Carbon Fibre Wishbone study

[Kaizoku]

hi everyone...i'm new at this forum so i don't know much about FSAE but i've got a problem related to this topic. Can the wishbone in the suspension system be made of carbon-fibre? if it can, how should the design be done? n how do i calculate their capable loads?

if there is any book regarding to wishbone design, could u please recommend it me

Thank you for your attention

best regards

Ferdy Djafar

[D-Train]

Delft have had a lot of carbon fibre on previous cars, and some other universities have run carbon fibre wishbones before. Try using the search function in the forums, there should be a fair bit of info around the place.

By capable loads, i'm assuming you mean how much can a given peice of carbon tubing withstand in tension/compression. You'll have to run the calcs for that if you're purchasing carbon tubing, or just do some testing if you're making the tubes in-house.

[ben]

Originally posted by Kaizoku:
hi everyone...i'm new at this forum so i don't know much about FSAE but i've got a problem related to this topic. Can the wishbone in the suspension system be made of carbon-fibre? if it can, how should the design be done? n how do i calculate their capable loads?

if there is any book regarding to wishbone design, could u please recommend it me

Thank you for your attention

best regards

Ferdy Djafar

There isn't a book on wishbone design as such but this is basic mechanical design.

You will need to assume a vehicle mass (wet including driver) 300Kg would be sensible conservative estimate. You'll need to estimate the mass distribution and calculate the vertical load on each wheel.

The tyres we use a good for sustained cornering at 1.5G without any aero, call it 2G as a safe estimate. Multiply the vertical load on the wheel by 2 and this will give you the in-plane load that the suspension has to resist. Multiplying the vertical load by 3 would be sensible to cover bump cases.

Now that you have a series of loads at the contact patch you need to use your suspension geometry to calculate the chassis reactions to these forces and this will give you the loads in you wishbones. You then need to design the section modulus of your CF tubes to take these loads.

Ben

[Lukin]

Im a bit disappointed Ben. You give a great rundown of tackling an engineering problem do not once say 'free body diagram.'

HAVE YOU LEARNT NOTHING!

http://fsae.com/groupee_common/emoticons/icon_smile.gif

[jack]

bladder molding is the way to go--but very difficult to get right.

[Chris Razl]

We had/have carbon tube a-arms, with machined Al end lugs. Under normal use they were fine, including a few laps with a notch worn right through, from the rim (steering stop came loose).

However, eventually we snapped all of our lower front arms, by hitting cones. We chose to run steel for endurance. Something to keep in mind when you pick your design safety factors; when you wedge a cone between the lower arm and the ground, that member sees a large bending load and will buckle at a *much* lower axial force, orders of magnitude lower. Carbon is far less tolerant to this abuse than steel.

You will find that you end up with massive safety factors anyway if you make it stiff enough to avoid excessive camber change. We went through all of that, and we still broke arms when running over cones.

As a side note, I am most impressed by the safety benefit rather than the weight+eyecandy aspect of carbon vs plain steel: Shish-kebabing my leg with a steel rod if I spin into a wall doesn't sound like fun...whereas carbon would shatter nicely in that situation. I've noticed that no one here discusses safety much.

Chris Razl
Queen's University FSAE
Kingston, Ontario, Canada eh

[ben]

Originally posted by Lukin:
Im a bit disappointed Ben. You give a great rundown of tackling an engineering problem do not once say 'free body diagram.'

HAVE YOU LEARNT NOTHING!

http://fsae.com/groupee_common/emoticons/icon_smile.gif

That smiley needs to be bigger :-) I mentioned finding the loads in the tubes, obviously this will involve some free body diagrams somewhere!

How's the new job going BTW?

Ben

[Buckingham]

As a side note, I am most impressed by the safety benefit rather than the weight+eyecandy aspect of carbon vs plain steel: Shish-kebabing my leg with a steel rod if I spin into a wall doesn't sound like fun...whereas carbon would shatter nicely in that situation. I've noticed that no one here discusses safety much.

Isn't the carbon more likely to shatter when it hits a cone, where steel will simply bend a little?

You can drive a very long way on a bent control arm, but not very far on a shattered one.

Additionally, If you design the control arm to be weaker than its mounings, the steel arm is less likely to end up inside the cockpit. Rather, it will stay a mangled piece of steel still connecting the tire to the chassis, while half of the carbon arm is flying everywhere and the other half is attached to the wheel (which is now sent rolling at a courseworker).

