flexures?

TWTW · 18 Mar 2002, 01:38 (Ref:237992)

Hey fellas,
I have recently become intrigued by the use of flexures by the F1 teams. I know that Jordan uses them in the back A arms. I am trying to design some as a project. I know that the A arm is acting as a beam which is being loaded at one end where it is connected to the upright. and that the flexures are only being deflected where they mount to the chassis. Can anyone steer me in the correct direction to figure out the deflection and criteria?

enzo · 18 Mar 2002, 05:31 (Ref:238101)

They're being used on the Reynard Indy car also.

To design them, you'll need to decide just how much suspension travele will be allowed. From thatyou can easily calculate the deflection at the arm end of the blade. The calculations are for a simple cantelivered beam. The "I" you'll use in the equation will be determined by whether or not you use a constant crosssection - I'd suggest that you do so as it is a heck of a lot easier for the calculations and manufacturing.

From the beam calculations, you'll determine the stresses. From there, compare those stresses to the fatigue strength of the material selected - if you want an almost infinite life, keep the stresses well below the fatigue stress levels. If you plan on a finite life, the stresses can be above the fatigue stress.

Once all that is sorted, look at & calculate the spring rate of the blades, and their contribution to the wheel rate. I'm not really sure, but I suspect that most F! cars set the neutral blade angle to coincide with full droop, otherwise the wheel rate will get fouled up in droop. if the rate is too high for your application, then start over!

enzo · 18 Mar 2002, 05:34 (Ref:238102)

Oops - he actual deflection that the blades will see for full suspension travel will be somewhat lowered by flexing of the a-arm. You'll need to model the arm also, and treat the assembly as 2 springs in series.

TWTW · 18 Mar 2002, 05:52 (Ref:238111)

thanks man,
you think its possible to design the flexure so that it adds almost no spring? Im not sure this is possible because of material limitations. I am heavily leaning towards steel due to its high mod of Elasticity, I am leaning away from Aluminium because it fatigues, Titanium is hard to machine... Any suggestions?

enzo · 18 Mar 2002, 13:24 (Ref:238351)

All material fatigues, including steel and Ti.

To decrease the spring rate of the blade, the requirements are a lower "I", lower "E", and/or greater length. Ti would be a better choice in this regard as it is about 1/2 as stiff as steel for the same strength.

Also, do't forget that part of what you'll have to sort out is the compressive strength requirements (column effects) for the compressive loads that will be applied.

sporty.dave · 18 Mar 2002, 13:27 (Ref:238357)

We have a programme her called CMS (i think it stands for cambridge material selector) if you can devise a ratio system of the properties the material needs the programme can tell you what material will work best.
Oh and sorry if this sounds a little vague but I have only used the programme twice and it was a while ago)
maybe this helps..
good luck (I know what a pain these simple sounding things can be.)

THR · 18 Mar 2002, 23:41 (Ref:238819)

The Jag i saw at autosport had carbon flexures and the front wishbones anyway.
i think they moved away from carbon cos it didnt last as long and cost more to make..
but they also had problems moulding in the tit into the carbon wishbones so it didnt pull out and was properly bonded.
flexures are a great idea really! simple and lighter than a rod end would be i guess to!

bobdrummond · 19 Mar 2002, 20:22 (Ref:239585)

Hi,

Carbon 'Flexures' were introduced by john Barnard on his '95 Ferrari 412 T2 (or was it the '94??). Anyway, he explained that their main advantage was that there was none of the tremendous friction associated with heavily-loaded rose-joints. At the time the 412 was introduced, Johns ex-colleague Patrick Head dismissed the flexures as a gimmick. Barnard responded by reminding everyone that a similar statement was made about semi-automatic gearboxes. The number of current F1 cars employing both tells you who was right. In an interview later in that same year, J.B commented that you have to get your buckling calculations right. Given the resources at his disposal, I assume the sums are complicated. Advance with caution. I wouldn't try and do composite ones if I were you.
As an aside, there were sketches of the Jordan ones in a contemporary issue of 'RaceTech' magazine.

KC · 20 Mar 2002, 16:00 (Ref:240220)

They are also used for the lower suspension pickup points on the MG-Lola LMP675.

TWTW · 21 Mar 2002, 23:45 (Ref:241216)

seems like the largest problem i am going to have is the bucking effect, im not worried about the vertical deflection, buckling will be what does them in it seems. Also seems that Ti is the best material due to its E

av8rirl · 22 Mar 2002, 00:24 (Ref:241237)

For the beginner here, someone please explain flexures...

enzo · 22 Mar 2002, 04:15 (Ref:241329)

They are those flat "blades" at the inboard end of the a-arms that are used instead of rod ends. The purpose of them is that hey do not have the friction that rod ends do, allowing the suspension to move more freely. The friction in a rod end has a "stick-slip" characteristic - it takes more force to get them to start moving than it does to keep them moving (remember "static" and "dynamic" friction and from grade school physics?). That static friction reduces tire grip.

av8rirl · 22 Mar 2002, 09:28 (Ref:241425)

Are there any close-up pics please?

KC · 22 Mar 2002, 16:15 (Ref:241702)

The term used to describe the stick-slip condition is common referred to as "stiction" in motorcycle front forks. The intial friction must be overcome and then the mated surfaces move more freely. There was a in depth article on the MG-Lola in RaceTech two months ago.

bobdrummond · 29 Mar 2002, 13:08 (Ref:246560)

The current issue of 'Racecar Engineering' magazine has a feature on Barnards B3 technologies company and the use of Titanium in F1. There are several pictures of flexures and wishbones.

I wish that Racecar could show the drawings that the parts are displayed on in the pictures. For example, there's an upright on top of a corner-assembly drawing that I'd love to see.

Griff · 30 Mar 2002, 00:28 (Ref:247029)

It does make a real change to read a tech. forum
with people on it who do actualy know what they are
talking about!! when I have somthing to contribute
I'l be back, many thanks
GRIFF.

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