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Thanks for the encouraging remarks. I have a thread in this section with more pictures and info. I'm pushing 2400lbf at each corner in this analysis, so it's quite a lot of force. I'm trying to get a robust frame that will keep the occupants safe in bad situations and also handle a lot of power and grip.

I ran it again locking the front shock mount locations and applying the force to the shock mounts in the back. Here is what it is showing me.



It looks pretty solid to me. though I'm concerned I should change my main roll hoop configuration, or beef it up. Seems the beams running from the rear lower control arms up to the edges of the main hoop create quite a local bending moment that could be reduced by just relocating them to the intersection of the roof and main hoop.

This thing will be a doozy to build, but I think the result will be worth the trouble.

This is over my head but? What about using more tetrahedrons? Some of my "OLD Race Chassis Books" used a lot of them.

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American 7 5.0 T5
Lotus 15 ish?
914/H6

This doesn't tell us what your torsional rigidity is. You have more steps to go through. Can your shock mounts physically handle 2400lbs each? Why would they need to as opposed to your control arm mounts?

Another little shot of what it will do in a very nasty T-bone.

6000lbf yields 29ksi peak axial and 18ksi peak torsional.


Some of you guys snuck in some replies: So the shock mounts physically can't handle that force I don't think; they are 11gauge plate. I just picked a force value that would far exceed what might be seen on the road to see how the frame might react. I haven't figured my torsional rigidity since the last round of changes a while ago; let me do it and get back to you guys.

So doing some math. I have 4800lbf causing 4.58in displacement at a distance of 35.73in (2.978ft) from the centerline of the fixed points. This is 7.3deg of twist (arctan (4.58/35.73)).

So: 4800lbf x 2.978ft / 7.3deg = 1955.9 ft.lbf/deg

This seems a bit low...

EDIT: alright, I think I found the error. So this isn't a right triangle, and the Law of cosines needs to be used to find the true degrees of twist, which is in fact 3.26. Also, it isnt 4800@35.73 inches. The true torque is 4800@ 22.27 inches. If I carry this through, I get 2728 ft.lbf/deg which seems more realistic.

Are you using 4,800 lbs or 2,400 lbs at each shock mount?

The equal but opposite loads create a couple so you only use one for the ft-lbs/deg calculation. I want to say you are more in the 4,000 ft-lbs/deg range.

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-Andrew
Build Log
Youtube

Wouldn't you sum the torque about the center axis (defined by the fixed points)? Or should I rerun the FEA with only a single force applied to one side and the other three sides fixed?

No since its a couple. One force twists the chassis and the other reacts the first force. If you did not have the second force then the front of the chassis would want to rise up and it would rotate about the constrained rear shock mounts.

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-Andrew
Build Log
Youtube

If I run it with three sides fixed, and 2400 on a single shock mount at the rear, I get 1.12 deg twist, 2400@22.27in which yields 3977 ft.lbf/deg.

I have no feel for what realistic numbers should be.

Look back a few pages. 4,000 ft-lbs/deg is in the ball park of what many were getting.

If you fix three corners and apply a load to the fourth, I do not know what you will be testing. Doing it with a couple at one end twists the chassis in torsion and measures the stiffness of the "torsional spring" that connects the front and rear suspension.

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-Andrew
Build Log
Youtube

First the proper calculation for the simple couple is M = d * F where d is the distance between the equally opposing forces and F is the force at one application point. Example A spells this out pretty clearly. http://www.google.com/url?sa=t&rct=j&q= ... WU&cad=rja

F= 2400lbs
d = 2.978ft x 2 = 5.956ft

M= 14,294 lb-ft.

TR = 14,294/7.3 = 1958 lb-ft/deg

Fixing 3 corners and loading 1 can be done but you have no basis to judge from so 4,000ft-lbs/deg for that won't be comparable because most everyone else with those numbers only fixes 2 points of the chassis. Fixing 3 should require higher stiffness values than 4000.

Finally, I don't think you're modeling the test correctly for what you actually care about. 2400lbs is an awfully large input force for an on-track cornering situation and could cause abnormally large deflections in the software that aren't real. If you are trying to find out how stiff your chassis is on a race track as opposed to some dangerous situation I would bring that number down to something more realistic which you should be able to calculate starting off with a 1.0G cornering load. As an example, I gradually increased load from 25lbs to 125lbs on each side of my FSAE chassis. We used measuring tubes that stuck out 24in. from centerline at specific points along the chassis for very fine measurements of deflection. You can see the real world example in the attached pic but in the model you can easily just output the node displacements of the dummy tubes.

Our chassis was pretty stiff for a tube chassis without any stressed panels with maximum angular displacements of less than 0.5 deg in all situations. 2000ft-lbs/deg sounds like a decent start but it means nothing because 7.3 deg of angular deflection is extraordinary which may just be a byproduct of your high input forces.

Oops I goofed on the couple. Do what he said. ^

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-Andrew
Build Log
Youtube

Alright, I ran it again with 500 on each side, front two shock mounts fixed. My new displacement is .296" and if I run through the math I come down to 1850 ft.lbf/deg. I think the deformation in my roll hoop is really downplaying the rigidity in the rest of the frame. I'm going to move my two rear diagonals (connecting rear lower control arm to outside top edge of roll hoop) and see if my rigidity doesn't improve.

For a more realistic stress distribution, especially under braking, and not a lot of extra work compared to what you've already done, add the the suspension arms and a few beam elements for the shock/spring and a connection to the contact patch.

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Noah

For comparison with a real chassis test, have a look at my build log (http://www.locostusa.com/forums/viewtopic.php?f=36&t=3241&start=75) and my post from 26th July last year, when I did the Torsion and Beam strength test mandated by our local registration requirements for homebuilt cars. My chassis reached 5828 Nm/degree or 4299 ft.lb/degree.

The test was done with 2700ft.lb of torque (285kg of weights on a 1.33m beam). Total chassis movement was measured in millimetres at most.

Dominic