model for some reason skipped 1939 so the difference was between 1938-1940-1941. Makes me wonder why they skipped 1939 then mysteriously came out in 1940 with the oddball design?

July 1940 service bulletin - only place I have seen the chassis drawn out - not in service manuals.

1941 totally new, with wheelbases 3" longer than previously. All-steel welded bodies (my 1930 Dodge has that!), concealed running boards and the front seat was 3" wider. Convertible 125 lb. heavier too. They made over 15,000 that year. They probably put out a 1940 convertible because there had been demand in 1939, after there having been one in 1938 and 1937.

Hey, just wanted to pop this back to the top to see if there are any more takers on theories as to why this odd chassis configuration for the '40 Chevy Conv.?

1941 totally new, with wheelbases 3" longer than previously. All-steel welded bodies (my 1930 Dodge has that!), concealed running boards and the front seat was 3" wider. Convertible 125 lb. heavier too. They made over 15,000 that year. They probably put out a 1940 convertible because there had been demand in 1939, after there having been one in 1938 and 1937.

Spinney,

I will readdress this and ask… is it possible the odd design was due to as you said, a “rush” to get a convertible out the door? That statement in itself is a bit perplexing since they had an entire model year to think it out (no convertible for 1939) and even more so when you see that all of the other GM cars had a standard X-brace frame compared to Chevrolet’s back-to-back K design where the legs of the brace do not converge in the middle as a standard X does. And, the reason for the double-stacked side rails under the passenger compartment area due to using the standard flimsier than other GM car’s frame (not as deep of rails as other GM models)? The 1938 was basically a 2 seater instead of a 4 seater model like in ’40. Then the plate is also a puzzle. I have seen modified cars remove it to install larger engines and transmissions with no ill effects.

It just seems odd they did not utilize a X design or simply retool the chassis as they did with the body and changed the frame anyway the following year. They could have used the 1941 design on the 1940 model year. Or considered during the retooling of the 1940 body to fit around a modified Pontiac, Oldsmobile, Buick, or LaSalle/Cadillac chassis already in production for several years.

Just also wanted to add that the 1941 chassis frame is close to weight but only slightly wider in the rear than the previous year. The side rails are shallow being only about 4.5" tall for both years. Compared to 1940's Cadillac frame being 5.5" tall and 7.5" tall where the X meet in the middle (no idea what it is on the Chevy Conv as there are no dimension charts for it but not much different in height than the side rails). It is the Y-K design X for 1941 that makes the difference I believe.

Eric

just very curious, why are you so obsessed with the x frame. Are you an engineer ?

Wayne

I have always been fascinated with automotive construction. For about 23-years of my working career I have done just about everything connected with the automotive industry. But one "obsession" if you will is the X-frame. I don't know why but it has and I use to look under cars, buy models just to see what frames they had, etc... But for the past 5 years or so a colleague and myself have been collecting data related to the history and design of vehicle frames mainly X-designs for a book on the subject. The X design was used worldwide for quite a while and a great majority of automobiles post 1932 had them for the next 30 years. We found that there are restorers and historians alike interested in our venture because most marque books omit chassis information, skim over it, or have incorrect information. I want the work to be as accurate as possible and not simply go by hearsay. There has been a couple of chassis puzzles like the 1940 Chevy Cabriolet... a one year, one car only design not even listed in their catalogs.

I agree with Wayne.

OK, how long does it take to develop a new model? 18 mo.? more? When did they decide to make an all-new Chev for 1941? I suspect the planning would be 18 mo. to 2 years ahead. So once the 1941 decision is made, what do they do about 1940 (1939 already committed)? Rush something, like the new chassis? No. It probably would not fit under the older body shell. So they use the existing stuff and "facelift". Is that what happened to the other Chev models?

