Spindle Dimensions

[Gary]

Hey Guys,
We are currently doing the calcs on the front spindles and using a max cornering force of 1.7g we cannot seem to reduce the spindle shaft size from 35mm OD/20mm ID to anything smaller without reducing the factor of safety. Just wondering what shaft sizes other teams run and if they have had failures in the past and where the spindle has failed.

thanks heaps guys

[Amos]

What sort of factor of safety are you using???? Thatsa chunky meatball!!!!
I'm assuming you're calculating the moment at the centre of the spindle and then finding the maximum compression tension in the spindle?? Or are you considering the shear from the resultant vertical load also?

I'v just calculated for max compression tension ie stress = M*c/I where c is the maximum distance from centre line. Using this only have a spindle thickness of 3mm.....

Otherwise i can't help ya much, i'm new to this too!!

[Ashley Denmead]

factor of safety? hmmm

aim for a FOS of about 1.00001 http://fsae.com/groupee_common/emoticons/icon_smile.gif so it breaks as you cross the finish line!!....otherwise it may be considered over designed...haha or maybe you wish to take your car overseas?

i'de be interested to know what people use as a FOS for certain areas of the car.

[Denny Trimble]

It depends if you're using a fixed or rotating spindle (fatigue calcs, bearing locations). Also, heat-treated 4340 can be trusted to very high stress levels. Of course larger-OD, thinner-wall sections are more efficient, but you have to add the weight of the required wheel bearing and hub sizes into this too.

Don't forget to add the brake caliper reaction force into your calculations. It may add or subtract from the moment on the spindle. And, finally, if you have a CNC lathe, make sure you do lots of tapering.

For FOS, 1.0 at 3/3/5 g's (lateral/braking/bump) has served us well, except when we forgot to analyze something. (http://students.washington.edu/dennyt/fsae/a-arm_failure/)

[Akos]

Gary,

Your spindle dimension is about right for 13" wheels.

The biggest problem with sizing spindles is with flex. You would be surprised how much a 1.25" solid spindle can flex with 1.5g cornering load (ie brake disk bolt rubbing on upright).

Most people break spindles around the weld at the base of the spindle (assuming sheet metal uprights). If you are doing something like this make sure the spindle diameter at the weld is larger by about 1/4" and heat treat the whole thing after welding.

Generally speaking you are better off with large diameter hollow spindles (remember the air inside the spindle doesn't weigh anything http://fsae.com/groupee_common/emoticons/icon_smile.gif ).

The bearings will weigh more, but you will end up with a lighter hub and upright. Also both spindle and hub will be a lot stiffer.

Bottom line, generally a spindle that is stiff enough will be plenty strong.

Cheers,

Akos

[Gary]

Thanks heaps guys for all your help and information. Yeah with modelling stuff on paper there is always a hint of uncertainty, that's why testing is sooooo damn beneficial. Any other teams willing to shed some light on their spindle shaft dimensions and whether they have had any failures in the past? They are a high risk component that's why i am pretty concerned about them. thanks guys.

Good luck to all the teams competing in the US!!

PS: To clarify we are using 4140.

[clausen]

I have a 1970's Mclaren F1 front spindle at work, which is about 35-40mm diameter and 3 or 4mm wall thickness, at the most. (from memory)

If a 40mm diameter, 2mm wall thickness, 4340 rotating spindle on an FSAE car broke I would be very surprised

[Ryan Schoffer]

we have had some 'surprising' spindle failures as a result of a lack of wall thickness and non radiused bore base (from the back to make it hollow) on a 4340 spindle

we run soild spindles now - the upright is still hollow all the way up to the base of the bearing fit, but outboard of that the spindle is soild HT 4340

it's a pound or so extra we can deal with to ensure the wheels dont fall off!

[Denny Trimble]

We're selling our souls for increments of 0.1 lbs.

[Frank]

we're trying 35mm OD * 3.5 mm wall thickness on our live spindles this year

I'll keep you posted

[Steve-Imperial]

Im just designing my spindle too. Its gonna have a 40mm OD and a wall thickness of 4mm I reckon. Ive done some FEA tests on an axle and at the stress conc at the built in end the stresses were in the order of 200MPa (under worst possible loadings), so it should be OK using 4340. Im also planning on using a large castle nut on a thread turned on the end of the axle to hold the hub in place, has anyone used this before and had problems? Wheres a good place to buy lightweight nuts?
In response to Ryans post, surely making your spindles solid isnt the way forward? Even if you had a 10mm wall thickness you would save a chunk of weight http://fsae.com/groupee_common/emoticons/icon_smile.gif

[clausen]

Steve,

Castle nuts on the end of the spindle is fine. I work part time for a company that makes uprights for australian touring cars and thats exactly what we do. We did have a second nut that screwed into the inside of the spindle to act as a locknut, but now we'eve discarded that in favour of a trick (if i may say so myself) vernier type key locking thingo type setup.

obviously our sae car will feature something similar http://fsae.com/groupee_common/emoticons/icon_smile.gif

As for the lightweight nuts, turn them up yourselves.

its probably a good bit of design to have the retaining nut threads on the spindles handed so that the nut could never loosen itself under bearing drag.

[Steve-Imperial]

Sounds like a good part time job you have there! I thought the point of castle nuts though was to put a split pin through holes in the axle, locking the castle part of the nut and stopping it from coming undone?
I havent got the workshop time to make the nuts myself, so i need to find a supplier really. We're being pretty screwed by our department because in a couple of weeks time the student workshop will close for four weeks to us, not what we need in the run up to the competition!

