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Top ball joint - direction of force is inline with the arm, angling either forwards or backwards under brakes (x / y direction). There is no shear (z direction) other than MoI of the arm mass and suspension bushings and there is no rotational loading due to the ball joint.
Lower ball joint - Forces here act in all directions (x / y / z). The entire weight of the corner of the car goes through this point, plus the majority of the cornering load.
Steering arm - Forces are only towards and away the steering rack (y). There is no rotational loading other than the offset due to the pivot point of the rod end being offset from the arm.
Hub bolts - Forces here are in tension and compression only. All shear loading should be taken by the press fit of the hub into the spindle.
Caliper mounts - All forces here act axially around the centerline of the hub. The top caliper mount will be in compression under braking, whilst the bottom mount will be in tension. There needs to be enough sideways rigidity to stop any vibrations of the caliper.
not quite. For loading I like the standard 2g braking, 3g cornering, 4-8g bump. That way it should have enough margin of safety to even work on a crazy non-dot aero machine, at least for a while.
Upper ball joint will see some of the cornering load I'd give it 800 lb pull and 300lb push, and it will see the braking load but its very strong in that direction compared to what its going to experience so I'd probably not even FEA it.
lower ball joint will see a lot of the cornering load, give it 1.5k lb push and 800k lb pull, again for the braking. I'd give it 4k lb in the bump direction.
steering arm will see the steering thrust forces, which if you've ever driven on 275 hoosiers with a manual rack you'll know they can be enough to make your wrist sore for a week. I cant even come up with a number here, just its more than you're expect.
hub connection. Bolts should only ever be loaded in tension, just make sure your torque spec on them is high enough to ensure that. The mr2 hub itself shouldnt be press fit, it should be a slip fit into that hole for alignment only and should take the majority of its force in friction between the spindle and the hub, again make sure your torque spec produces a sufficient amount of clamping force to generate enough friction.
you forgot the hardest stress to account for, and thats the brake mounts. Those ******* are the hardest thing to make happy in the whole damn spindle when playing with the parts in fea.
steering arm will see the steering thrust forces, which if you've ever driven on 275 hoosiers with a manual rack you'll know they can be enough to make your wrist sore for a week. I cant even come up with a number here, just its more than you're expect.
It's still only as much force as you can apply via a steering wheel though x leverage. Say I'm super buff and can hold 200 lbs through the wheel... whats the steering ratio? 4:1 plus a bit of extra leverage. so max of 1000lbs.
Originally Posted by Leafy
hub connection. Bolts should only ever be loaded in tension, just make sure your torque spec on them is high enough to ensure that. The mr2 hub itself shouldnt be press fit, it should be a slip fit into that hole for alignment only and should take the majority of its force in friction between the spindle and the hub, again make sure your torque spec produces a sufficient amount of clamping force to generate enough friction.
You can get a pack of ARP SS bolts that hold up to 170,000 PSI for around $40... something like that should be suitable I would guess. M10 x 1.25 is the thread. I'm not changing the hub mounting style so how they are mounted from factory should be ok... subject to big Hoosiers and aero of course.
Originally Posted by Leafy
you forgot the hardest stress to account for, and thats the brake mounts. Those ******* are the hardest thing to make happy in the whole damn spindle when playing with the parts in fea.
The braking force isn't that massive given you have 4 of them working at once. I think the main thing here is to make the mounts rigid enough to stop the caliper moving around under load. I've found some references to high end spindles and the braking mounts aren't overly large, in fact some are on slim stalks. I figure as long as my mounts are larger and beefier than what is currently on the car I'm all good.
If you manage to put more force into the arm by moving the steering wheel than the ground imparts into the arm from cracks and bumps then you probably just had the tire stuck against something and reefed the **** out of the steering wheel with PS.
I'm slowly working on my design. Not sure if it's still the right direction or not but I may as well try it and see how it goes. I figure I'm just going to start machining stuff and see how it turns out. I always find it easier to judge weak points in a design when I have it my hand. I'll probably start with the billet UCA given it's easy to make first and I can run it on my existing hub.
Here are a few key design features.
1. +3 degrees camber built into the spindle
2. MR2 hubs
3. Lightweight - approx 700 grams
4. Ducted 2.5" vent through hub to cool bearings and rotors (see below)
5. Spherical bearing for UCA connection
6. Billet UCA with 5/8 rod ends. Weight of the arm approx 550 grams
7. Rod end for steering arm
8. Solid mounts for radial caliper
All up it looks a bit too light to me. I think I'm going to add a bit more meat by increasing the radius of the hub as well as the wall thickness of the inner and outer rings. I think the steering arm connection looks too large, so I might drop the height of that a bit.
Questions:
Should I include a small spindle drop or keep the factory hub location?
Running the vent through the hub I think is a good idea. It should route air right past the bearing which will be nice to keep the temps down in the bearing itself. I might need to add a small shroud out to the rotor surface. Any issues with this?
What kind of wheel/rotor offset are we looking at here for these? Or, how much farther does the hub face protrude compared to stock?
I run 6UL 10" rims with 245 Hoosiers with 11.75 Wilwood rotors and dyapro radial mounts.
Placement is exactly the same as a stock hub. The MR2 hub is shorter by a small distance. From memory I think the outer surface of the spindle is around 5 - 10mm further out to make up for the shorter hub.
I think if I wanted to run wider track I'd lengthen both the control arms rather than push the hub out further but that would mean making a billet LCA as well which is a whole stack harder.
Machining the spindle will be fun. It will need to be made from a 75mm thick billet. I'm going to need to machine up a tool holder for my CNC machine to mount up a 10mm bit. I'll need the length.
I'll get a new control arm scan done this weekend and send it over along with the upright scan I have. Sorry, that totally fell off my radar this past few weeks, been very busy with work.
I think I mentioned this already and I don't mean to sound blunt but compare the size of the MR2 wheel bearing to the stock Miata if you have a chance. The Miata wheel bearing is too small for race cars evidenced as a known failure point. Moving the tie rod mount to help bump steer might also be cool in addition to adding negative camber which you mentioned.
I'll measure the bearing at some point. Now I have one I'll pull it apart and have a look. There is a whole range of timken hubs based on these bolt centers so there are more options for stud number and spacing. Ideally for a race spindle you'd use two offset bearings but thats a whole level of extra work.
The tie rod can be moved down by changing the spacer amount. If I make it so that the rod end center matches the stock position and then it can be packed downwards if required.
I forgot if this got asked - is that your personal CNC equipment or are you borrowing time on someone else's? Just because I'd *like* to step up to having the ability to do stuff but relatively few people do.
09-07-2016, 11:16 PM
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