When was the first 1940 cabriolet made? Early in the production run? or later? If later, I would suspect there was buyer pressure to produce a cheap cabriolet and they kludged something together with what they had, to keep buyer loyalty and to keep their space in the market. They must have run it hard on the proving ground and made improvements to rectify problems found. The model sounds like an intermediate version, converting from 2 to 4 seats using the existing production as far as possible.

You say the plate is often removed by hot-rodders. Do they strengthen the chassis, such as box the side rails, when they do that? I would imagine the car will flex a bit more with a larger engine and heavier brakes, undercarriages and so on. How many miles have these hot rods done? Any done 100,000 mi.? Can you look at the chassis for cracks? I would be reluctant to "improve" something by removing something the factory put in. They never add things for fun - it has a significant cost and that is everything to the management, to the shareholders and to the buyers.

One more thing. Perhaps you would like to look into how GM ran its brand divisions. How separate were they and how much sharing was there, including in the design devisions? How do the cars compare in size, weight, performance and so on. Longer span (=wheel base) = heavier beam (=chassis) to mount the car on. How many of their brands had the same wheel base? When did they introduce all new cars in the other GM brands? I think there would be some corporate planning to roll out new designs across the brands in a controlled sequence and manner. Production costs and the market place (and their spin = advertising into the market place) must govern everything they do if they are to maximize profit. I think you are obsessing about this chassis in isolation. You need to consider it in the light of all GM brands and models and the planning and design thereof, as well as how they were competing in the market place. Responding to Ford and Chrysler, for example.

It would be useful if you including something about its structural design and why it was such as efficient design. In seismic structural engineering for buildings, cross (X) bracing is essential for strength. Look at post-earthquake photos of buildings, esp. masonry, and you will see X-shaped cracks between adjacent windows, for example. Masonry has little tensile strength, so when the wall deforms in-plane (top moves along relative to the bottom) one diagonal is in tension and the other is in compression. The one in tension cracks. The X-frame under a car operates in this way. The diagonals (Xs) stiffen the chassis against sideways sway of front relative to back. It also provides deep beams to resist sagging.

you are truly committed to the cause. Hopefully you will find someone with an equal or superior knowledge base to satisfy your inquisitiveness. Good luck !

Wayne

Have you approached GM or Chevrolet to see what historical records they have? I would imagine a board meeting would approve the production of the model after a 1 year hiatus. They may have the board paper about it and maybe even some engineering records. Good luck!

Have you approached GM or Chevrolet to see what historical records they have? I would imagine a board meeting would approve the production of the model after a 1 year hiatus. They may have the board paper about it and maybe even some engineering records. Good luck!

I will get back later on the other questions and comments but as far as contacting GM... I did contact the Heritage Center but they did not have anything in their archives about the 1940 Chevrolet Convertible. I don't really think they tried looking because I have seen on a post where they told someone the number of units of a certain paint code produced on the convertible. Any other suggestions along these lines?

And no, I don't want to dwell on one particular car since this is a small passage but what can be said in a paragraph could be condensed and fact filled. I just want to make 100% sure what facts are said are correct or why bother writing a book like this? So I appreciate ANY kind of feedback positive or negative because this is still a learning curve for me.

And yes, there will be a "technical" chapter giving the scientific reasoning behind the design. That is already covered.

And the 1940 Chevy frame is a box girder design so is stronger than the channel design and in 1941 Chevrolet claimed that the new frame was stronger than their competitors. It was 23% stronger than the 1940 model. And that the 1940 design did its purpose but was not as strong as the non X frame on a closed body so this redesign was needed. Dimensions was altered to accommodate the new body shape but was basically the same base perimeter frame.

I think I now answered most of the questions I was going to answer later - LOL.

Eric

OK, ask the historical centre different questions? Ask for board meeting papers, engineering dept papers, anything that might discuss the development and production of 1940 models. You can be specific and also try being non-specific. They do have production figures, coz they are in the book I have. Maybe even ask them how to find out what you want to know.