[Brent Howard]

God Ryan...stop writing such stupid stuff. Our spindles have never been 4340 until now, they were 1020 steel with no heat treat. Talk about systems you actually know about, instead of trying to look like you are the entire team. Oh, and by the way, I´d love to see your calcs for the exhaust and intake, which you had no part in designing.

Brent

[NovaCat2005]

Originally posted by Denny Trimble:
For FOS, 1.0 at 3/3/5 g's(lateral/braking/bump) I found the reactions at the wheel bearings using the 3/3/5 loading condition at the tire contact patch. Now do I add all of the loads together? Or do I just look at each individual case and only take the highest loads. Basically what I am asking is do we analyze the car for turning, braking and bump at the same time? You know, for when we're spinning out of control and running over cones... http://fsae.com/groupee_common/emoticons/icon_smile.gif And for FOS, my advisor asked some other professors, and they all agreed 3.0 was best... So basically I'll just make all my estimated loads smaller so I'm essentially using a 2.0 FOS when I present my calculations. I guess they think this thing will be similar to the bathtub racer http://fsae.com/groupee_common/emoticons/icon_wink.gif.

[Frank]

"live axle"

spindle = 35mm OD 28mm ID

4340 "U temper" (ie.. how you get it in stock)
tensile approx 925 MPa = 135 Ksi

[Frank]

"live axle"

4340 U-Temper (ie as recieved stock)

Tensile = 925 MPa = 135 Ksi

no sharp radiii. We use spacers against the bearings. Smallest radii = 5mm

car has driven + 25 Hrs

regularly checked by NDT (fluorescent magnetic particle)

35mm OD 28mm ID

220kg car

[Nate Notta]

Is there any problem with welding the spindle to the upright... or simply put having the spindle as part of the upright? i.e. "Spindle-rights".

[fade]

no but its definately not as cool as our "steering-pedals" which combine the brake and throttle on the steering wheel http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Cement Legs]

Originally posted by Denny Trimble:
For FOS, 1.0 at 3/3/5 g's (lateral/braking/bump) has served us well, except when we forgot to analyze something. (http://students.washington.edu/dennyt/fsae/a-arm_failure/)

So Denny if I understand correctly you are using these loading conditions which inherantly have a FOS built in. So by computing loads with these elevated G forces what value for you FOS are you expecting? Also when you are modeling the lateral forces are you setting them up to come into the car perpendicular to the line of travel? Did you also calculate those forces with full steering, jounce and rebound? If so did you apply all the different load possibilities throughout the rest of the outboard parts? Just curious to find out how detailed people are trying to be in their force analysis.

[Denny Trimble]

Mike T could answer this better, he's been the suspension leader this year and did most of the work last year. He has a giant spreadsheet that checks for worst-case loadings on each member and joint, but I don't think it incorporates steered angle of the wheel yet.

If your factor of safety vs. yield strength is 1.0, you still have another 0.1 or 0.2 to go until ultimate failure http://fsae.com/groupee_common/emoticons/icon_smile.gif

Usually, stiffness considerations cause the large parts (a-arms, uprights) to be overbuilt if you look at strength.

It all comes down to how well you know your loads, material properties, and manufacturing process (tolerances, heat treatment, polishing, shot peening, etc). And don't design-in stupid stress concentrations, as in the a-arm failure photos http://fsae.com/groupee_common/emoticons/icon_smile.gif

I've experienced 4 major suspension failures while driving, one of which was my fault (insufficient toe link strength on my first car). But, I have to thank the "formula" for FSAE, for keeping speeds and power fairly low. Where else can you learn this kind of stuff and not get killed?

[Mike T.]

This year we designed for a FOS of 2, with 2g/2g/3g as worst case expected loading. This is loading we expect the parts to see in normal cone slammings and the like, and allow the FOS of 2 for any unforseen conditions that they might be subjected to. It ends up working out to roughly the same as FOS at 1.0 and 3g/3g/5g, but to me it makes more sense to design for expected loading and have the FOS as a FOS. And generally, when a part is designed to meet its stiffness requirements, its FOS will be right around what it should. The 'giant' spreadsheet takes the geometry point locations, tubing, fastener, and spherical bearing/rod end properties, spring/ARB stiffnesses and such, and the assumed g loading, and spits out the loads at all suspension points for each case(braking, cornering, acceleration, bump, and combinations), finds the maximums and calculates the stresses in the rod end shanks, fasteners, and control arm tubing members, and calculates the delfections of the outboard points, from which the stiffness of the entire linkage system is calculated. The loads are then used for analysis of the different components, such as the chassis attachment brackets, uprights, etc, and yes, each case of loading is examined, since even the case with the highest loading may not cause the highest stresses in your parts. I have a solidworks assembly and a VB macro set up to obtain the point locations, and can input them for any case of suspension travel, roll, or steering displacement. However, after experimenting some, I found that the loads don't change that significantly for different cases, it is just the lines of action that do. So, if it seems that the strength of a part will differ considerably with changes in suspension travel etc, the same load is used, just with a line of action corresponding to the case that is being examined. I haven't experimented with inputing loads at the tire in non orthogonal directions, since any angle changes should be relatively small.

Mike Trumbore
Team Co-Captain
University of Washington FSAE

[Cement Legs]

Very cool, thanks Mike. We haven't setup our matlab code yet to calculate under various conditions so as of right now we are still doing them by hand. Keeping everything orthogonal will save us some time and when our code is ready we can go back and chechfor more info to fill in the gaps.