Wheel Hub design considerations.

[Toolish]

I am designing a wheel hub for an off road buggy. I am trying to work out what factors I have to take into consideration and what the system interactions will be.

The standard stud pattern for off road wheels is VW Wide 5 (5 x 205) so I will be using that stud pattern.

I am not sure what brake rotor I will be using. I am hoping to find something useful off a common car (any suggestions given I am Australian) if not them I will look at something from Wilwood probably like

http://www.wilwood.com/Products/002-Rotors/006-ULD/index.asp.

This would be better performance wise but cost would be greater.

So that leaves wheel bearings. I am undecided as to if I should go for an aluminum or a steel hub. Steel makes the bearing selection process easier. If I go aluminium it will save weight butas far as I can tell I have to use a hub bearing unit to allow for expansion but I can not find technical details on these units (sizing available etc). The SKF website has a CAD library of their bearings but I could not find the hub units in there. Any details on this sizing information and how these units should be mounted would be greatly appreciated.

Other than that it all has to be strong enough to take the weight of the car. Any help in any area would be greatly appreciated. Is there any other factors I need to take into account? Seen as I don't have a design team to bounce ideas off then this forum is going to have to be it!

 

[patprimmer]

I would approach DBA about their range of rotors.

Regards
Pat
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[bentwings2]

We have a sand buggy with a 954 Honda and about 800 pound weight rolling. We tried one of the larger ATV hubs and stub shafts on the independent rear suspension. It lasted about 30 seconds. Sheared the spline right off. The second generation was a larger ATV stub axel which lasted about 100 yards before tossing the wheel another 100 yards.

I then set about desiging my own hub and stub shaft. The first was aluminum which I optimized for light weight and strength. Unfortunately the CNC shop wanted way too much to machine them. Next I designed a fabricated steel hub and stub shaft. Since I also TIG weld for a living (in my retirement) I was able to weld TIG weld the entire hub and stub shaft. The ahrdest part was to get the machine shop to rough machine the hub before welding then finish machine it after welding. I had to go to the shop and show him how to set it up for the final boring to size in one set up.

The final weight of the steel hub and stub shaft is less than 10 pounds each and has been bullet proof. We use a single inboard mounted disc and single Willwood 4 piston caliper. With a 3/4 inch master cyl it is great. We use MC calipers and custom discs on the front and a proportioning valve. I had to use a toggle action on the foot brake due to space requirement so the brake leverage is very high. this give feather light brake pedal. It's a little bit tricky to adjust but it really grabs hard when needed.

I did all the design work and analysis on Solidworks and Cosmos.

Here is where I got the splined shafts. These were very inexpensive and easy to work with.

http://www.grobinc.com/coldrolled/std_spline_shafts.htm

 

[PedroCG]

dinamic forces will probably be more important than just the weigth of the bugy

 

[Toolish]

"I did all the design work and analysis on Solidworks and Cosmos."

What sort of input loads did you use to validate the design. I have no idea what sort of road load profile would be required to be honest.

"dinamic forces will probably be more important than just the weigth of the bugy "

What do you mean by this?

I have decided to head down the path of a motorcycle rotor and caliper on the front. Not sure exactly which one yet, time to go hunting at a wrecker I think!

 

[PedroCG]

forces that actuate on the tire under driving, like lateral acceleration, braking, acceleration, bumps etc

http://www.dpcars.net/

click on the ariel atom picture and go to page 11 migth be of some help.

 

[GregLocock]

He just used 4g in x y and z.

That is neither efficient for a circuit car nor realistic for an off road vehicle.



Cheers

Greg Locock

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[Toolish]

"That is neither efficient for a circuit car nor realistic for an off road vehicle."

That is the problem I am having. What is realistic for an off road vehicle.

Worst case scenario I can think of (aside from hitting a tree or something, which can't be designed for) is coming down from a fast off axis jump with the wheel turned so that the vehicle weight/momentum is acting across the tyre not along it.

Either way I am not sure what sort of loads I should be designing around.

 

[GregLocock]

Well, do you have a prototype car? Could you estimate its suspension travel and from the spring rates and bump stops work out some forces?

Alternatively if you have some broken parts and some parts that work then reverse engineer the laods from them.

The 'proper' way of doing it is to strain gauge the suspension arms and so on, and then measure the forces when you drive the car.

Otherwise you are just guessing. If this is for FSAE Baja for example then as a design judge I would give a team a very hard time if they hadn't come up with a better method of designing the parts than asking around for loads.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Toolish]

I don't have a prototype car or any broken parts. This is a from scratch design that I guess will become the prototype car. I know the initial design is not going to be the best it can be but I am looking for a starting point.

Perhaps the answer is to make it, break it, then make it again !

 

[GregLocock]

The way we do it is to overdesign the prototype, strain gauge it, and then we know the loads for the proper design.

Another approach is to underdesign a part built from welded steel, and to reinforce it until it stops bending. Mr Chapman was fond of that approach.

Either way you need a series of events that you want to survive - you might specify a 6 foot drop for the vertical loads.

Another approach is to borrow a succesful product and reverse engineer the loads off that.

Incidentally unless you really need to, I'd stick with a welded steel fabrication rather than a cast aluminium part, certainly for prototyping and even in production.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Before the days of FEA and CAD, the race cars I worked on, including several off road buggies were designed by building light then reinforcing areas that break to often.

Experience showed that as you built it tougher, the driver just went harder. The approach of spring rate and bump stop rte and travel plus a bit is a good starting point I think. The grip of an off road tyre is VERY variable depending on the alignment and composition of the surface it acts on.

Regards
Pat
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[PedroCG]

Greg he talks of hiting a bump while cornering, i posted the website to show where he aplied the loads.

This is not my area of strength (i dont have one in fact). making a model of the buggy's suspension in working model or some similar software and them droping it and measure forces that actuate on the hub, would this be a valid method?

 

[GregLocock]

Yes, but the trick is finding events that represent the strength you need.

I picked on dropping a car from six feet, because I know it is survivable, but only if you have the right suspension.

If you did that to a road car it would break.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Greg

One major factor in winning off road races is to be able to hit very large bumps at higher speeds than your opposition and not break nor become unstable.

I expect that includes being able to drop the car from a greater height than you competition.

Obviously making everything heavier does not work as it increases strength, but also increases load.

Regards
Pat
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[sszuch]

Since you are using a stock VW wide 5 bolt pattern wheel, I would suggest looking at CB Performance (

http://www.cbperformance.com).

They already manufacture a front disk brake kit specifically for this wheel pattern and stock spindle using stock bearings. I believe they have used these on off road type buggies, but since they are the manufacturer, they could better tell you the limitations of their product.