Hubs with built-in tripod joint

[Z]

Josh,

To clarify, you have a VERY BAD DESIGN.

Part of the reason so many other Teams persist with such bad designs is "because Racecar". I have heard too many FSAE students say "But it's supposed to fail like that, because it's a racecar...". Yes, literally!

But the main reason for your bad design is that you are simply copying everyone else. If this is not so, then please provide a comparison of above design against another ~half-dozen obvious alternatives, with OBJECTIVE NUMBERS for cost, strength, stiffness, lifetime, etc.

Anyway, here are some aspects of the design that are crucifiable offences (my dear old dad would have nailed me to the wall if I suggested them ). Worst offender first, then in no particular order:

1. COST - You start with two ma$$ive billets of expen$ive alloy (upright + axle), then machine away almost all of them! The end result is a flimsy upright/wheel-hub assembly that, despite still being overweight, would quickly fatigue fail from the stress raisers you put in it (Goost's #6, at the highest stress-reversing area of the axle!). Except that the wheels will be flopping around so much that you won't be able to drive the car hard enough for the loads/cycles to do their work (see why below).

In short, a very expensive way of building heavy and weak parts, which are also perhaps the most crucial parts of the car! "But, hey, that's how everyone else does it!"

I most certainly would not buy a part like that. And any cost-conscious Production Engineer at any (good) company that you might work for in the future would not like it either. The profligacy you are practising here is common in the Racecar world, but NOT good elsewhere.

2. UPRIGHT - Too much material in the wrong places, and not enough in the right places. So, overweight + understrength!

How did you come to such a design? Did you have any strength/stiffness targets? (Yes, I know, cost was no issue...) More importantly, how many alternative designs did you consider? What are the numbers?

3. AXLE & BEARINGS - Just a "...light press on the spindle and a slight slip in our uprights", eh? And "...an inner race spacer to snug up against.", with the snugging-up done with an aluminium nut? And you are "...following SKF reccomendations for this [shoulder] diameter", but seem not to have noticed the word "minimum", which probably applies only to HARDENED steel axles...

I very much doubt you would get through a single 22 km Enduro with the design as shown so far, let alone any significant testing. And that is even if you use the world's most expensive aluminium alloy (the usual "racer" approach to solving such problems).

4. BRAKE DISC - Is it a good idea to drill a "speed hole" in the most highly stressed part of the disc? Or, put the other way around, where do you think the first cracks will start to appear in the disc?

5. OVERALL - You are a very-first-time Team. So why not buy one of the many small-car axle/bearing/hub units that are available off-the-shelf, in a size just right for your needs? These are much stronger and stiffer than yours, would have a lifetime of around 100,000 miles (cf. MegaDeath's ~2k miles), have a similar mass to yours (or perhaps less, after some machining?), and be much, much quicker and cheaper to get done.

If you think you can do a better job, then try next year. But compare objectively, with real numbers.

Z

[Jay Lawrence]

Josh,

Don't get disgruntled by taking the above as how it comes across ("EVERYONE EXCEPT ME IS A FUCKING MORON!!!!").

I completely understand the desire to 'engineer' your car as much as possible, because being a student engineer is what that environment promotes. This creates many FSAE cars with many 'highly engineered' components that get cobbled together to create many crappy cars. As a first year team, just make the thing as cheaply as you can. This should drive you to simplicity, which will drive you to lightness and a quick build. You will learn the most (and earn the most points) by driving/testing the car. Unfortunately with designs like yours, it's very hard to have a backup plan and typically the lead times and costs can mean that you have designed yourself into a corner (in which you can't drive the car).

[Joshkb]

Thank you for all of the feedback, everyone.

The radius on the back of my rotor hat could have been larger, I agree.

For the bearing shoulder on my hub, it is small to fit our rotor buttons. A radial circuit analysis showed that 5mm of clearance between caliper and wheel + the radial distance required for our brake pads to not interfere with the buttons left a small gap between that point and our 80mm spindle. The brake buttons were then located as far out (radially) as possible, and the remaining gap was the space available for this shoulder. My though process may have been flawed, but that is what I did to determine these dimensions. I probably should have valued this shoulder size more and modified our buttons.

Our main goals for the upright was to keep the stress under 300 Mpa during simulation, which we did in SolidWorks Simulation. Z, where do you think I have too much material, and not enough?