I think the reasoning on the frame design is more engineering than scientific. Science is the strength of materials, metallurgy and so on. Engineering is the use of mathematics and science and reasoning and judgement and technology to arrive at a design.

I thought you might like this picture of a 42 Buick.

Thanks Dave.

Spinney... Another thing I had found is that the 1940 itself was not rushed. It was a facelift and enlarged but so were the other GM brands. I can not find anything yet that says why for the design and it seems even more strange that Chevrolet head Marvin Emmett Coyle aka "Mr. Facts and Figures" was frugal with designs and did not even like the YK chassis introduced on the 1934 Master model. He had engineer James "Jim" Mark Crawford come up with a more traditional design almost as soon as the YK was introduced. The 1934 design was already planned by the time Coyle took the reins saying it was too expensive and cumbersome to work around but the current suspension design needed it.

I contacted the GM Heritage Center and asked the head honcho there Christo Datini if they had anything on the 1940 Chevrolet Cabriolet chassis? He responded back with page scans of a book that was published and sent to dealers called Engineering Features for that model year. But in true fashion about chassis-frame design mentions - it was barely so in the book. In fact it was so vague that anyone reading it would have gotten a misinterpretation as it basically said that the side rails were reinforced by steel channels that make the rails double box sections from the dash to the front leaf spring shackle in the rear. And continues to say it was reinforced in other points on the frame but doesn’t mention how or where? In fact the “double box section” is actually a U channel added to the bottom of the rails making it look like a double stacked frame which drops below the runningboard. Again, questionable and puzzling design plus the vagueness of the frame even in technical manuals???? He did send me a clearer diagram of the shim points on the frame that I found in a July 1940 service bulletin but it too is not 100% accurate. They made some changes on the actual car since it was drawn. I do have a couple of more leads courtesy of author Mike Davis who also contacted me but he himself had no information.

This discussion puts me in mind of the late '30s GM cost saving measure of shortened frames at the rear of the car.

the shortened frames resulted in sagging and warping of the rear body, trunk lids that were hard to open/close.

Perhaps, this fiasco was in mind when the reinforcing of the Chevy frame was thought of

This discussion puts me in mind of the late '30s GM cost saving measure of shortened frames at the rear of the car.

the shortened frames resulted in sagging and warping of the rear body, trunk lids that were hard to open/close.

Perhaps, this fiasco was in mind when the reinforcing of the Chevy frame was thought of

James... you must be referring to another oddity called the Rigid Transom X-type Frame used one year only and only on the 1939 Buick Special and Century models (40 and 60 Series)? They said it was to reduce overhang weight due to the rails but as you said it must have failed miserably. The body was supposed to have been altered to compensate and the gas tank placed in the overhang area. Here is what it looked like:

Eric

That's the one Eric.

That's the one Eric.

Like the Cabriolet frame this one makes no sense. Didn't they do testing and the math to figure out that an overhang without support especially where you have a 150 pounds of fuel in a tank and the added weight of luggage as well as steel, will eventually flex? I would have thought it would be detected on the proving grounds?

There was one other GM car that had a large plate in the X center area and I am going through all of my hundreds of frame alignment charts I have starting in 1934 onwards. It again was just one car need to post it here as well.

And don't forget that Chevrolet experimented with a conventional X design in 1935 on their Standard model and not sure why they didn't simply switch over to conform with other GM models but simply dropped any center bracing in favor of boxed side rails. That was a result of the Coyle demands for cost cuts that eventually led to Chevrolet not using a X in the late '30s until the mid '50s except for convertibles, unlike other GM cars. (not talking about the Y-K type).... The 1935 X was one year one Chevy car only again but wasn't an oddball design like the 1940 Cabriolet.

Did anyone come up with a definitive answer for why Chevy used this highly specialized and unique frame design for the Cabriolet and only on that one car, one year only?

