Tubular Wishbone Project

[Cameron 16]

Imperial rod ends are cheaper and more widely available, american market drives that, personally I have imperial sized bodies and balls ground with a metric bore - nearly as cheap as imperial ends and ease of use with Metric bolts.

You can see why I had using rod ends on the outers and prefer a ball joint or spherical bearing then?

 

Yep. It's all about the double shear.

[Rippthrough 98]

they look close to what Compbrake sell at £245, though they may actually fit with an Anti roll bar..

 

 

Similar, but both as s*it as each other

[ravydavy 0]

Similar, but both as s*it as each other

 

I agree...

[Batfink 201]

whats wrong with the second one?

[ravydavy 0]

Can't get enough castor with the compbrake ones. Catches the ARB.. quality is good though. Even with machining new spacers so the arms sit further forward they still catch.

[Rippthrough 98]

That's 'cause really they should have another pivot for the rear stay.

[Mandic 0]

Don't forget that 4130 is going to need both pre- and post-weld heat treatment, and you need to factor in the fatigue effects on the wishbone, especially around the weld areas, where you will want twice the factor of safety over the rest of the design due to the risk of micro-cracks forming in the welds when they are being made.

 

 

T45 would be a much better bet for the tube.

 

Does that apply also to thin walled 4130 (25CrMo4) tubing as well? As I was told that under 3mm of wall thickness post heat treatment is not necessary and that MIG with SG-CrMo1 wire is best to use, as it heats more surrounding metal and there is less temperature gradient present as opposed to TIG.

 

Thanks!

 

Ziga

Edited January 20, 2010 by Mandic

[Rippthrough 98]

There's a lot of debate on that subject with many roll cage builders, but although the weld itself is relatively ductile, the metal surrounding the weld pool still experiences a high enough quench that it would be worth treating, in my opinion.

[Mandic 0]

Agree, but on structures which can easily be heated that's not a problem, but heating welds inside the car is another matter.

 

Comparing CE (have taken average values) of 4130 (0.635) and T45 (0.594) there is not much difference. Both are above 0,5 and therefore poor, according to AWS standards. There is supposed to be a new standard for TIGing Chrome-moly steel, which advises the use of ER80S-D2 filler rod. This eliminates the need for the post weld heat treatment.

 

I believe same goes for MIG, it's down to wire choice.

 

I would really like to get to the bottom of this, as I'm about to start making front subframe and need to be sure.

 

Cheers

 

Ziga

Edited January 20, 2010 by Mandic

[Rippthrough 98]

There is supposed to be a new standard for TIGing Chrome-moly steel, which advises the use of ER80S-D2 filler rod.

 

I haven't seen that yet, but most people welding tube generally use an ER70 based rod - although it's not as strong as the parent steel, the weld cross-section is higher than the tube is anyway, and the extra ductility usually helps prevent fatigue/stress cracking - that's a sort of work around for not heat treating the area on thin walled fabrications.

Every time we've had to use 4130 we've always given the surrounding tubes a bit of preheat with an acetylene torch to prevent quenching of the weld area.

 

Ideally, if you're having to conform to any regulations, you'd be better of brazing it.

Edited January 20, 2010 by Rippthrough

[Cameron 16]

Similar, but both as s*it as each other

 

So glad somebody finally agrees with me!

I got seriously slated on another site for saying Compbrake arms were s*ite.

[Rippthrough 98]

[Rippthrough 98]

whats wrong with the second one?

 

 

Here you go, this explains it better than me typing it out, but basically, both those designs incorperate every single factor of bad design they mention here:

 

http://www.formulastudent.de/academy/pats-...nds-in-bending/

 

And then adds heavy steel wishbones with residual stresses from bending to the pot too.

[Cameron 16]

Woo FSG! We're there this year.

[Rippthrough 98]

Woo FSG! We're there this year.

 

 

I'll fetch a bin bag and a brush

[Cameron 16]

 

Wont be necessary, we won it last year!

 

Edit: No we didn't. We won FSUK class 1A with our electric car, and came 2nd overall at FSG.

Edited January 30, 2010 by Cameron

[Rippthrough 98]

 

Nothing quite like seeing number 1 on your car!

 

(Well, unless it's eleven, or one hundred and one...)

[davenport 51]

been a while but this is STILL a work in progress..... lmao

 

got a seriously thick big ass bit of steel plate to start jigging a std wishbone...

 

my mate spanky has started CNC`ing the rearmounts that will house the sheprical bearing..

 

the 16mm pins that go up into the hub have been ordered from TRI in France...

 

just need to decide whether to use T45/EN14 or CDS and EN8 to make them...

 

 

a cheeky pic i got today:

 

[Cameron 16]

Nice.

 

There's something strangely awesome about CNC'd parts.

[davenport 51]

yup... i would LOVE a cnc mill..

 

god only knows how long it would have took me to do these on the manual mill...

[Batfink 201]

Here you go, this explains it better than me typing it out, but basically, both those designs incorperate every single factor of bad design they mention here:

 

http://www.formulastudent.de/academy/pats-...nds-in-bending/

 

And then adds heavy steel wishbones with residual stresses from bending to the pot too.

 

That was a very good read! thanks

I think my wishbones are using rose joints so I might need to look at some redevelopment at a later stage!

 

K.

[davenport 51]

i do agree that the rose joint in bending isn`t ideal but from reading that article it seems they are against it more because the joint has to be "over spec`d" increasing weight rather than being an accident waiting to happen...

 

there are too many aftermaket wishbones and not to mention all the competition cars using them in that aplication for it to be totally ruled out...

[Batfink 201]

True. I also think with the right suspension and wheels (i'm eyeing up some 13" wheels that weigh 3kg each!) the risk will be reduced as I attack the curbs...

[davenport 51]

i would guess running slicks would be the major factor as the loadings applied through the joint will be greatly increased... side loadings wont be a problem as the joint will be working in its preffered diection but heavy braking would create extra bending force in them... especially in a heavy car..

 

 

just aswell i have road tyres and cheap aftermarket brake pads!! lmfao....

[Cameron 16]

The SAE absolutely detest components loaded in single shear, that's the reasoning behind that article. And yeah the reason they are so hated on race cars is that they aren't designed to be loaded in shear so you have to massively over-specify the component.

 

Here's a really nice example for you..

 

 

 

In the first picture you can see a typical loading under hard braking, and the location of peak stress has been hilighted (global maximum) at the rear rod end. It was seeing peak stresses of over 800MPa which far exceeds the yeild stress of the material - in other words the rod end would have failed straight away!

 

Compare this to the second picture which shows a typical cornering load, and you can immediately see the peak stress is much much lower - only 100MPa and is located in the wishbone rather than the rod end. This is because the rod ends are being loaded in compression.

 

Note that I've had to use a very large outboard rod-end there! And so have Compbrake etc. The problem is you can't use spherical bearings as you then lose all your adjustability! So the easiest workaround is to fit a very large rod-end.