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Old 08-13-2007, 03:41 AM   #1
VitaminG
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Question For the engineers out there...

I am currently debating two frame options for a car project that I will soon be entering the design phase on. I am not sure which frame to go with, I would like to hear some outside input. Note that this will be a small open-wheel track car, rwd with a Subaru flat-4(~125 hp). Target weight with driver is <800 lb.

Frame 1: A triangulated box-style frame, all square tubing cut and welded except the roll hoops directly around the driver, which will be bent round tube. See here for a general idea.

Frame 2: Similar to above, only using round tubing bent hoops welded to a square tubing base instead of cut and welded square tubing. Engine compartment would still be square tubing.

It isn't so much a matter of performance as safety. My main mental battle is that, while the round tubing hoops will do better in a roll-over, they wouldn't do quite as well to keep their shape in a normal sliding collision. I am mostly aiming for a design concept similar to what F1 uses, where the driver is in a "survival cell" that will not crush or break in any collision(Just ask Robert Kubica how well it works) and using the rest of the frame as collapsible energy absorbers. Well, let me hear any ideas, I can use as many as I can get going into design.
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Old 08-13-2007, 04:26 AM   #2
windsonian
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Quote:
Originally Posted by VitaminG
I am currently debating two frame options for a car project that I will soon be entering the design phase on. I am not sure which frame to go with, I would like to hear some outside input. Note that this will be a small open-wheel track car, rwd with a Subaru flat-4(~125 hp). Target weight with driver is <800 lb.

Frame 1: A triangulated box-style frame, all square tubing cut and welded except the roll hoops directly around the driver, which will be bent round tube. See here for a general idea.

Frame 2: Similar to above, only using round tubing bent hoops welded to a square tubing base instead of cut and welded square tubing. Engine compartment would still be square tubing.

It isn't so much a matter of performance as safety. My main mental battle is that, while the round tubing hoops will do better in a roll-over, they wouldn't do quite as well to keep their shape in a normal sliding collision. I am mostly aiming for a design concept similar to what F1 uses, where the driver is in a "survival cell" that will not crush or break in any collision(Just ask Robert Kubica how well it works) and using the rest of the frame as collapsible energy absorbers. Well, let me hear any ideas, I can use as many as I can get going into design.

so... are you looking more at strength or stiffness... a big performance consideration with this is the stiffness of the structure.

Bear in mind that the round tubing will have similar properties from multiple directions, while the SHS will vary greatly with direction.

the weld joint angles may be a bit easier to cut into the pipe too. ... not that this should be your final decision maker
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Old 08-17-2007, 12:32 AM   #3
rudypoochris
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I would suggest buying the book "Chassis Engineering" for starters. You might want the GRAPE chassis design (space frame) software and possibly solidworks to run addition stress analysis. I would use a mix of circular and square tubing depending on the area. Remember you will want certain zones to bend in an accident. Having all stiff zones will cause injury to you at a much higher rate than having crumple zones. I would highly recommend using alluminum sheet or something similar in shear to augment the use of running extra diagonals, 800lbs with driver is light especially if the motor alone is 250-300lbs. Are you planning on running suspension, if so what kind of setup? What do you want in the way of a trans? Imho you would be better off strating with a motor that has a lower base weight or a better power/weight ratio. 125hp from 250-300lbs isn't a very good start, the weight it low though.

I would think a motorcycle engine to be a much better starting point.

Last edited by rudypoochris : 08-17-2007 at 12:40 AM.
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