The material for each hub was $80, the machining, yes, quite expensive had I not done it myself.. and even then, it cost us time.

Josh

[Z]

Josh,

This would be easier if you gave more details. This is supposed to be "engineering", after all. So,

1. What is your engineering specification for "a light press on the spindle and a slight slip in our uprights"?

2. Do you you realise that there are proper standards for describing such "round-peg in a round-hole" fits? (Hint: look in the bearing handbook...)

3. How "snug" is the pre-load compression on the two inner races? (FWIW, many cars' workshop manuals have the torque figure for this "nut that does the snugging-up" as the highest of the whole car. Typically, several 100s of ft/lb or Nm.)

4. More importantly, what is the difference in widths between the Upright's shoulder (ie. that separates the two outer-races), and the "inner race spacer"? (This spacer is not shown in your CAD image, so I guess its importance is a recent discovery? Its width is important because it detemines how floppy your wheel will be.)

5. Have your teachers given you any formal "Design of Bearing Assemblies" lessons? (I guess probably not, and this thread is a result of their failing, not yours. You should tell them that.)

6. What are the uprights made from? (If not good quality steel, then what aluminium-alloy do you plan on repeatedly stressing to 300 Mpa?)

Anyway, you have now backed yourself into a corner that is difficult to escape from. So many little details that need fixing. I would almost be inclined to start from scratch. Perhaps just buy the core parts off-the-shelf...
~o0o~

...the upright ... where do you think I have too much material, and not enough?

You lost this race as soon as you chose to put the tripods INSIDE the uprights. Even with the "thinnest ring" DGBBs, namely the "68xx" series, you now have well over a foot (314 mm+) of circumference of bearing that has to be wrapped in upright. So no matter how thin you make the part of the upright that surrounds the bearings, there is inevitably a lot of it!

Worse yet, if you do not make that bearing support reasonably stiff, then the flimsy outer-race of the thin-ring bearing deforms, and only a very few of the balls must carry all the load. Rapid failure follows. (Followed by students cheering "Yey! At last! It's now a real racecar."!)

Look at how your upright transmits the many different loading patterns (= cornering, braking++) into the bearings. Look at how your crude "slabs" connecting the upper and lower BJs to the bearing-support-ring feed the loads into that bearing-support-ring, Specifically, look at the deformations!

A folded sheet-steel upright, similar in outside dimensions to yours, but fully "closed/boxed" and made out of maybe 0.6 - 1.0 mm thick sheet, would give your 68xx bearings a much better chance of survival.

Then get rid of the stress-raiser in the live-axle (ie. between outer-bearing and disc-brake flange). Look at the abundant prior-art that shows how to do this. (Hint: large fillet radius between axle-OD and disc-flange ... and then an appropriately shaped SPACER between outer-bearing and flange.)

And, your "toe-base" is probably much too small...

Z

[Joshkb]

Z,

1 - The bearing inner races have a .0002-.0005" interference fit on our spindles. For our uprights, they have not been made, but we do not have an internal mic that large, so i'll have to measure some other way, or go by feel using an op-stop program that adds .0003" to the diameter per pass + spring pass. Any smaller and I don't get good tool engagement.

2,5 - I believe the standard fit for this bearing is a k5. Sadly, no, our professors and program in general does not cover bearing assemblies, different fits etc. We have a helpful machinist who we can ask these types of questions, but honestly, I'm not sure I could name a faculty member I would go to expecting a helpful lesson on picking fits.

3 - Rather than controlling the torque at the nut to provide axial preload, I have been guided towards a length-based tensioning method, which makes more sense to me. When tensioning the nut, i'll have force coming from the fit between my inner race and spindle, which leaves me guessing what component of that torque is actually from the bearings being preloaded. Let me know if you have a correction on this, I fell I am missing something.

4 - Bearing center-to-center distance is 35mm (~1.38") and the upright has a total width of 45mm (~1.77"). I've been guided by a SKF applications engineer to make my spacer on the order of .001" smaller axially than the outer race shoulder. Yes, you are right, I had initially planned on preloading the system using the nut.

6 - We had 7075 - T651 donated for these uprights and are planning on using that material, but then you knew that..because racecar. Our simulation stress resulted between 220 and 240 Mpa.

