Review this chassis design please

Clay · 05-20-05, 12:23 AM

Ive started working on a custom chassis on paper. This is not a gt40, but it is similar. This chassis will be made to specific length/width/height to fit within a pre-existing body.

Ive reviewed some other chassis designs and it seems that where I put X-brace supporting, I could have used 1 diagonal bar, but would 1 diagonal bar really give as much support as the X? Am I just adding dead weight by using so many X's?

One of my main concerns is chassis rigidity, but another main concern is to make the chassis as light as possible.

Another problem is that under the passenger and driver's seats is one large X on each side...is this sufficient for a rigid structure?

Clay · 05-20-05, 12:25 AM

here is the top view

Clay · 05-20-05, 12:28 AM

Sorry for the poor quality these are digital pictures of a drawing.

The center of the chassis has a structural back-bone to provide more chassis rigidity, where the cooling pipes will be stored.

Bob Putnam · 05-20-05, 08:17 AM

[ QUOTE ]
Ive started working on a custom chassis on paper. This is not a gt40, but it is similar. This chassis will be made to specific length/width/height to fit within a pre-existing body.

Ive reviewed some other chassis designs and it seems that where I put X-brace supporting, I could have used 1 diagonal bar, but would 1 diagonal bar really give as much support as the X? Am I just adding dead weight by using so many X's?

One of my main concerns is chassis rigidity, but another main concern is to make the chassis as light as possible.

Another problem is that under the passenger and driver's seats is one large X on each side...is this sufficient for a rigid structure?

[/ QUOTE ]

X's are not necessary unless the ratio of the length to tube diameter excedes roughly 30. Your design's main problem is that the section under the door feeds the loads into unbraced points, putting the vertical tubes into bending mode..
Your tunnel does very little if it doesn't feed loads into the front and rear sections.

Unfortunately, there seem to be many other problems too...

At this point, you might consider investing in Racing and Sports Car Chassis Design by Michael Costin. It's the best basic book for chassis I've seen. Amazon's got some used ones.

Clay · 05-20-05, 10:30 AM

Thanks, I've redrawn the chassis without the X-Braces, replaced by a single diagonal support.

The center section is lowered like that because i wanted to leave room for the driver to enter and exit the car. Should I make the upper brace of the driver area match the upper braces in front of and behind it? and hinge it to swing out with the door but lock solid when closed?

also should i form a bar that links the two verticals like the upper (front to back) supports in a roll cage?
I will purchase that book tonight, as well as post another revision of the drawing.

Clay · 05-20-05, 01:02 PM

Lets say I were to make the car a convertible with a removable hard top, and didnt need space for the driver to swing into the car from the side, would the best design be this one?

Clay · 05-20-05, 01:03 PM

This was a design revision made by a fellow builder for if the driver needs space to get his legs in and out of the car over the side bars. Thoughts on this?

I think this is one of the best ones yet. If this design proved to be good enough, I would most likely use it over the others.

Chris P. · 05-20-05, 01:09 PM

Clay - seriously - buy some balsa sticks (1/8" square will suffice), some thin cardboard (like manilla folders), and some CA glue (you can get all this at any hobby shop. Try different ideas of frame design with this because even if you are very very good at visualizing in 3D, you will miss some very important design points drawing in 2D. Not to mention the fact that it's fun too! You'll be surprised how much you learn and how many other ideas you'll get. You can easily set up stiffness tests (torsion, bending) with it too and compare design ideas.

Judging by these drawings, it looks like you are going to have a wheelbase over 110". That's getting pretty long for a sports car. It also means that the cross bracing you are worried about will become even more important. The 3D balsa modelling will also clear up issues like those braces above the engine. Those may have to be removeable. Not that removeable braces are an absolute no-no - they are fine, but one should try to design without using them and only resort to them if proven to be needed.

One quick pointer on cross bracing. An ideal tube frame has tubes only in compression / tension. (This rarely ever works out perfectly in real life - there will be some bending here and there in final designs but don't design that in if you don't have to.) Look at the horizontal tube below the doors as Bob pointed out above. They should connect to the ends of other tubes, not the middle of other tubes. The "V" shaped bracing is not required - you can typically get away with one diagonal brace from corner to corner if the tube ends all meet up properly. You're goal is LOAD TRANSFER. Look at the load path through the tubes in your mind. This is where the balsa model will really come in handy because there are loads not purely on plane with the paper.

I'm at the same stage you are with my design - I get just as much fun out of designing as I do building, modifying, and driving. Cheers!

Clay · 05-20-05, 01:16 PM

Chris, I tried constructing a wood model with stuff i had around the apartment. 12" Long wooden matches, but superglue was not holding them together, i dont know if the wood was soaking the glue in and keeping it from sealing or what, but it was annoying. Maybe the balsa wood and hobby shop glue will prove more useful.

Also, my drawing isnt exactly to scale, its just a rough design drawing to setup my supports. The car will be as short as possible while allowing a v8 and audi transmission just behind the drivers back, and just enough room in frnot of the driver to allow for foot room, pedal box front suspension etc. The drawing might make it look long, but it will only be as long as it has to be once i decide how to brace everything and get some real measurements to work with.

Trevor Jones · 05-21-05, 05:05 PM

Speaking of chassis triangulation, I've attached a photo of a 1/5 scale Porsche 906 chassis I made out of old coathanger wire welded together. Was very useful to see where flexing was occurring.

Chris Melia · 05-24-05, 03:47 PM

hi Trevor

Nice model of the Porsche chassis I have to build a similar one for my 908 Porsche body. Porsche really did know how to triangulate for strength.
The photo below is of a 917 chassis.
Regards

Chris

Chris Melia · 05-24-05, 03:54 PM

my Porsche 908K body.

Chris Melia · 05-24-05, 04:14 PM

Lets try another

**F3L** · 05-25-05, 01:07 AM

[ QUOTE ]
Speaking of chassis triangulation, I've attached a photo of a 1/5 scale Porsche 906 chassis I made out of old coathanger wire welded together. Was very useful to see where flexing was occurring.

[/ QUOTE ]

You know, when I look at that, I see a thing of beauty...

Keep us posted on your progress with the Porsche, Chris, please and thanks [img]/ubbthreads/images/graemlins/smile.gif[/img]

Keith Hardy · 05-25-05, 07:38 AM

Now that's what I call a tube frame chassis! [img]/ubbthreads/images/graemlins/laugh.gif[/img]

Trevor Jones · 05-25-05, 11:25 AM

Chris,

You're right about Porsche knowing how to triangulate chassis. I was surprised how stiff the model structure was. Can you give us any background on the origins of your 908 body; looks beautiful.

Chris Melia · 05-25-05, 02:54 PM

Hi Trevor

I'm having problems getting my file size correct for posting? the body is from original moulds, I intend to build the first car using a 1984 3 litre 911 as a doaner vehicle using the engine and trans, drive shafts and brakes ect. however the uprights and wishbones will be original pattern. some good friends are helping with various aspects of the project.

GraemeS · 05-25-05, 05:58 PM

Hi Clay, the easy way is to mock it up with balsa sticks, and a hot melt glue gun, dries in seconds, i have attached a couple of pics of a similar chassis i found on the web maybe it will give you a few ideas, also have a look at some of the Lambo/ diablo build sites, they usually have pics of chassis that are rear engine etc, if you need links let me know.
Best Regards
Graemes

GraemeS · 05-25-05, 06:00 PM

and another view