Upright/hub design

[Mark Fleenor]

Hey. I was recently elected president of the Mercer University FSAE team (aka Bear Motorsports). The team has done nothing for the two years it has existed and the Dean is cutting funding unless we can design the front suspension quick. We have already designed the frame and most of the geometry of the suspension and steering, but we lack crucial information about the uprights/hubs. We are a new team and we honestly have no idea how to design an upright/hub. If anyone has any info or papers on this we would greatly appreciate it.
Thanks,
Mark

[John McCarthy]

Mark,

A few questions. Why have you designed your frame before the suspension, and suspension before the Hub/Upright?? Have you looked at brakes? Have you chosen your tyres?

I do not profess to be an expert on how to design an FSAE car, but starting at the tyre and working back to your chassis seems to make sense...

Refer to any reputable Text you can get your hands on...Carrol Smith, Milliken, etc. I am sure other people will add to this list.

If you need to get a suspension design to get money, try looking at a suitable software package (we use SUSprog 3D, but others will have input here also) and ask your Dean why he is imposing such a impractical and illogical condition on funding...

[Denny Trimble]

If you already have your suspension geometry, then the design work left is to come up with parts that perform well, fit with everything else, and that you can make / buy / have made.

For loading, figure out your worst case lateral / longitudinal / vertical accelerations of the car (point mass at the CG, 2/2/3 to start with). Then, do the load transfer calcs to get worst case contact patch loading (normal, lateral, longitudinal at each tire).

For the hubs, you're primarily concerned with bearing loads, which are the reactions to all the external forces on the hub (contact patch forces in all 3 directions, plus brake caliper tangent force reaction). Make sure to look at stresses at the brake rotor mounting points too. Once you know the bearing loads, you can pick your bearings and seals and preload system.

For the spindle and upright, take the reactions from your hub bearing loads, plus the caliper bolt reaction forces. You should also look at camber and toe stiffness from loads applied at the contact patch, if you have time.

Other decisions to make:
-Live axle (rotating hub/spindle with bearings in the upright) or conventional fixed spindle / rotating hub
-How to attach your spindle to the upright if it's not a live axle (we do a press-fit from the inside)
-Brake rotor mounting: direct bolting is the simplest, floating is better (read up on it)
-Materials: if you use aluminum for the hubs, beware of high thermal expansion (think about your bearing race press-fits at operating temperature), and dramatic reduction in yield strength at over 200 degrees F (search the forum, I posted a cool graph a while back). Steel is great if you use it effectively in thin sections. Quick and dirty, I'd go with some rectangular tube, cut diagonally to make tapered box sections, and welded at the seams. I haven't tried that method yet, so it might not be as easy as I think.

Make sure to run the suspension through its full range of bump, droop, and steering in CAD before you build it, and check for interferences of all kinds (wheel/control arm, upright/control arm binding at the balljoints, etc). You want to allow enough clearance with the wheel shells for some deflection to happen without finding out you have "self machining wheels".

I hope you have people who can do FEA, otherwise you're in for a long ride, a lot of guesswork, a heavy car, and a sneaking suspicion that one of the wheels might come off on the next corner. Reminds me of my first year or two in FSAE... back in the days of Windows NT http://fsae.com/groupee_common/emoticons/icon_smile.gif

Also, don't forget about alignments and take-up of manufacturing tolerances. Alignments are hard enough with an "easy" car, let alone if nobody thought of how they would be adjusting camber and toe when they designed it.

Is that long enough? I hope this helped, I'm not trying to give all the answers, just my perspective on how to get started. Good luck!

[Mark Fleenor]

Thanks for the feedback. You are right that we should have started with the wheels and designed back. The problem lies in the fact that our car is partially composed of senior design projects. One of the projects was a 4130 space frame for the car. Another project was a cooling system which led to the purchase of a F4i engine. The other projects, however, were unsucessfull and can not be used on the car. Thus we were left with a frame, engine, and no idea about where to go now. So, we have decided to work our way out to the wheels first.

We are not going to try to compete this may, instead we have decided to shoot for the 06' competition. To give you some kind of idea what we are capable of I'll briefly explain our current team. We have 5 members (1 Junior, 3 Sophomores, and a Freshman) None of us has any excperience with designing a race car. Oh, and we have money but it has been cut off because of the incompetent past presidents. Our goal is just to build a car, not even a great car. All we want is to have something to show so we can gain the schools support once more. So if you have any tips, tricks, or ideas please share them with us.
Thanks again,
Mark
BearMotorsports@gmail.com

[TA]

Mark, what level of knowledge is your team starting with?

Do you have a good idea what you're looking for as regards suspension geometry?

Or are you starting from scratch?

The model car folks have a number of good sites online that explain the basic geometry concepts well.

http://www.rctek.com/handling/index.html

Is an OK place to start -- There was another better site posted before but I can't find the link ATM.

Once you determine where you want your links to fall I would start looking at other peoples uprights as inspiration -- there is a photo gallery here somewhere on the FSAE site, there are also commercially available uprights from larger formula cars, I think Pegasus has pictures in their catalog and there are numerous others as well, let google be your guide.

For a first time effort I would suggest simple and robust.

Dennis Palatov has designed very nice, very simple, uprights for his project. His are, with different bracketry and links, universal for all four corners of the car.

(quick note: his suspesion joints are in single shear, which upsets a lot of folk here on the board -- my comments are i) With beefier joints and fixing bolts it's a perfectly acceptable solution ii) Take a look under a "real" car iii) You can easily redesign to place everything in DS if you choose)

http://www.dpcars.net/dp1/dh.htm

(scroll down about 3/4 of the way)

There are other views of them on his site as well as well as a fantastic, learn by example, overview of his car design.

http://www.dpcars.net


Cheers, Ted

[buggaero]

Originally posted by Denny Trimble:
-How to attach your spindle to the upright if it's not a live axle (we do a press-fit from the inside)
-Brake rotor mounting: direct bolting is the simplest, floating is better (read up on it)
-Materials: if you use aluminum for the hubs, beware of high thermal expansion (think about your bearing race press-fits at operating temperature), and dramatic reduction in yield strength at over 200 degrees F (search the forum, I posted a cool graph a while back). Steel is great if you use it effectively in thin sections. Quick and dirty, I'd go with some rectangular tube, cut diagonally to make tapered box sections, and welded at the seams. I haven't tried that method yet, so it might not be as easy as I think.


I am in high confusion regarding the effect of relative expansion of steel bearing with respect to aluminium upright/hub/spindle thus affecting the press fit shear.

On reviewing what we studied in thermal expansion of cylindrical bodies, the outer side expands away from center while the inner side expands towards the center (radial expansion).
So that mean the bore of the bearing is going to have a more sheared fit and even it's outer ring is going to press harder onto the inner side of the housing and same with the housing to the outer ring.

How is the fit anyway going to loosen??
can't catch where am i missing out...

[Drew Price]

Coefficient of expansion of the (usually) aluminum upright means that it will expand dimensionally more than the bearing will expand, resulting in a looser fit.

[buggaero]

Originally posted by Drew Price:
Coefficient of expansion of the (usually) aluminum upright means that it will expand dimensionally more than the bearing will expand, resulting in a looser fit.

Yes. I just caught my mistake. A simple application of expansion formula proves the loosening.

Thanx anyways.