Excel Torsional Stiffness

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DavidOliver
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Excel Torsional Stiffness

Post by DavidOliver »

While following postings on jacking points, which included a photograph of an Esprit jumping and suspension hanging, I
got to thinking about Body to Chassis fastenings and what element constitutes car torsional stiffness, Body or Chassis, and if
both how the connection points/bolts are effective. The Excel backbone is hardly torsionally stiff being a U section of thin sheet.
The Excel Body is very stiff but is it connected sufficiently to the chassis to provide front/rear torsional stiffness between the suspensions?
Hence these questions to Forumers,
1. How stiff is the chassis on it´s own?
2, Are the attachment points at each corner (just above the suspension spring towers) taking all the torsional loads?
3. For track does anyone plate the open portion of the backbone (to go from U to closed section)?

All of this is relative as to how we jack up our cars, whether garage car lift or garage floorbody jacking.
Opinions please.

Dave the cog.

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Re: Excel Torsional Stiffness

Post by richardw »

You're right - the chassis has little torsional stiffness. The body is an important part of the physical structure of the car, which is why you don't see (m)any 'specials' on an Elite or Excel chassis!

The loads are fed into the body via the mounting points - I guess most come through the points closest to the suspension towers.

No idea about your third point!

Cheers, Richard
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Re: Excel Torsional Stiffness

Post by Pete Boole »

The chassis is not a "u" section it is a box section with thick anti-lozenging plates at either end (even more in the Elite). The forward and rearward sections are inherently strong and stable as they are triangular in shape - box section at the front, flat plate with returned edges at the rear. It is similar to an Elan chassis which was praised at the time because of it's torsional stiffness. It's not like an Elise chassis, but it is not weak. The body and chassis work in combination to give the overall stiffness. The loads fed back from the suspension are shared by the chassis/body.

Pete

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Re: Excel Torsional Stiffness

Post by richardw »

Pete Boole wrote:
Wed May 06, 2020 14:17
The chassis is not a "u" section it is a box section with thick anti-lozenging plates at either end (even more in the Elite). The forward and rearward sections are inherently strong and stable as they are triangular in shape - box section at the front, flat plate with returned edges at the rear. It is similar to an Elan chassis which was praised at the time because of it's torsional stiffness. It's not like an Elise chassis, but it is not weak. The body and chassis work in combination to give the overall stiffness. The loads fed back from the suspension are shared by the chassis/body.

Pete
Interesting - thats not what I believed but I bow to your superior knowledge! I guess it's partly down to the definition of 'strong' - the complete car seems to be exceptionally strong - but would the chassis alone be strong enough to be used without a strong body?

Back to David's point, I'd certainly avoid jacking the car up on the chassis - not that there are many suitable places anyway
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Re: Excel Torsional Stiffness

Post by bash »

Just to be a complete pain, my understanding is that the Excel doesnt have a chassis.... its a subframe, like most other Lotii. Thats why elans can have a new 'subframe' and keep the same reg number. On the subject of specials, Mike taylors Micra and sunbeams have Excel 'subframes' which is why they are so good and why he can develop really quick bits for Excels.

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Re: Excel Torsional Stiffness

Post by Pete Boole »

You're right Bash - it is a subframe!

Pete

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Re: Excel Torsional Stiffness

Post by ianhateswork »

Reading these would seem to suggest a chassis.

https://www.european-aluminium.eu/media ... frames.pdf

https://en.wikipedia.org/wiki/Vehicle_frame

Whatever it is, its worth checking the brackets that hold the two together. Not made to the same angle as metal to fibreglass, and chassis/subframe
had been distorted as the two were bolted up, nasty.

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Re: Excel Torsional Stiffness

Post by DavidOliver »

For Pete Boole, the Excel is a U and the Elite is a full box. Surprising. I was!

When you jack the Excel at the front body point the effect of engine and partial gearbox weights are acting directly through the
attaching body/chassis bolts. Fibreglass is not good in shear. I only body jack at the front for wheel change or brake to-do, short time only.

At the rear there is only diff weight from the chassis, but imagine the load produced on the body by Spare wheel and Battery when
you ride a bump. Is this why Lotus changed the connection from horizontal to vertical bolts on later models, and is this a cause
of the frequent fibreglass fractures around the fuel pump area.

Consequently I would not like to leave my Excel suspended by a garage carlift at body points for any length of time, not overnight.

Regarding subframes etc., as the Excel has minimal torsional stiffness I believe we have two subframes separated by a backbone horizontal strut.
The body provides the stiffness between front and rear subframes, through the connecting body/chassis bolts. Worth a look?


Dave the cog

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Re: Excel Torsional Stiffness

Post by AndyC »

My first reaction is that the Excel subframe is a box but thinking back to engine swaps,the bottom of that "box" is very open perhas too open to really be called a box, it certainly has some bracing across the bottom between the two sides but not a lot, so it's probably closer to U than it is to a true box. Either way, I believe it's defined as a subframe in that the VIN is defined by the fibreglass tubs as mentioned before, the bit that carries the drivetrain can be swapped out for another without corrupting the VIN. There was a big piece of work by Lotus to get that agreed with the DVLA albeit many years before the Excel was produced.

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Re: Excel Torsional Stiffness

Post by Pete Boole »

Looks like we'll have to disagree on the box/u-section thing Dave! :wink:. The bottom of the "U" is completely panelled in - the bottom panel just has flanged access holes in it to get to the prop etc.. It only has one more hole in the bottom than the Elite, and I don't believe Lotus would weaken the Elite chassis to design the Excel. By extension the top of the chassis isn't complete either because of the handbrake hole in it, and that isn't flanged all the way round either. If you look at the design of air frames/early race cars they are similar - the anti-lozenging is done by bracing around the periphery of the open space, like the flanged holes in the bottom of the chassis.

There's no question though that the body/chassis/subframe work in combination.

Pete

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Re: Excel Torsional Stiffness

Post by Lotus-e-Clan »

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I cannot see an issue with suspending the car all day by the floor.

The subframe/chassis is bolted to the body at 16 locations (2 x 1 to 8). The mass of the power train acts as a unit (engine bolted to gearbox) which is attached at three locations within the chassis and cannot escape from a hanging chassis -so no danger there. The hanging chassis acts as a unit ...you couldn't easily bend it in half longitudinally and the mass of the drivetrain is certainly NOT capable of bending a suspended body-less chassis in half. The V-twin deep-section front beams in combination are probably at least as stiff - if not stiffer - than the single lifting beam on my engine hoist and I can't see the engine bay beams suddenly detaching themselves from the central backbone and, in any case, the gearbox end of the power train is attached to the backbone thus spreading the power train load along the length of the chassis. Even accounting for the extra mass/load of the suspension and wheels, a suspended body-less chassis isn't going to bend or shear at the central backbone.

I'd be happy to undo both locations @ 1 and 2 and stand all day under the car suspended by the floor of the body. :D

:idea: :?: Imagine undoing the remaining locations - at what point do you think the chassis would fall out of the suspended car? :wink:
Peter K

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