[Chris Razl]

Originally posted by dhaidinger:
Isn't the carbon more likely to shatter when it hits a cone, where steel will simply bend a little?
You can drive a very long way on a bent control arm, but not very far on a shattered one.


Yes, precisely, that's what I was saying we found to be the major drawback of carbon. That's why we used steel for endurance, and why I'd rather not use carbon again.



Additionally, If you design the control arm to be weaker than its mounings, the steel arm is less likely to end up inside the cockpit. Rather, it will stay a mangled piece of steel still connecting the tire to the chassis, while half of the carbon arm is flying everywhere and the other half is attached to the wheel (which is now sent rolling at a courseworker).

If you hit a cone, its not like the wheel breaks off, the rest of the suspension will still be intact; Sending the wheel flying isn't an issue.

I neglected to mention that our chassis was a carbon-fibre monocoque without an internal steel frame. If your a-arms are welded, and bolted to a space frame: in a crash, intrusion into the cockpit is not likely, as you say. Things would just get mangled. However on our car, a steel arm would punch through the monocoque and cause injury, whereas the carbon would just shatter.

The carbon is safer in a crash, but much less reliable in normal use. We saved about 8lb in the suspension vs steel though.

Chris

[Scott Cranston]

We ran a carbon/kevlar hybrid tube for our steering arm and front lower fore wishbone this year. The kevlar is only on the outside layers and is thicker than the rest of the cars tubes. It hasn't hit a great deal of cones but easily survived what it has done. Also, does anyone coat their composite tubes for protection from rocks and debris?

[Wright D]

What is an actual Cost/Weight savings of carbon a-arms?

And wouldn't steel a arms, being smashed in an accident absorb more energy then carbon ones, and so be safer for the driver? Plus, a steel arm might hold the wheel/upright tethered better then carbon a arms.

[Erich Ohlde]

You know a front wing would do a marvelous job of keeping the cones off of the suspension...

[TomF]

As was mentioned above, Delft has indeed used several carbon fibre wishbones the past couple of years. Not one of them ever broke due to a cone getting stuck/wedged. On the other hand, we did not manage to run full endurance runs either, so if we had a cone stuck we kept it there for not more than a few seconds. However, it does seem to be a good idea to use carbon. The only problem we encountered were adhesive bonding of an aluminium insert and rod end to them, but that was due to the quality of the process... Also our car is very light, meaning that the forces in the arms are lower and therefore smaller chance of breaking them of course. But we did run oval shaped carbon fibre wishbones on a 275 kg car. That one did have a cone stuck, but it never broke...

[Hammock]

Just curious what your thoughts are on the use of flexcore and unidirectional fabrics around a firm core, or even no core. I think a hybrid arm could resist the forces of a cone strike, while still retaining the desired weight and strength. Of course a braider and autoclave would be very sweet, especially if any pre-preg was going to be used, but I think rather primative methods could still offer great results.

[Scott Cranston]

We looked at using some kind of core to improve buckling strength. It is a good idea as you can improve global buckling strength and reduce weight. As far as cone impact goes, the trouble is not the whole thing shattering but multiple impacts. Carbon composites get small delaminations every time they are struck which over time reduce the structural integrity. Having a core would reduce the likelihood of a very thin walled tube from shattering, but would still not prevent continual impacts from doing their damage. I am of the mind that the leading arms should be periodaically replaced to prevent their fatigue failure.

[Hammock]

Does anybody have experience or knowledge about the use of expanding foams. Seems to me they could dampen the vibrations and provide structural support. Also, for the drivers state of mind, perhaps they could prevent the carbon from completely shattering and implanting itself in their legs. I see it is used in many other sports and industries.

[joesnif]

just wandering what diameters and wall thickness those who have built successful cfrp sus parts have used. i have found the range of sizes avaliable commercially is fairly limited (cant find any wall thickness over 1.5mm) so am thinking of building and testing some but need a benchmark to work from.

[SpdRcr]

Originally posted by jayhawk_electrical:
You know a front wing would do a marvelous job of keeping the cones off of the suspension...

Up in New England we prefer to use a snow plow. Does a number to our front to rear weight ratio though...