KNUCKLE ANALYSIS

[A]

hullo guys,
i am working on the force analysis of our car and following is the procedure i followed:
1 i considered corner+breaking condition and calculated the forces and moments as follows:
A.i assumed breaking acceleration to be 2g and wt of the car to be270 kg so friction force between tyre and ground is 5298N
B.Then wt force is 2649N and i assumed breaking force by the caliper to be 1200 N
C.then i assumed cornering force to be 2g and got force to be 5298N....i assumed d extreme case and applied all these forces on 1tyre as in extreme case(corner+ breaking) car will be on 1 tyre
MOMENT calculations:
A i transferred these forces to the knuckle centre .i got 132 Nm of moment because of wt force, 84Nm because of caliper ,265Nm because of friction (bt grnd n tyre)and 1345 Nm because of lateral force i didnt consider trailing here.


2 Then i applied these forces in ansys
A i applied the cylindrical constraint on the a arm pivots with only tangential motion free and i applied cylindrical constraints on the steering joint with all the motions fixed( i am doubtful about it)
B i got factor of safety around 1.5 ....guys correct me if i am wrong and any suggestion is appriciated ....as for now we r thinking of using aluminium but can we use tin??which 1 is better??/..thank you

[Zac]

Please use something close to proper grammar. Your post is difficult to follow.

Your load case is unrealistic. 2g braking and cornering? at the same time? on a single wheel? While overestimating loading is more desirable than underestimating it, you're too far off for your simulation results to be very useful. The conclusion will be, "hey, if we use a lot of material it doesn't matter what the geometry looks like."

You mentioned that you have a FoS of 1.5, what criteria is this based on?

How would you plan on making an upright out of tin? You might be better off with Mahogany.

[Marvel]

Originally posted by A:
hullo guys,

1 i considered corner+breaking condition and calculated the forces and moments as follows:
A.i assumed breaking acceleration to be 2g ..............
B.....................
C.then i assumed cornering force to be 2g ...........i assumed d extreme case and applied all these forces on 1tyre as in extreme case(corner+ breaking) car will be on 1 tyre
MOMENT calculations:
A ...................


2 Then i applied these forces in ansys
A .................
B i got factor of safety around 1.5 .................


2g of both lateral and longitudinal acceleration...???
"1.6g of longitudinal or lateral acceleration is enough to make your neck shore"- Tune To Win
And 2g is seriously way too high..
..........
Applying entire load on a single wheel itself???
100% lateral as well as longitudinal load transfer at the same time is very difficult(not impossible though) to get..This means that under cornering+breaking, your car will corner on the outer-front-wheel only and rest three will be completely unloaded..!!!
..........
According to me(people may debate http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif ) a "bit" of over-designing is OK(and safer) but you have considered a lot of it..
Check out weight of your knuckle.
..........
And seriously Tin??Never thought of it...

[exFSAE]

Troll?

[A]

sorry...i wanted to say titanium...our wt is 1kg each knuckle

[A]

are constraints correct????plz reply guyz....

[Claude Rouelle]

Marvel,

Good FSAE cars with wings can now get 2.2 to 2.3 G of lateral acceleration (not peak, sustained acceleration). Without wings 1.7 to 1.8 lateral G is a good target; if you design you suspension to use the tire efficiently that is possible.

A,

350 grams (without bearing) for an upright (that what I guess you call knuckle) is reasonable target especially if you use 10 " rim. I have seen many CNC or cast + machined aluminium uprights under 450 grams in FS /FSAE paddock.

Titanium is marvelous but you will feel the heat from the design judges and even more from the cost judges when you will be asked to justify the choice of such material. Better have good arguments. Th worse one I heard from a team using titanium hubs and uprights a few years ago was "because we can!"

You also may want to calculate the load on your upright and wishbone when your car goes backward (car spinning for example) and the driver hits the brake pedal ... but the brake balance is still 70 % front. I have seen a few suspension collapsing because this "unusual' case was not taken into consideration! Aame thing when you go backward, put the first gear and release the clutch.

Claude

[Marvel]

Originally posted by Claude Rouelle:
Marvel,

Good FSAE cars with wings can now get 2.2 to 2.3 G of lateral acceleration (not peak, sustained acceleration). Without wings 1.7 to 1.8 lateral G is a good target; if you design you suspension to use the tire efficiently that is possible.


Agreed!
But, 2g of lateral as well as longitudinal acceleration at a same time??
Isn't that too high to expect??
(well I am not sure..just inexperienced to comment on this... http://fsae.com/groupee_common/emoticons/icon_frown.gif )

[Z]

A

2G lateral + 2G longitudinal + 1.5FoS (= 4.2 x weight of car) is easily exceeded during ... cornering + braking + small pothole!

Z

[Claude Rouelle]

Marvel,


But, 2g of lateral as well as longitudinal acceleration at a same time?

No not at the same time but look at your tire model traction ellipse and it will give you an answer. The traction ellipse is neither a perfect circle nor a rectangle: it is somewhere in between and its size and shape change with other factors such as vertical load and camber.

Of course you should take lateral and longitudinal weigh transfer into account but you should not apply the whole car load on one tire!

3 G ( or 2 G and 1.5 safety factor) of additional vertical acceleration (additional to static load + weight transfer) is a seasonable assumption. However you do not have infinitely stiff suspension springs and ARB and there is a tire between the rim and the ground. That will change your calculations on the load your suspension members force will reach on a pothole. If you would not take the suspension and the tire into account you could come with pushrods / pullrods which are 30 mm diameter bars http://fsae.com/groupee_common/emoticons/icon_smile.gif

Claude

[Charles Kaneb]

You may want to calculate your a-arm loads and stresses, and make sure that your (cheap) a-arm bends/breaks before your (expensive) upright.

This can save you when you, say, hit a curb doing 40 mph head-on.

[Claude Rouelle]

Yep!!

Wishbone breaking before the uprights... Or the chassis. That what we call a fuse!

Would be nice that students show these calculations to the design judges.

Claude

[Yogesh]

Dear Sir,
May i know how to calculate stiffness of knuckle in CAE?


Originally posted by Claude Rouelle:
Marvel,

Good FSAE cars with wings can now get 2.2 to 2.3 G of lateral acceleration (not peak, sustained acceleration). Without wings 1.7 to 1.8 lateral G is a good target; if you design you suspension to use the tire efficiently that is possible.

A,

350 grams (without bearing) for an upright (that what I guess you call knuckle) is reasonable target especially if you use 10 " rim. I have seen many CNC or cast + machined aluminium uprights under 450 grams in FS /FSAE paddock.

Titanium is marvelous but you will feel the heat from the design judges and even more from the cost judges when you will be asked to justify the choice of such material. Better have good arguments. Th worse one I heard from a team using titanium hubs and uprights a few years ago was "because we can!"

You also may want to calculate the load on your upright and wishbone when your car goes backward (car spinning for example) and the driver hits the brake pedal ... but the brake balance is still 70 % front. I have seen a few suspension collapsing because this "unusual' case was not taken into consideration! Aame thing when you go backward, put the first gear and release the clutch.

Claude

[Yogesh]

Please suggest how to calculate stiffness of stub axle?

Yogesh,
Tata technologies

[Claude Rouelle]

First calculate the forces and moment acting on your hub. That simple. Do that and we will go from there.