Jim

A factor not often mentioned is the influence of the all steel body on chassis design. Up to 1930 or so bodies were usually open touring or roadster type, or  wood framed sedans and required a stiff frame for support. Luxury cars had steel channel frame rails up to  7" or 8" deep.

Then came the all steel, welded sedan body. It  was so strong and rigid  they could eliminate the chassis frame entirely - this was the first stirring of unit construction design adopted by a few makers between 1934 and 1942.

Others compromised, using the strength of the all steel body to reinforce a comparatively flimsy frame. By the late 30s typical  frames had gone from 6 or 7 inches deep, to 4 or 5 inches.

With a convertible you lose the strength of the body when you cut off the roof. All the strength must be in the frame, or in the sills or rocker panels.

Different makers got around this problem in different ways, by reinforcing the frame, reinforcing the body, or adding a sub frame between body and frame, or to the frame itself.

Convertibles were something of an afterthought. The bulk of production was sedans and coupes. Convertibles accounted for less than 5% of sales, in some cases no more than 1%. And some cars  did not bother offering a convertible at all.

It appears the frame you refer to was a rush job at the last minute. Given the small number produced, it was quicker and cheaper to reinforce an existing structure than design one from scratch.

It appears the frame you refer to was a rush job at the last minute. Given the small number produced, it was quicker and cheaper to reinforce an existing structure than design one from scratch.

Then I would have thought they would simply add a standard X brace that Cadillac, Oldsmobile, Buick, or Pontiac were using (standard I beams) adapted to the closed body Chevy frame rather than some random design?

Jim

It looks to be an afterthought design but what is puzzling is why this design when there were other GM cars already using traditional X designs on convertibles with no issue as well as other makes for 8-years, and why tool this heavy monstrosity for 1 year use only for this car then not even list it in any published catalog as existing under it?

Here is the 1940 Pontiac (often thought of as a Chevy sister):

248807.attach

Even Cadillac doesn't have an odd frame but traditional:

248828.attach

Even though Pontiac uses a "A" body, it doesn't mean the frames would be the same. In fact for 40-41 Pontiac can be had in the "A" body shared with Chevrolet and small Olds, "B" body with Olds and Buick and "C" body Cadillac.

Also the 40 Chevy may have a different frame for it's Special Deluxe and Master Deluxe, and a different one for the Master 85. Special Deluxe and Master Deluxe have independent front suspension while Master 85 has a straight axle.

Even though Pontiac uses a "A" body, it doesn't mean the frames would be the same. In fact for 40-41 Pontiac can be had in the "A" body shared with Chevrolet and small Olds, "B" body with Olds and Buick and "C" body Cadillac.

Also the 40 Chevy may have a different frame for it's Special Deluxe and Master Deluxe, and a different one for the Master 85. Special Deluxe and Master Deluxe have independent front suspension while Master 85 has a straight axle.

I think the chassis is across the board for Chevrolet except the Cabriolet in question. 

I am still baffled as to why they could not have simply inserted 4 I-Beams connected in the center and attached to the side rails rather than the random size odd pieces of beams, braces, and plates that they used to come up with a K-like design that makes no structural sense and even apparently as massive as it looks, did not do the job.  The design concept, testing, and history of working for 10 years of a simple X brace in-between side rails, should have been a no-brainer which is why I am wondering what they were thinking and why with the oddball layout they ended up tooling and only used on one year, one car, one model only?  They went to the more traditional X I am mentioning for the following 1941 year so, why didn't they do that for 1940?

Why does anybody do anything? The junior engineer assigned to quickly whip up a convertible frame did not have 70 years of hindsight. He had to work with what was available, and do something that would please his boss, who probably had his prejudices about chassis design dating back to the horse and buggy age. Said designer may have had a few whims of his own. And they had to take into account what the factory could do, and how fast and cheap they could do it. Have heard of odd designs made to fit the tools available, the materials available, and unnecessary restrictions imposed by management who didn't know what they were talking about.