SkinnyG Posted January 7, 2011 Share Posted January 7, 2011 The Lotus 7 chassis has a 1" square tube running the full width of the chassis at the bottom footwell. The floor is also a full sheet riveted to the bottom. The Caterhams now seem to have 1/2" (or 5/8"?) square tube running only on each side of the transmission tunnel, and not all the way across. The floor no longer spans the full width, but only under each passenger. How does Caterham's version provide more torsional rigidity than the Lotus? Link to comment Share on other sites More sharing options...
escondidoron Posted January 7, 2011 Share Posted January 7, 2011 I'm not sure that the additional tubes in the central tunnel were added to increase torsional rigidity so much as to provide a stronger transmission mounting structure. The transmission mount for the S2 and S3, as built by Lotus, was pop riveted to the aluminum floor panel and to the transmission tunnel sides. The pop rivets for the mount are prone to becoming loose over time with the result that the transmission is not firmly secured to the chassis. These structural changes have been evolutionary with several iterations by Caterham and various tuners (like Dave Kaplan of DSK) over time. There is a very good illustration of these and other structural revisions/iterations in the book, "Lotus Seven: Restoration, Preparation & Maintenance", by Tony Weale. The book is out of print but does come up on ebay a few times a year. Link to comment Share on other sites More sharing options...
11Budlite Posted January 7, 2011 Share Posted January 7, 2011 Here's an illustration of a DSK chassis for your viewing pleasure. Lot's of extra bracing but no framework in the trans tunnel area. Link to comment Share on other sites More sharing options...
Alaskossie Posted January 8, 2011 Share Posted January 8, 2011 On the DSK frame, there is a hoop under the trans tunnel linking the two footwells, that is absent from the Caterham frame. This hoop is in the same general position as the cross-brace in the original Lotus frame. Link to comment Share on other sites More sharing options...
MichaelD Posted January 8, 2011 Share Posted January 8, 2011 http://www.nasasevens.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=207&g2_serialNumber=5 These cross bars in the Hamlin cage add significant ridgity to the seven frame. More photos here: Competition Hamlin roll-cage (low-front hoop, NASA approved) See http://www.nasasevens.com/index.php?q=gallery&g2_itemId=151 - Competition Hamlin propshaft safety loop (NASA-approved). See http://www.nasasevens.com/index.php?q=gallery&g2_itemId=253 Link to comment Share on other sites More sharing options...
DeanG Posted January 9, 2011 Share Posted January 9, 2011 I never could understand the attaching a 2" roll bar to a .75" frame member. Link to comment Share on other sites More sharing options...
SkinnyG Posted January 9, 2011 Author Share Posted January 9, 2011 I like how the Hamlin cage was done - it looks easily removeable if need be. Though ~any~ type of bolt-in cage is iffy on a 7. It's not a fair comparison, though - while it does replace the wee crossmember under the bellhousing/transmission, it also adds a full cage or something "cage-esque" which would add considerable strength to the tub - it's not just a replacement tube under the frame. It would seem to me, that keeping that bottom tube in the footwell 1" square and all the way across would be more rigid than without. Tony's Weale's book says that the curved tube at the base of the gearbox was deleted in 1990 to fit the Vauxall engine which had to sit 3/4" lower. Apparently torsional testing revealed the bottom tube was redundant. I just have a hard time picturing this. Perhaps it is because the transmission mounting crossmember is significant enough to make up for the missing tube at the front of the tunnel. Link to comment Share on other sites More sharing options...
Birkin42 Posted January 10, 2011 Share Posted January 10, 2011 I modified the frame of my Birkin to accept an S2K engine and transmission. This required a complete rework of the members in the engine and transmission tunnel area. I also reworked the rear of the chassis to convert from a live axle a DeDion. I modeled all the changes in CAD and calculated the change in torsional rigidity and found it was improved by nearly 50%. This was validated by physically testing the stiffness before and after the mod's. In doing the mod's I removed the bolted in connection under the transmission as well as removed one of the 2 upper engine bay brace tubes. I did take a lot of care in triangulating the frame tubes I did add try to maximize the strength and stiffness. I didn't analyze as to where the major gains were made, though I suspect it was likely a combination of a number of small gains more so than mostly due to one of the changes in particular. I do know in analyzing the stock frame, cutting out 1 of the 2 upper engine bay diagonals maked practically no difference to the torsional rigidity even though visually you'd think it would have made a big difference. I did check this single change both before and after and confirmed this. Ultimately there are countless ways to design a frame. I would think in most cases you could leave out a tube that seems important and design other parts of the frame to carry this load. Of course there are lots of ways to design the frame poorly as well. Here's a pic of the modified frame. Link to comment Share on other sites More sharing options...
escondidoron Posted January 10, 2011 Share Posted January 10, 2011 How many lbs/degree of torsional stiffness? Link to comment Share on other sites More sharing options...
lotusman1951 Posted January 10, 2011 Share Posted January 10, 2011 The DSK folks did some calculations on a standard S2/7 frame and the number was about 300 pounds per degreee, the DSK frame was more like 1600 per degree-and Elan frame was almost twice that around 3000 per degree-I am away from my car files and my numbers might be off a bit but not by orders of magnitude. A stiffer frame allows softer suspension-less cracking. Kevin Kearney Link to comment Share on other sites More sharing options...
a.moore Posted January 10, 2011 Share Posted January 10, 2011 If removing that tube did reduce rigidity, Caterham got it back and more by adding the diagonals within the engine bay (the ones commonly left out since they interfere with the engine block, exhaust, and intake). Its surprising how much those stiffen that whole bay. These tubes also provide a good load patch to a structural transmission tunnel which acts as a torque box or a backbone. Link to comment Share on other sites More sharing options...
MichaelD Posted January 10, 2011 Share Posted January 10, 2011 Kevin, How are things in Cabin John? Want to get together for a ride after dinner before the snow comes? Link to comment Share on other sites More sharing options...
lotusman1951 Posted January 10, 2011 Share Posted January 10, 2011 Michael, My other pastime is long distance "trawlering" and I am engaged with that for a while-haven't been in a car for 2 months-my 7 is under covers and I am headed for guatamala-have no idea when I will get back, presently in Flordia-miss home and the river roads-don't miss the cold. Kevin Kearney Link to comment Share on other sites More sharing options...
MichaelD Posted January 10, 2011 Share Posted January 10, 2011 See you in the spring. Happy sails. Link to comment Share on other sites More sharing options...
Birkin42 Posted January 11, 2011 Share Posted January 11, 2011 Ron, I measured the stock Birkin frame at 1100 ft-lbs/degree and my modified frame at 1600 ft-lbs/degree. In both instances this is the stripped frame with no aluminum panels riveted to them, which I believe would add some stiffness. Link to comment Share on other sites More sharing options...
SkinnyG Posted January 11, 2011 Author Share Posted January 11, 2011 This is very fascinating. Thank you very much for the pictures and sharing your experience. Are there others with more to share? Link to comment Share on other sites More sharing options...
escondidoron Posted January 11, 2011 Share Posted January 11, 2011 Birkin42 said: I measured the stock Birkin frame at 1100 ft-lbs/degree and my modified frame at 1600 ft-lbs/degree. Thanks. Link to comment Share on other sites More sharing options...
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