Chassis weight
ProjectLMP - 19/1/03 at 02:49 PM
From my model you can see that I used three sizes of balsa to represent the different tube sizes. I have calculated the total weight of the chassis
with a few combinations of tube sizes as follows:
All tubing is 16G unless otherwise stated.
Thick balsa = 1 1/4"
Medium balsa = 1"
Small balsa = 3/4"
Pontoons = 1"
Rollbar and back braces = 1.5" (0.120), 1.25" (0.120)
Total weight = 181 lbs
Thick balsa = 1"
Medium balsa = 1"
Small balsa = 3/4"
Pontoons = 1"
Rollbar and back braces = 1.5" (0.120), 1.25" (0.120)
Total weight = 163 lbs
Thick balsa = 1"
Medium balsa = 1"
Small balsa = 3/4"
Pontoons = 3/4" or 1" (0.049)
Rollbar and back braces = 1.5" (0.120), 1.25" (0.120)
Total weight = 155 lbs
Switching to a 0.095 rollbar would remove 2.5 lbs
Anyone got any ideas on which option would be most suitable. I had budgeted about 150lbs for the bare chassis. However, 15 or 20 lbs doesn't exactly
seem the difference between success or failure. Hell skip a good meal and you've recovered 5lbs 
Spyderman - 19/1/03 at 04:20 PM
An interesting comparison.
I think Rorty would be the expert here with his FEA, to see what differences there would be in strength between the different sizes.
However from what I have learnt from the general concensus is that the larger tubes do not necessarily give more strength in same proportion to extra
weight gained.
I would go for second choice of 163 lbs.
Terry
Alan B - 19/1/03 at 04:48 PM
A couple comments Terry,
Does Rorty have FEA? ....maybe a question for Rorty really...I wasn't aware he had.
Tubes do give more "strength" in proportion to weight whether that is a wall or O/D increase.....However, a wall thickness increase only increases
"stiffness" proptionally (approx) wheareas a diameter increase improves stiffness enormously. Hence bigger diameter, thinner wall is much better for
stiffness.
It is important also, to consider not only the indvidual tubes, but also their combined effect within the structure....but I'm sure everone knows that
For anyone who is not familiar with the subtle yet vast difference between strength and stiffness there is a good article here:
http://www.erareplicas.com/machdes.htm
It explains it far better than I could.
Paul, 5 pound meal?????
What do you eat?
Joking aside, your point is well taken, and although there is weight saving scope it easy to get carried away when not really required.
Rorty - 20/1/03 at 04:57 AM
quote:
Does Rorty have FEA?
I do (actually I got Grape a while ago too, but haven't had a chance to give it a run), though I seldom use it, as (and I'm not being blasse), after
doing this sort of thing for years, I've got a pretty good idea what works and what doesn't. I'm also in the position of being into racing off-road,
where I can safely test designs stripped to the bare minium in a harsh environment, until they fail, then go back one step to the lighest/safest
combination.
LMP, I would choose 1 1/4" X 16g for the bottom of the chassis, as it serves well for picking up wishbone brackets, and also because the bottom
wishbones (asuming your shocks are attached to the bottom wishbones) will take the most punishment.
With the amount of "tube" in your model, I would say use 1" for the remainder of the main chassis tubes, and thereafter, I would use 3/4" for any
bracing and for the pontoons.
I would also use round tube, as you get an almost 25% free weight saving, and a similar figure of increased strength, for the same nominal sizes.
IIRC, you're in the States (imperial system would be the clue.....duh!). What is the standing on rollbar material and dimensions over there? I would
be very happy with the rollbar proportions you propose, but in my experience, most authorities require rollbars of ridiculously gigantic proportions
and wall thicknesses. Here, the powers-that-be demand rollbar material to be 48mm OD X 4.8 for a 795kg (1750lbs) car! AND that's irrespective of
whether you're using black pipe or CroMo!
Spyderman - 20/1/03 at 01:31 PM
Cheers Alan, I stand corrected!
However I noticed you wouldn't be drawn in to answer which of the designs you thought would be best.
Maybe you are as wise as you are knowing!
They say with age comes wisdom!
Just kidding!
Terry
Alan B - 20/1/03 at 02:03 PM
OH nutsack 4th time trying not delete my post....GRRRRR
The tubing I used on Meerkat are the sizes that Rorty suggests, and the rollcage tubing I used was 1.5 x 0.095 DOM as per SODA spec.....
This was MUCH longer but I keep deleting tryint to insert a link....
[Edited on 20/1/03 by Alan B]
Alan B - 20/1/03 at 02:14 PM
BUGGY link...
Is my buggy link if you are interested.
I've seen MANY rolls and never any structural failure with 1.5 x 0.095 DOM..
ProjectLMP - 20/1/03 at 06:00 PM
Thanks for the advice Rorty. I am going to go with a bit of a hybrid approach similar to what you suggest.
kb58 - 28/1/03 at 03:41 AM
I'll also add that SCCA in the U.S. specifies 1.5" x 0.090" wall for the roll cage structure, for cars in "our" weight class.