Lightest Cayman S
Discussion
grantsfo said:
If I go to smaller wheels and tires than 18x11 with 335/30/18 DOT R in favor of 16x10 with slicks and go for CF roof, fenders and rear quarters I'd get this car to 1000 kg no problem. For steel chassis car the cayman is very efficient once you get all crap off the car
With 350 HP 3.4 liter 6 cylinder my car is becoming very quick!
Yes but you are lumbered with all the stiffening (in the wrong places) needed for the Boxster, which probably costs 50-75kg (maybe more) - or have you managed to cut some of that out?With 350 HP 3.4 liter 6 cylinder my car is becoming very quick!
anonymous said:
[redacted]
I suggest you take an empty cardboard box (of say chocolates), hold it at each end and twist. Try it with the lid open and then closed. You will find it is vastly stiffer in torsion with the lid closed. That is why closed cars are almost always stiffer or lighter (and often both) than their open counterparts. In the case of the 981 the Cayman is (needlessly)over twice as stiff torsionally as the (adequately stiff) Boxster - and inevitably heavier than it need be.
It's why when you take a saloon (almost invariably designed as a closed box) and make a convertible out of it a lot of extra weight (50-150kg typically) is added to make it stiffer - but it's rare that even then it comes close to the stiffness of the saloon.
A few examples coupe vs convertible
BMW 2 series 1415 vs 1570kg
BMW 6 series 1825 vs 1895kg
C Class MB 1605 vs 1735kg
You won't find the reverse - except when a metal hardtop is added to a car designed as a roadster.
Porsche could have chosen to make the Cayman lighter than the Boxster - but, presumably for development cost reasons chose not to.
You acknowledge in a previous post relating to the Alpine that you don't know why the Cayman is so heavy - it's because of the degree of commonality with the Boxster. The fact that the Alpine is designed as a coupe only is a major part of the reason it is 200+kg lighter than a 718 (yes the fact that it's all aluminium obviously helps - but a Cayman is 44% aluminium according Porsche)
Just to give you an example - the Alpine would almost certainly have had a closed "transmission" tunnel if it had been designed as an open car - the transmission tunnel, is a major contributor to torsional stiffness in most open car designs.
I could go on - but please start with a chocolate box and (I sincerely hope) you will appreciate how important the roof is.
anonymous said:
[redacted]
As often the case, your example to support your case, nobbles it! You omitted to notice the VAST (much criticised) sills - you can create stiffness with vast (wide) sills (and keep overall width in check) a big transmission tunnel or a modicum of both. Or let torsional stiffness go hang - plenty of examples of that. Did you not notice that the Alpine manages without either a "transmission" tunnel or vast sills? Ask yourself why? Just perhaps, given their clear focus on weight reduction, it's because, for a closed car, it's not the most weight efficient way...Just try the basics with a chocolate box - with or without a central tunnel!
Anyone who thinks, all things being equal, an open car can be as stiff and light as a closed one (or that a rear engined car naturally understeers) really doesn't understand the basics.
Edited by bcr5784 on Friday 24th March 19:33
Edited by bcr5784 on Friday 24th March 19:37
Edited by bcr5784 on Friday 24th March 19:48
Edited by bcr5784 on Friday 24th March 19:53
bcr5784 said:
Yes but you are lumbered with all the stiffening (in the wrong places) needed for the Boxster, which probably costs 50-75kg (maybe more) - or have you managed to cut some of that out?
Having torn apart both Boxster and Cayman making both lightweight racers , the Cayman is worlds stiffer . I had to weld in full cage with door sill reinforcements into Boxster to stop front end from twisting so much. Weight in Cayman comes from roof structure as opposed to Boxster that can loose its top and wind screen However the Cayman chassis is still quite well engineered . I will be carving more weight out. I think people will be surprised at how low I can go with this car.
To give you a sense I built lightweight Nissan 370z and literally made semitube frame with aluminum floors , CF roof, doors, fenders, rear quarters , hatch, front hood and the Nissan still weighed 150 kg more than my Cayman weighs now. The 370z was a 400 hp beast but is still a bit hefty more like Porsche GT3 weight.
