Designing a Roll Cage for an Electric Car

[Abhishekdas]

I am a bachelors mechanical engineering student with a decent factual knowledge about cars and how they work. But i am a novice with no experience when it comes to design. Now I (along with 3 other guys) have been assigned the task of designing the roll cage of an electric car. I did not get any relevant theory material in the net and I do not have any books covering roll cage design. So I request you all to guide me through by referring some websites or books which covers this topic and how to go about with the design in general.
The details which have been decided upon are :
1. Car for 2 people
2. Range - 100 kms in one charge
3. Max speed- 6 mph
4. 4 batteries weigh total 40 kgs
5. Motor + transmission = 30 kgs
6. Rear wheel drive (single wheel at the rear) and two wheels at front with steering

Thanking you in adavance...

 

[berkeman]

I am a bachelors mechanical engineering student with a decent factual knowledge about cars and how they work. But i am a novice with no experience when it comes to design. Now I (along with 3 other guys) have been assigned the task of designing the roll cage of an electric car. I did not get any relevant theory material in the net and I do not have any books covering roll cage design. So I request you all to guide me through by referring some websites or books which covers this topic and how to go about with the design in general.
The details which have been decided upon are :
1. Car for 2 people
2. Range - 100 kms in one charge
3. Max speed- 6 mph
4. 4 batteries weigh total 40 kgs
5. Motor + transmission = 30 kgs
6. Rear wheel drive (single wheel at the rear) and two wheels at front with steering

Thanking you in adavance...

We do not do your schoolwork for you here on the PF. You need to show some effort on the project before we can be of tutorial help. What kind of things have you looked at so far? How are roll cages designed for different existing vehicles? What kind of things will you need to take into account when you chose the materials and structure?

 

[Abhishekdas]

As of now my only concrete source were SAE Baja Team reports. But My situation is different. Out there they had fixed the thickness and outer diameter of the roll cage tubes and the basic structure of the roll cage was fixed - All part of the rules. In my case i don't know how to start. All I gathered was that in these sort of cases you are better off using triangles to complete the structure. We did not get details of the design of roll cages in the internet anywhere. So we decided to start by making random base designs in Solidworks and now we plan to put loads and analyse these base structures using Ansys. So our approach right now is vague and I don't even know if its right. So I was hoping for some guidance.

 

[AlephZero]

Start by thinking about what you want your roll cage to do. An electric car with a max speed of 6mph isn't going to do a high speed rollover crash on its own, but it might get hit by another vehicle.

On the other hand, if it does roll over, the roll cage needs to be strong enough to support the vehicle upside down without collapsing and crushing the passengers, for example. It might need to be strong enough so the passengers can roll the vehicle back onto its wheels, without needing to use a crane. Etc, etc...

 

[nvn]

Abhishekdas: You may have seen this already, but check out this http://www.dezertrangers.com/vb/fab-shop/23738.htm thread. It contains lots of advice. It is a lot to assimilate, but you can start. And here appears to be another copy of post 1, without all the additional, helpful comments.

However, although I am not an expert roll cage designer nor builder, I currently would not prohibit all non-triangulated members. I currently would say there is one exception. If I recall correctly, I think I have commonly seen a U-shaped tube roll bar, somewhere in the vicinity of above the seat headrests, something like as shown in Figure 1 in my attached file (?). I currently would say, if you wish to use this member, you can make the arch (the U-shaped roll bar) quite strong, by increasing its wall thickness until it becomes resistant to collapse (including lateral shear distortion, during rollover), which is a somewhat natural function of an arch shape.

But if the pink tubes in Figure 1 interfere with your frame design, etc., then you could potentially, alternately replace the pink tubes with some variation of something like the green tubes in Figure 2 (?), if it would make the roll bar strong enough.

And I think I sometimes have seen a second of these roll bars in the vicinity of the windshield, which can optionally be shorter than the rear roll bar, if you wish.

Keep in mind, these diagrams are in no way intended to be complete. You can attach other things to these members (although I think I have occasionally seen them alone, if I recall). Also, the attached diagrams are in no way intended to imply that every detail should be exactly as I drew. I just arbitrarily drew those. I do not know the exact best, optimum shape factors, nor details.

 

[Abhishekdas]

I would like to thank you both for your replies... But before I go ahead I would like to correct myself... Max speed is 60 mph and not 6.
So coming to what I did. I think I should go ahead by building a Base first. We are planning on going for something with a rectangular outline with a front U as nvn suggested and within it i will include cross (2 triangles) or triangular members. On top of the framework loads (Uniformy distributed) will have to be applied at the nodes(using ansys). Loads will be static ones(mentioned in my fist post). Am i doing it right? Am I missing out on anything

 

[Baluncore]

There are two distinct approaches to designing roll frames, (ROPS).

Elastic. The first is to design a frame strong enough to remain elastic under all possible situations. That is a heavy design and will be uneconomic as part of a high efficiency vehicle. This first solution is applicable where protection will be required to operate several times in the life of the vehicle. This design will typically have straight lengths of large diameter tube with heavy common joints.

Plastic. The second approach is to design a frame to absorb energy by plastic deformation. That is a low cost, low weight, highest economy solution. The frame should be designed so it will crumple progressively. This solution requires total replacement if it is ever deformed once. Variations of this plastic approach are used in current vehicle design. The components will typically be curved with thicker walled small diameter tubes that meet at separate points.

 

[geoff033]

Look at specifications from international motorsport organisations such as FIA, CAMS etc, example.

Packaging the batteries so they aren't dislodged in a rollover is critical.