I will thicken the walls of the bearing housing and do a study on deflection of the area.

Thanks for the input, Z.

Josh

[Z]

Josh,

The most disappointing thing about your approach to this design is that, regardless of the hub-assembly's eventual performance, your Team is now very likely locked into this particular design forevermore. Each year from now until the end of time will see essentially the same design repeated, albeit each year having some small and inconsequential changes. This despite the fact that absolutely no objective engineering analysis of the pros/cons of your current design against the many alternative designs was made, or will be made. No comparative analysis of total mass, strength, stiffness, lifetime, cost, etc...

Of course, this is not unusual, because it is pretty much exactly the same approach taken by most other Teams. Namely, copy what everyone else is doing, and when the wheels fall off, cheer "Yeay! Perfectly optimised!!!"

Your current design can be made workable, and done right you should be able to complete a comp with it. To that end I was going to give you a few more suggestions on fits, preloads, slight changes to parts, and so on. But I really do not think it is a good idea to keep perpetuating this irrational approach to design.

So good luck, and let us know how it works out.

Z

[P^squared]

Josh,
How much do your uprights weigh right now ? You should not mind adding material to places where you cannot project what the actual loads/deformations might be.

Z,
The reason teams follow this 'tripods inside the hub' design is because then they don't need a lot of steel in their hubs to accept splines.

The obvious, simplest way of doing this is to use solid axles, but that will need a lot of systems to be modified/simplified, and not an option for Josh right now.

Using flange couplings(welded on the tripod side) to connect the tripods to the hubs is far simpler, but that won't greatly reduce the hub OD/ bearing ID.

Any designs that might be missing out ??

[Will M]

Josh,

1. COST - You start with two ma$$ive billets of expen$ive alloy (upright + axle), then machine away almost all of them! The end result is a flimsy upright/wheel-hub assembly that, despite still being overweight, would quickly fatigue fail from the stress raisers you put in it (Goost's #6, at the highest stress-reversing area of the axle!). Except that the wheels will be flopping around so much that you won't be able to drive the car hard enough for the loads/cycles to do their work (see why below).

In short, a very expensive way of building heavy and weak parts, which are also perhaps the most crucial parts of the car! "But, hey, that's how everyone else does it!"

I most certainly would not buy a part like that. And any cost-conscious Production Engineer at any (good) company that you might work for in the future would not like it either. The profligacy you are practising here is common in the Racecar world, but NOT good elsewhere.

Z

That's a negative Ghostrider.

I cannot speak to the floppiness of such a design but is IS well designed for manufacturing.
During my FSAE time (2011) we arrived at a near duplicated design (see pics) precisely for its manufacturability.
And now I do this kind of stuff professionally.

The key is to assess FSAE designs relative to the real world industry that FSAE most closely resembles.
And it is not the automotive market but rather low-volume high-mix (LVHM) manufacturing.

So yes machining it from billet 7075 is expensive material wise and possibly machine time wise.
However you will only be making one of each of these so your most of the processes traditional used in the automotive world don't work well.
In a LVHM environment material cost is only one factor; tooling and design are often the biggest cost.

For those uprights I would estimate less than $75 of material and about 45 min of CNC machine time.

So let's see what our options are.

For a larger volume a near net shape process would be a good idea but not here.
So no die casting or stamping because of the tooling costs.
These would still need secondary machining so your equipment costs will be higher.

Investment casting could work if you 3D print the pattern, no way if you mold a wax pattern.
But this is rarely a cost effective process for one off parts unless it absolutely must be cast.
Still have secondary machining.

Sand casting with a wood pattern.
Cheap but typically would not be acceptable in quality or performance for this kind of part.
Secondary machining and embedded silica particles! yay!

Electron Beam Additive Manufacturing.
Could make it out of stainless steel or titanium, so that is cool.
But either way the cost will be an order of magnitude higher.

Fabricated steel "box" upright
Sheet steel and a small thick wall tube for the spindle bore.
Welded together, either self fixturing or a small jig.
You probably can reach a weight parity, and material costs will likely be lower.
But now your design is more complicated (a welded assembly vs billet).
How much did that extra design time cost you? A good designer can cost over $600 per day burdened rate.
Designing for a single item made from a homogeneous material is more straight forward than a welded structure.
Especially if you want to set up a simulation or need to create drawings and BOMs.
Remember you only have $75 of material to start with.
Also you now have 3 process steps (sheet metal cutting, welding, final machining) compared to just one.
If your labor is free like in FSAE this might be a great idea, but a skilled welder is also expensive.

tl/dr version.
Or you can just hog it out of billet.
Yes your material costs are higher but you will have less cost from tooling, equipment, labor, and design time.
In the LVHM world making parts from billet is a totally reasonable thing to do.

