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i have a question. How stiff is stiff enough perhabs for a Hub and Upright loaded with the critical forces? Is there a simple way to estimate if it´s ok?
A friend of me design a Upright for a FSAE car and did an FE Analysis. The displacement on the wishbone brackets was i think 0,2mm is that stiff enough? What are general way to check this and what are your goals to achieve?
I know it has to do with experience but i think is not that easy so say ok it is ok or it has to be more stiffer. The Same on the Hub.
Well you kinda have see what design case you are talking about, what load you are inputing and what might be your weight target. If you are looking at your max load cases, being as full braking, full bump, cars lifting wheels...etc. Based on your car weight and what not, and you get 0.2mm of deflection, its probably good enough, especially if your upright is of a reasonable weight based on the most feasible way of making it. For me at the time of my first upright for a welded steel upright replacing a CNC alloy one, I am starting at a weight disadvantage but we were trading that for the fact we can get the car done a month earlier(in house manufacturing vs sponsor machining), that was acceptable, and my work was to make that as less as possible and hopefully get a stiffer design. So I ended up with a design that was ~50% more weight, but about 300% stiffer(~1.7lb per stripped upright, all figure based on CAD/FEA), and based on my load cases it was ~0.006" deflection at peak braking + bump. The next year I was able to shave about 0.3lb off due to some different design changes in tri-pod/hub design, maintaining the stiffness.
Its also simple enough to test an upright to correlate to FEA, design an appropriate fixture and a simple load case to run through, and replicate that in real life and you can see how that looks. Machined one will be much more accurate than a welded design simply because FEA suite don't deal with weldment too well, but you take that into account when you design as well. In our case in the end the part didn't break, the car responds well to setup changes, and we know its "good enough"...
You may consider writing a program that translates Your deflection into resulting steering & suspension geometry. Then determine if You (and Your team) are willing to tolerate the resulting geometry. Be sure to consider each particular load case on each upright.
This can be calculated using pencil & paper, CAD, or any numerical computing software (MATLAB/ MathCAD/ spreadsheet/ etc). It can also be translated in reverse to set upright stiffness targets (e.g. if bearing deformation is a high priority).
I would add all the deflections from the front wheels to the rear wheels under worst load case. Then check how this will affect the position of your tires (chamber, toe,...) and your performance. When you know how much total deformation you want to allow you can define a max deformation for every single part in the chain. 0,2mm doesn't sound much but when you add up all the parts this can cause a significant deformation. I think 0,2 is to much for the upright.
You can do something similar for manufacturing tolerances.
Ultimately with hub / upright / bearing compliance you're trying to control wheel orientation.
You have to make an engineering decision to figure out how much toe and camber compliance is significant. Tire data definitely helps... only real way of making a legit decision.
What is 1 degree of camber compliance going to do to your grip level? Make or break? Insignificant? How about 0.5 degree? 0.1 degree? Same with slip angle.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.
@RacingManiac what have you used for your Upright? Normal steel or like the 15CDV6 with a high tensile strength? What sheet thickness you have used?
It is in interresting thing to weld an Upright an do only a little bit of milling after welding insted of milling a full one.