Check out my old lightweight 370z with new owner chasing a GT3 cup car.
https://youtu.be/eRILfG2IsQ8
grantsfo said:
Having torn apart both Boxster and Cayman making both lightweight racers , the Cayman is worlds stiffer .
https://youtu.be/eRILfG2IsQ8
I know - 40500 vs 17000NM/degree (981). Thats the point I'm making. Either the Cayman is stiffer than it needs to be (and heavier as a result) or the Boxster needs to be stiffer (and therefore heavier). Both can't be true. Actually the Boxster is unusually stiff for an open car and (on the road) suffers none of the scuttle shake or shudders of (say) an MX5 or 911Cab so my conclusion is that the Cayman is needlessly stiff. It's as heavy as it is because it shares the same floorpan as the Boxster - and you would design it differently if it didn't have an open sibling, as Alpine have done. It would be interesting to know what torsional stiffness they have achieved. https://youtu.be/eRILfG2IsQ8
http://blogs.youwheel.com/2014/04/25/car-body-tors... has a fairly comprehensive list of torsional stiffness. The Cayman would come almost on the top. Coupes are generally about twice as stiff as their roadster siblings - and it's interesting how much difference folding rear seats make!
Edited by bcr5784 on Saturday 25th March 08:21
Edited by bcr5784 on Saturday 25th March 09:14
I must say that I've really enjoyed following this thread. I am surprised how heavy the basic shell of the Cayman is though. After all the stripping and it's still over 1,000 kg. I guess the 987 hadn't started on the route of more advanced materials like the 981, and now 982.
Keep up the good work
James.
Keep up the good work
James.
finestjammy said:
I must say that I've really enjoyed following this thread. I am surprised how heavy the basic shell of the Cayman is though. After all the stripping and it's still over 1,000 kg. I guess the 987 hadn't started on the route of more advanced materials like the 981, and now 982.
Keep up the good work
James.
981 and now 718 haven't really moved away from similar steel chassis. They use aluminum doors and hood . Those were and option in 987 cars with R model. Porsche has stuck with fairly conventional steel in Its cayman and boxsters.Keep up the good work
James.
For a modern steel chassis 6 cylinder car I find them to be fairly light weight. I'm not nearly finished and suspect I ll have car in 950 kg range eventually . Heck my old bare bones 914/6 weighed same and was much smaller. And 914 was a flexy flyer
Thing that amazes me is that Porsche can bolt on so much heavy crap on a car that is fairly light . It was as if good engineers designed chassis and then allowed marketing to bolt a bunch of heavy stuff onto platform.with no regard to how it weighed.
In approximate order
Replacing steel doors with composite doors
Ceramic brake rotors
OEM power seats to race composite seats
Front and rear bumper bar removal
OEM wheels/tires to lightweight race wheels/tires
Rear hatch from OEM to composite
Removal of interior - seats backward
Lightweight battery
AC delete
OEM exhaust to headers and lightweight mufflers
Lightweight flywheel
Airbag delete
Replacing steel doors with composite doors
Ceramic brake rotors
OEM power seats to race composite seats
Front and rear bumper bar removal
OEM wheels/tires to lightweight race wheels/tires
Rear hatch from OEM to composite
Removal of interior - seats backward
Lightweight battery
AC delete
OEM exhaust to headers and lightweight mufflers
Lightweight flywheel
Airbag delete
Edited by grantsfo on Wednesday 12th July 11:48
ajondyh said:
Should consider a 5 speed box, stronger, good final drive ratio and at least 50lbs/22.5Kg lighter 
Interesting. My experience with 5 speeds in past was that they were weaker than 6 speeds I broke a 5 speed and know of others who broke them as well.. No issues with 6 speed. What data do you have that indicates stronger ? Might be an option.grantsfo said:
Interesting. My experience with 5 speeds in past was that they were weaker than 6 speeds I broke a 5 speed and know of others who broke them as well.. No issues with 6 speed. What data do you have that indicates stronger ? Might be an option.
Hmmm, generally in motorsport I have found the consensus is the 6 speed is weak. The place fitting mine now (EMC) also endorses the 5 speed over the 6 speed. I have broken a few 6 speeds myself. Also you can easily get a much better diff for the 5 speed without having to break the bank shipping a very expensive one from USA.grantsfo said:
In approximate order
Replacing steel doors with composite doors
Ceramic brake rotors
OEM power seats to race composite seats
Front and rear bumper bar removal
OEM wheels/tires to lightweight race wheels/tires
Rear hatch from OEM to composite
Removal of interior - seats backward
Lightweight battery
AC delete
OEM exhaust to headers and lightweight mufflers
Lightweight flywheel
Airbag delete
tx. so the replacement of steel doors with carbon ones loses the most weight but I suppose must be done in conjunction with a full cage? Replacing steel doors with composite doors
Ceramic brake rotors
OEM power seats to race composite seats
Front and rear bumper bar removal
OEM wheels/tires to lightweight race wheels/tires
Rear hatch from OEM to composite
Removal of interior - seats backward
Lightweight battery
AC delete
OEM exhaust to headers and lightweight mufflers
Lightweight flywheel
Airbag delete
Edited by grantsfo on Wednesday 12th July 11:48
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