-William

(Also Josh you do need positive retention for the hub. Check you the linked photos)


Originals here of the 2011 parts:
http://media.wolfpackmotorsports.com..._display_media
http://media.wolfpackmotorsports.com..._display_media
http://media.wolfpackmotorsports.com..._display_media
http://media.wolfpackmotorsports.com..._display_media
http://media.wolfpackmotorsports.com..._display_media
http://media.wolfpackmotorsports.com..._display_media

[Z]

William,

I have no idea what a "negative Ghostrider" is, but I am certain that "Profligacy 1.01" is indeed now a core subject of the modern Engineering curriculum (which explains why it is so widely practised in the western world's "LVHM" industries = shiny junk that doesn't work well). And "Critical Thinking 1.01" has long been abandoned as too "old-school" for this modern age...
~o0o~

I cannot speak to the floppiness of such a design but ...
...
The key is to assess FSAE designs relative to the real world industry that FSAE most closely resembles ... low-volume high-mix (LVHM) manufacturing...
So yes machining it from billet 7075 is expensive material wise and possibly machine time wise. However ...

I would say that the key to assessing FSAE designs is that the BEST DESIGN is the one that helps the Team score MOST POINTS at the competition. (Unless your goal is to lose? Is it? )

So the most important factor is reliability, which here includes fatigue-life and strength (because no-finish = no-points). Next in importance are things like stiffness (important for hub-assemblies), speed of manufacture, mass, and cost, with all of these having to be assessed very objectively with carefully estimated NUMBERS that reflect "point-scoreability". Least important, although too often given undue weight by FSAEers, is "bling".
~o0o~

Anyway, your analysis of the various options is, sadly, and very typically in Engineering these days, a distorted and subjective one with next to NO NUMBERS. For example,

Fabricated steel "box" upright...
You probably can reach a weight parity, and material costs will likely be lower.
But now your design is more complicated...
How much did that extra design time cost you? ...
Designing for a single item made from a homogeneous material is more straight forward than a welded structure...

It only takes a bit of experience making these things to know that the subjective bias you have given to the above analysis is hogwash!

Specifically, your suggestion that the design of a folded-sheet-steel upright is somehow "more complicated" than that of a billet-machined upright is baseless. Why? I am sure that many people who have done it both ways will argue the opposite.
~o0o~

And another example,

Also you now have 3 process steps (sheet metal cutting, welding, final machining) compared to just one.

Since when is it NECESSARILY the case that "A + B + C > D"!?

This is a really STUPID ASSUMPTION, although, again, very common in modern Engineering. (For those struggling with this, try A = 2, B = 7, C = 0.0015, and ... D = 9,274,357!)

The above is the sort of nonsensical argument used by soap salesman flogging their product to dimwits, or politicians trying to get you to vote for them. Yes, sadly it does work well, and on too many people. Ahh..., H. Sapiens, the "wise one"!

But you will never win an FSAE comp using that sort of reasoning.
~o0o~

Or you can just hog it out of billet.
...
In the LVHM world making parts from billet is a totally reasonable thing to do.

That "LVHM world" is mostly about profligacy and bling. Selling junk to dimwits.

FSAE is supposed to be about engineering a fast car that scores maximum competition points. Many different ways to do this (eg. as one alternative not covered above, buy hub-assembly off-the-shelf...). Following the latest profligate fashion is not one of them.

Z

(PS. I remember this issue coming up before. There I suggested you students look at the countless, cheap, "Made in China" bicycles available in your supermarkets. These have many parts that look as if they are billet-machined, or forged, or investment-cast++, but they are actually sheet-metal fabrications. These parts are lightweight, strong, stiff, (so structurally very efficient), but obviously also very quick and cheap to make. And they look good. Go look at them, and learn...)

[BillCobb]

Negative, Ghost Rider, the pattern is full. (Request for a fly-by to an Air Boss).