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Monocoque Design


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#1 blueprint2002

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Posted 25 June 2020 - 12:37

The success of the pioneer monocoque Lotus 25, and its derivative the 33, has long been the stuff of legend, and rightly so. But I sometimes wonder if the BRM P261, which of course followed the leader (as did Ferrari and others), has not been given the credit it deserves.

For one thing, they avoided the “bathtub” design, wrapping the outer shell all the way around ahead of the dash panel, and thus enclosing the driver’s legs, which is inherently both stiffer and stronger, hence safer. Restricted access to the pedal area was a small price to pay for such advantages, I should think.

Somewhat less obvious is the way in which the other end of the monocoque was also strengthened and stiffened. By arranging the V8 with intake ports between the camshafts, and exhaust ports inside the V, instead of outside as is usual, enough space was left for the “wheelbarrow arms” cradling the engine to be made deep and hence stiff. Which should have increased the stiffness and strength of the chassis as a whole, both in bending and in torsion. It would be interesting to see some comparative figures for torsional stiffness of the P261 vs the T25/33. As I understand, measurement of this parameter was not unusual, and, assuming a similar test procedure, such a comparison might be meaningful.

Tucking the exhaust pipework behind the driver’s head, instead of low down on either side, may also have given a small improvement in aerodynamic drag, considering the fairly narrow wheels and tyres of that era. Figures for this might be hard to come by, however, as I am not sure how much wind tunnel testing was done, and if so, whether the tests featured a moving ground plane and rotating wheels.

Anyone have any information on these? Thanks in advance.



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#2 Greg Locock

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Posted 26 June 2020 - 00:17

"assuming a similar test procedure,"

 

That's your problem. When  I did this with a customer I ended up having to FEA model the loading rig as well as the chassis before I got good agreement, assuming the rig was 'stiff' was a major error.



#3 blueprint2002

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Posted 26 June 2020 - 00:46

"assuming a similar test procedure,"

 

That's your problem. When  I did this with a customer I ended up having to FEA model the loading rig as well as the chassis before I got good agreement, assuming the rig was 'stiff' was a major error.

Thanks Greg, nothing like learning the hard way. I was just being cautious in a general sort of way, so it's good to hear, from one who knows, that it was justified.



#4 Charlieman

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Posted 28 June 2020 - 09:34

BRM had used monocoque principles when designing the original P57 which had a stressed skin for the cockpit zone. However it was a complex and expensive design, and BRM reverted to more conventional tubular construction.

 

Tony Rudd performed torsional tests for many designs, including an F2 Cooper. Reports can be found in the BRM volumes by Doug Nye. Racing and Sports Car Chassis Design by Costin & Phipps contains first hand data of the Lotus 25/33 design.



#5 blueprint2002

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Posted 01 July 2020 - 07:20

BRM had used monocoque principles when designing the original P57 which had a stressed skin for the cockpit zone. However it was a complex and expensive design, and BRM reverted to more conventional tubular construction.

 

Tony Rudd performed torsional tests for many designs, including an F2 Cooper. Reports can be found in the BRM volumes by Doug Nye. Racing and Sports Car Chassis Design by Costin & Phipps contains first hand data of the Lotus 25/33 design.

 

Thank you, C'man.

Luckily I acquired a copy of Costin & Phipps years ago, just have to dig it out.

I also acquired copies of Theme Lotus and The Autocourse History of the GP Car 1966-85, both by DCN. 

While I'd say the first was a good buy, the second was a big disappointment. Good coverage of certain cars, such as the 66-67 Brabhams. But in far too many cases, "history" consists of a tedious catalogue of who drove which chassis where, and how numbers were perhaps switched from one to another; this might be of interest to someone who is in the market for those cars, but surely is no substitute for the real thing. For example the section on Arrows; technically two of the most interesting, A1 and A2, merit little more than this..

Racing cars are machines, and a history must cover their design and development, even if they were failures, as well as how they fared in the races, of course.

Since then I have become rather wary, but I'd like your opinion of the BRM history, in this context, if you are able to spare the time.

Thanks and regards.