Help with suspension forces

[wagemd]

This might sound harsher than I mean it to, but if you don't have tire data and you're not sure what your load cases are, why are you making carbon a-arms? It seems like the time (and money - ala "looking for funding of tire data") could be FAR better spent.

The weight savings are not huge and I have seen many of them fail on several other team's cars.

It's hard to argue with the weight (or toughness) of appropriately sized, thin walled, steel tube. What happens when you driver murders a wall of cones? How bad is it if you drop a corner?

IMO, figure out the full 3d truss, get the tire data, and put the carbon somewhere else...

Excel is a great tool, so is Matlab (and Octave - free if you cant afford Matlab, but uses almost identical syntax).

[Kevin Hayward]

Ceboe,

I didn't mean to be short on info. It is a fairly straightforward statics problem, even given there are more equations to solve than most textbook problems. Just about any of the standard engineering statics textbooks will be handy. This is generally taught in the first year of mechanical engineering.

When reduced to a statics problem in a typical double wishbone system you will have 6 unknown link forces (assume pure tension/compression). You will also have the forces and moments applied to the corner via the tyre contact patch. The locations and directions of all of these forces are known through the suspension points. From this it is a simple case of the sum of forces (x,y,z) and moments (x,y,z) being zero. You will have a few cross-products to solve for the link forces to get moments, but this is not anymore difficult than first year engineering problems, in this case it is more repetitive.

Make sure you put in some decent vertical loads as well to account for hitting bumps etc. I would also look at increased lateral and longitudinal loads for similar situations. It pays to be conservative with your suspension load calcs. A failure of a suspension system can be one of the most dangerous failures in a car, coupled with the fact that it can end up being very hard to repair. A FSAE track is well contained and doesn't have a lot of things to hit if things go wrong. Not all of our test venues are the same. Will your suspension handle hitting a pothole or a grate that no-one saw until it was too late?

Once you have a matlab or excel program sorted you can very easily see how the location of the suspension points affect the link loads.

This is part of the fundamental vehicle design process. If your team is unable or unwilling to do this sort of calculation of loads then you should steer well clear of carbon wishbones and go for something much more conservative.

Out of curiosity have you looked at quality control and fatigue of the glued joints in the carbon rods? If you want any confidence in the manufactured links at all you will need to do this testing.

Kev

p.s. the thread Z mentioned is gold

[Ceboe]

Originally posted by wagemd:
This might sound harsher than I mean it to, but if you don't have tire data and you're not sure what your load cases are, why are you making carbon a-arms? It seems like the time (and money - ala "looking for funding of tire data") could be FAR better spent.

The weight savings are not huge and I have seen many of them fail on several other team's cars.

It's hard to argue with the weight (or toughness) of appropriately sized, thin walled, steel tube. What happens when you driver murders a wall of cones? How bad is it if you drop a corner?

IMO, figure out the full 3d truss, get the tire data, and put the carbon somewhere else...

Excel is a great tool, so is Matlab (and Octave - free if you cant afford Matlab, but uses almost identical syntax).

We are getting our carbon tubes sponsored, so they don't cost us any money. Steel tubes on the other hand, we have to buy..

[Ceboe]

Originally posted by Kevin Hayward:
Ceboe,

I didn't mean to be short on info. It is a fairly straightforward statics problem, even given there are more equations to solve than most textbook problems. Just about any of the standard engineering statics textbooks will be handy. This is generally taught in the first year of mechanical engineering.

When reduced to a statics problem in a typical double wishbone system you will have 6 unknown link forces (assume pure tension/compression). You will also have the forces and moments applied to the corner via the tyre contact patch. The locations and directions of all of these forces are known through the suspension points. From this it is a simple case of the sum of forces (x,y,z) and moments (x,y,z) being zero. You will have a few cross-products to solve for the link forces to get moments, but this is not anymore difficult than first year engineering problems, in this case it is more repetitive.

Make sure you put in some decent vertical loads as well to account for hitting bumps etc. I would also look at increased lateral and longitudinal loads for similar situations. It pays to be conservative with your suspension load calcs. A failure of a suspension system can be one of the most dangerous failures in a car, coupled with the fact that it can end up being very hard to repair. A FSAE track is well contained and doesn't have a lot of things to hit if things go wrong. Not all of our test venues are the same. Will your suspension handle hitting a pothole or a grate that no-one saw until it was too late?

Once you have a matlab or excel program sorted you can very easily see how the location of the suspension points affect the link loads.

This is part of the fundamental vehicle design process. If your team is unable or unwilling to do this sort of calculation of loads then you should steer well clear of carbon wishbones and go for something much more conservative.

Out of curiosity have you looked at quality control and fatigue of the glued joints in the carbon rods? If you want any confidence in the manufactured links at all you will need to do this testing.

Kev

p.s. the thread Z mentioned is gold

Hi Kev, thanks for your response.
He also have 2 companies that will supply us with the glue and they our helping us the fabrication. We have done some testing already and indeed we can look at the quality of our glued joints, also at this company.

It's just that I'm the only person in the team doing this kind of calculation, the others are working on other stuff. I have already started with the excel and it is getting there but now I am checking all calculations again because I still get pretty high forces.

Also I don't know exactly for witch force I am aiming for, I think it should be around 6000N or something for the highest loads. This is just gut feeling.

[Kevin Hayward]

Originally posted by Ceboe:

We are getting our carbon tubes sponsored, so they don't cost us any money. Steel tubes on the other hand, we have to buy..

Ceboe,

Be wary of this approach. The carbon may not cost you money, but it will cost you resources. The time taken to develop the carbon tubes will be enough that you could have earnt the money (and plenty more) working at McDonalds to pay for the steel. Heck, put on a decent BBQ and you will pay for the steel and a few other parts. Alternatively you could have put the effort into finding sponsorship and got even more.

It is much better to trade money for time than the other way around.

Kev

[Markus]

I don't think carbon tubes are that dangerous approach but as stated you want to have some fatigue testing done and a hefty safety margin everywhere and especially in the bondline (>25).

Be aware that any metal to carbon contact will corrode and this might effect the service life of the a-arms.

You might also want to study how to spread the stressess in the bondline (insert design).

[Kevin Hayward]

Markus,

I don't think there is anything wrong with carbon tubes used in suspension when properly developed. Undersized steel tubes can be just as bad as poorly developed bonded ends. Although I would be using greater safety factors and a lot more testing on the carbon.

My last post was more about the dangers of assuming something is free just because the materials have been provided.

Kev

[Markus]

I see your point and you're very right on this.