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Rear Hub Design Forces

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    Posted: Jun/26/2013 at 7:55pm
Hello,
I am currently in the works of designing some front and rear wheel hubs for our car ( we have used modified polaris hubs till now). just wondering if anyone could give me a number that they have used in the past for a lateral force acting on the wheel and thus creating a bending moment on the spoke of the hub, lets say from the wheel impacting an obstacle from the side. im also wondering if anyone knows an approkimate max braking force to apply at the point where the brake disk connects to the hub, ie what force is required to lock the wheels while moving?

Thank you very much, any information is greatly appreciated.

Because Racecar
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Post Options Post Options   Thanks (0) Thanks(0)   Quote APhillips Quote  Post ReplyReply Direct Link To This Post Posted: Jun/26/2013 at 8:38pm
Why don't you tell me the numbers you think might be right and how you got them, then I'll tell you if I agree and why they may differ.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Jun/26/2013 at 9:15pm
I was thinking of applying a force equal to 2g (assuming 375lb car+200lb driver = 575 so give a bit extra(safety factor) and lets call it an even 600lb then force being applied is 1200lbf) and apply that at 11.5" from center of hub (where tire touches the ground). this creates the moment that would be created in the actual situation. however when I plug these numbers into solidworks simulation, fixing the spline and bearing surfaces, the result doesnt make sense. i ran the exact same simulation on the polaris RZRs hub that we used this past year and its telling me that that will fail as well, although the actual part has seen some hard hits and shows no sign of deformation.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote APhillips Quote  Post ReplyReply Direct Link To This Post Posted: Jun/26/2013 at 9:52pm
You're certainly on the right track and much further along than I assumed from your original post.

Typically, I would use 3Gs as a load case, certainly at least 2.5. Hubs aren't easily replaced/remade/repaired when they break the night before or during competition. You probably will see spikes of 2Gs, so 3 gives you a decent safety factor.

Next, I'm not sure I understand what surfaces you're constraining. A screen shot would go a long way here. I would constrain the surfaces where your wheel bearings are, as well as the "thrust face" that prevents the hub from pushing through the upright. Then, apply your load through the spokes where it bolts to the wheel, remembering that the bottom half will be moving inward, while the top is being pulled outward.

Secondly, while experiencing a 3G bump, your tire will be anything but round, and you're probably pretty much touching rim, so your moment arm is probably more like 6 inches (assuming 10" wheels.)

Lastly, remember that in the real world the entire car absorbs some of the load. Toe-links flex, rims bend, chassis deflect, etc. And getting good FEA results takes years of experience. So don't entirely trust the computer. I've seen lots designs in FSAE and Baja fail in FEA but live quite happily for years and years on a car, and those that pass fail. It's all about knowing how to understand your FEA results, rather than what they necessarily are.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Jun/26/2013 at 9:53pm
here is the Simulation done on a polaris XP rear hub with a solid aluminum 1"square bar representing the rim. if this simulation sas that parts of the bar are going to bend, i'm thinking that the forces are quite a lot more than whats actually going to happen as at that point your likely to de-bead a tire or bend a rim from the looks of this.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote APhillips Quote  Post ReplyReply Direct Link To This Post Posted: Jun/26/2013 at 10:02pm
I'd get rid of the bar - calculate what those forces end up being at the hub face, and split up the force over the face the bar is touching as I mentioned.

Unless you only have one lugnut tight the bar isn't very indicative of the loading scenario. And it's hard to tell how the bearing surfaces are constrained but it looks like you missed the aforementioned thrust face which is critical for absorbing the compressive force at the bottom of the hub flange, while the inner wheel bearing will counter the tensive force at the top of the hub flange.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RLM Quote  Post ReplyReply Direct Link To This Post Posted: Jun/26/2013 at 10:44pm
I actually discovered a function that works really well in solidworks simulation. it's the remote force/mass function. with it i am able to input where in space i want to apply my force and what direction the force is in as well as what faces it acts upon. when i use this, i find results that are just as i expected and i know how to make my design better. for anyone else who is designing things such as hubs or uprights, i highly reccomend this function to mimic a wheel acting on your parts. thanks APhillips for your advice and help.
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