Chassis Stress & Strain Calculations

[Jas1159]

Hi Guys,

So I am biting off more than I can chew with my latest project.. and looking for help.

I am going to fabricate a kit car chassis,

The suggested material is 1' x 1' steel box section but why?

Could anybody help by suggesting the kind of calculations I would have to research in order to proove such a chassis made from this material would not fail?

the reasons for this is I want to carry out similar calculations for Aluminium and possibly tubular sections...

I realize this is going to be tough to pin down as it relies on many aspects however a push in the right direction or points I should consider will be hugely appreciated.

Kind Regards
Jason

 

[xxChrisxx]

This kind of thing is a pain to workout by hand. It's far more simple so to an FEA analysis on the chassis. I appreciate this may not be available to you. As a properly designed spaceframe (ie al members loaded in either tension or compressions) is essentially a truss, you can use truss stress calculations.


A 1 x 1 section is fine for most spaceframe structures. If anything it's over kill if you are clever with triangulation. What's the wall thickness?

Tubes will almost always be better then box section becuase they are more efficient at carrying the load, they require a lot more skill to cut and weld correctly though. They can also be more difficult to hang brackets off. A well designed and made tube chassis will be stiffer per unit weight than a box section, but the box section will be for forgiving to a badly placed tube or poor weld.

You'll find a lot of Lotus 7 clones use 1 x 1 box section for the main structure and a mixture of varying thickness tubes and smaller box section to make up the rest of the structure (stiffeners, mounting areas etc). (See the Caterham for a mix of 1x1 0.75x0.75 and 0.75OD and 0.5OD tube)


Aluminium is a bad idea beucause it had no fatigue limit (so it will fail at some point). I don't think I've ever heard of an aluminium spaceframe chassis.

 

[Jas1159]

Hi chris thanks for the reply,

I should have mentioned this above the box section is 16awg approx 1.6mm thick which as you quite rightly said should be overkill on a lightweight chassis.

Thank you for the info regarding Aluminium I had questioned why it was not widely used on such chassis as a weight saver :)

At the moment I have almost completed a 3d model where I am calculating all of the cut angles and material lengths, I have a rough bill of materials and am confident that I can build the chassis.

I would like to teach myself a few things about stress and strain instead of pursueing My usual approach of building based on engineering experience.

My main concern is theoretical calculations that prove the steel structure will be strong enough even though I know it will be :D

Thanks again
Jason

 

[xxChrisxx]

Hi chris thanks for the reply,

I should have mentioned this above the box section is 16awg approx 1.6mm thick which as you quite rightly said should be overkill on a lightweight chassis.

Thank you for the info regarding Aluminium I had questioned why it was not widely used on such chassis as a weight saver :)

At the moment I have almost completed a 3d model where I am calculating all of the cut angles and material lengths, I have a rough bill of materials and am confident that I can build the chassis.

I would like to teach myself a few things about stress and strain instead of pursueing My usual approach of building based on engineering experience.

My main concern is theoretical calculations that prove the steel structure will be strong enough even though I know it will be :D

Thanks again
Jason

I don't know if you have any spare cash hanging about to buy it, but you could try a library somewhere. But try to get hold of Costin and Phipps - Racing and Sports Car Chassis design.

It was written in the 60's and it's been out of print for years now (so it's really expensive to buy - you'll be looking at £100), but if you want to build a spaceframe it's by far the best book I've come across.

Anotehr book to try is Chassis Enginering - Herb Adams. A really basic (it's a fairly crap book tbh, but ok for basics) book with do's and donts of spaceframe design (I can't remember if it's got calculations). It's cheap though.


It's been a few years since I've done any chassis engineering. I'll start reading up of some of my old notes.

 

[Jas1159]

Apologies for the slow response Chris, I have since taken your advice and purchased Costin and Phipps Chassis Design. I have to say it is an awsome book and covers pretty much everything i need to know in a firm level of detail!

Thanks very much for your help

 

[Jas1159]

Unfortunately as i said previously the design and build of such a chassis is going to be the easy bit for me, prooving that it is sufficient by means of calculations is something i would love to be able to do in the future.

 

[xxChrisxx]

Unfortunately as i said previously the design and build of such a chassis is going to be the easy bit for me, prooving that it is sufficient by means of calculations is something i would love to be able to do in the future.

Gald you got the book, bet it set you back a fair bit of cash (it's worth every penny though), there is a calculation section in the appendix.

People don't tend to do extensive stress analysis on spaceframes becuase the members can take so much more load than they would ever see in operation even using mild steel. Failure occurs at joints, which are pretty much impossible to calculate as no two welds are ever exactly the same.

Analysis tends to be more focued on how the chassis distibutes the load and tries to attain uniform stiffness. Preventing stress concentration and fatigue loading. That requires FEA (which never tells the full picture), or experience.


Is it a Seven style clone you are building?

 

[Jas1159]

Yeah i managed to get a paperback version for £60 the most i have ever spent on a book but as you say well worth a good read and the calculations are all there, my next step is simply to understand them fully :)

I can appreciate the difficulty in predicting maximum chassis loads and that they may not be entirely relevant but for me it is just about understanding the concept and hopefully using the internet and these books I can begin to understand the majority of these stress calculations.

I am a Mechanical Design Engineer however my main focus has been on design and build to specified parameters, hopefully a project like this can give me more knowledge of these parameters. Eventually I would also like to make the jump from Aero to Auto and therefore a project such as this one would look impressive in the CV.

Yeah I have seen the standard seven style drawings online and the locost builds they seem pretty interesting, at the moment I am collecting components, thinking about using an old Volvo 340 running gear as it can be used to make the Renault Williams F7R engine i am building transverse. I am thinking about going similar to the 7 series keeping the basic shape and making a few tweaks of my own.

 

[Ranger Mike]

Welcome and Hello..may I ask your plans for this endeavor? are you going to build a complete car from scratch?

 

[Jas1159]

Welcome and Hello..may I ask your plans for this endeavor? are you going to build a complete car from scratch?

Hi Mike,

Yep, although i expect my plans to change slightly as i do more research I am already collecting materials and reading up on a lot of the what to and what not to do.

I am very confident that the design and build project will be successful, I have an interest in the 1993 Renault Clio Williams F7R and wish to combine this with the volvo. I simply want to understand more of loads and stress data that will allow me to justify certain choices that today I would simply select out of common sense or run the risk of over engineering.

If you are interested in seeing progress i can upload my project as and when it is updated to a thread within this forum. I plan to document the project step by step with photos and release it for people to read if they wish.

 

[Ranger Mike]

Brother..you are in for a load of fun!
We have people here in USA who want to build kit car replicas of the Shelby Cobra..all I can i i would say that 90 percent end up on EBAY..there is a whole lot of form and fit to these things and this is with well developed build plans and complete parts lists and sources for the parts...
yes..let me know if i may be of assistance..
GOOD LUCK