Front Hub Load Input and Modeling

[Joest]

Would anyone care to share what they know about loading of automotive hubs for front suspension applications? I am interested in knowing if anyone has done testing during racing condition to see what sort of load spikes can be expected when a driver decides to drive over the berms during corner entrance and exit.

There are at least 3 types of loads seen by a hub: thrust, moment load from the tire gripping the ground, and radial load from the normal force on the tire surface from static weight and weight transfer. Obviously tire stiffness plays a significant role when impact loading is considered so, for conversion's stake, lets assume we are dealing with 35-40 series tires on 17 or 18" wheels. If anyone has any metrology data they are willing to share or out put from an ADAMS model I would be most appreciative. Thanks in advance.

-Joest

 

[JakubMech]

Hi,

on a few designs ive done ive used 4g from a bump during cornering.

Also dont forget about brake loads, and also to a lesser extend acceleration.

Jakub

 

[Joest]

Jakub,
Thanks, that's a good start. I've seen 3-5g quoted elswhere. How about lateral loading? As you probably know, the moment load on the bearing caused by a lateral force at the tire contact patch is the largest hurdle. Assuming full weight transfer of a heavy car (> 3000lb), a 275/40/17 tire, and 1g cornering, the lateral force can be as much as 1300-1500lbf, hence moment = rolling radius X lateral force = 12.5" x 1500lbf = 18750 in-lbf. I'm curious to know if this can spike higher such as when a driver curbs the outside tire on a berms such as that typically used on circuits to keep cars off the edges of the pavement.

Cheers,

-Joest

 

[funnelguy]

I suspect most of the loading from clouting kerbs is vertical loading. Kerb profile, vehicle weight, wheel rate and shock damping will determine the real numbers. Sorry, I don't have any for you. There is a data analysis thread on a forum I lurk at and I've never seen the issue discussed. A very good driver I worked with used to "straighten" or berm his 1300# sports racer on the exit curbing of the last turn at Watkins Glen. Typically resulted in a .2 or .3g spike in the lateral g's. More than that resulted in the car climbing the curb with the real possibility of high centering on the curb. Same course, I watched drivers bounce their cars over the curbs in the "Bus Stop" chicane. Sorry, we never measured vertical g's and I can't recall anything noteworthy about lateral g's through the chicane. I'd be very interested to see data from a trip into a gravel trap at speed! Good luck! I hope others will share some of their data...

 

[Joest]

Funnelguy,
Thanks for sharing some of your experiences. They are useful. I'm pretty sure that the load is kept within reason since the tire sidewall has give to it. I wouldn't expect it to be more than 0.5g increase lateral. We discuss many topics here on Eng-Tips, but often it is difficult to get specifics on the subtleties of suspension metrology data.

-Joest

 

[patprimmer]

I have no data, but in theory I agree with funnelguy.

If you mount the inside kerb, the wheel is already partly unloaded, so the loading in any but vertical planes is irrelevant.

If you ride up the outside kerb while sliding towards, the wheel alredy is loaded, and the maximum side and vertical loads that can be applied on a smooth surface will already be there. The extra traction obtained by both the vertical acceleration of the wheel, and the raising of that corner of the car relative to the other side, will have a substantial effect on both vertical and lateral loads

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Regards
pat

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[JakubMech]

As far as lateral and longatudinal effects, I have this if its any help:

for a kerb strike, 4g lateral.
for a bump or pothole, 3g longatudinal at effected wheel and 4g vertical.

These are more for consumer cars, not race cars, but it may give an idea. There was a post on here recently discussing suspension design and loadings for testing and how long it would last under those loadings, might try a search for that. It was along the lines of 3,2,1 which i assume is 3g bump, 2g corner, 1g longitudinal (although i would put more longitudinal), again production cars not race.

Jakub

 

[GregLocock]

2g was longitudinal, I think, 1g was lateral. Sideswiping a kerb could produce higher figures laterally, depending on the details of the event, but without revealing too much, most cars hop up over the kerb rather than slamming into it.

I picked up 3-2-1 for designing suspensions from two people who have designed a lot of suspensions- both for road and race cars. I suspect this means it is actually a bit excessive for race cars.

The peak loads for kerb strike, sqaure edge pot-hole and chuck hole events are also very event specific - hitting a 5 inch kerb square on will typically rip the suspension off after four goes, whereas a 2 inch step will be hit several hundred times during a standard durability test, with no significant effect.

I'd happily use 3-2-1 for bearing loads, but might want to get a bit cleverer if trying to optimise aluminium arms and things like that.

Cheers

Greg Locock