Board logo

Curve in wishbone plate
Slimy38 - 18/4/14 at 05:35 PM

Just a quick question, what's the reasoning behind the curve in the wishbone plate? Does it actually do something or is it decorative? My plates are currently straight edged from one side to the other.

I noticed Talon's MX5 wishbones replace the curve with a horizontal bar;




although he keeps the curve in for the Sierra wishbones;

jacko - 18/4/14 at 05:40 PM

I seem to remember reading if straight it puts strain on the tubing
Jacko

Slimy38 - 18/4/14 at 06:00 PM

quote:
Originally posted by jacko
I seem to remember reading if straight it puts strain on the tubing
Jacko


Seems reasonable. If I simply cut a curve out of the plate now that the wishbones have been made up, will that suffice? Is there any 'golden rule' as to how much of a curve is required?

I've just been reading the 'bent wishbone' thread, British Trident talks about dressing the welds to prevent any sharp corners, should I do that at the same time?

twybrow - 18/4/14 at 06:45 PM

I am not a stress engineer, so can't help on rules off the top of my head, but it is all about reducing stress concentrations where the two pieces meet. A sharp corner will typically give you a stress concentration, whereas dressed welds, and a curve to the joint will help to reduce this... I wonder also if Talon does this to reduce weight too?

mark chandler - 18/4/14 at 06:59 PM

Curve reduces the stress point, you should also weld in the middle of the tubing not top or bottom.

Slimy38 - 18/4/14 at 08:22 PM

quote:
Originally posted by mark chandler
you should also weld in the middle of the tubing not top or bottom.


Yep, already done that bit even if it was a complete pain to get everything fitting perfectly!

coyoteboy - 18/4/14 at 08:36 PM

Curved gussets spread the load down the weld more evenly rather than concentrating at a point.

PhillipM - 27/4/14 at 11:47 AM

quote:
Originally posted by mark chandler
Curve reduces the stress point, you should also weld in the middle of the tubing not top or bottom.


Welding a brace/gusset to the middle of the tube makes the stress even worse, it really should be almost flush with the top/bottom of the tubes, the middle of the tube is the weakest, least stiff part as far as a gusset is concerned. That's why when you see gussets on rollcages they're generally two pieces rolled around and welded either side.

Slimy38 - 27/4/14 at 12:18 PM

quote:
Originally posted by PhillipM
quote:
Originally posted by mark chandler
Curve reduces the stress point, you should also weld in the middle of the tubing not top or bottom.


Welding a brace/gusset to the middle of the tube makes the stress even worse, it really should be almost flush with the top/bottom of the tubes, the middle of the tube is the weakest, least stiff part as far as a gusset is concerned. That's why when you see gussets on rollcages they're generally two pieces rolled around and welded either side.


Mmm, I have to admit that's the first time anyone has suggested that, and the only time I've seen plate welded to the top or bottom was on the 'bent wishbone' thread which certainly isn't the best example.

Even a quick google image search for rollcage gussets shows both being used, although it does appear that the top/bottom gussets are much thinner metal than the centre gussets.

Would anyone else care to weigh in?

PhillipM - 27/4/14 at 12:36 PM

If you weld them in the centre your putting the stress into just the wall thickness of the tube - running the risk of kinking the tube (and hence a big loss of strength) with an impact, and cracking with fatigue if the ends off the gusset aren't rounded and ground flush.
If you weld them on the outside you're effectively using about a quarter of the tubes diameter as the effective wall thickness.

[Edited on 27/4/14 by PhillipM]

Slimy38 - 27/4/14 at 02:03 PM

quote:
Originally posted by PhillipM
If you weld them in the centre your putting the stress into just the wall thickness of the tube - running the risk of kinking the tube (and hence a big loss of strength) with an impact, and cracking with fatigue if the ends off the gusset aren't rounded and ground flush.
If you weld them on the outside you're effectively using about a quarter of the tubes diameter as the effective wall thickness.

[Edited on 27/4/14 by PhillipM]


But conversely, if the gusset is on the top (which would be the most likely due to it being a suspension wishbone), you've got the weight of the car (and the suspension) trying to do the same thing in a vertical plane. And suspension forces are guaranteed whereas an impact is (hopefully!) unlikely.

I do see your point, but for now I'll stick with the Haynes manual design.

britishtrident - 27/4/14 at 02:19 PM

quote:
Originally posted by PhillipM
quote:
Originally posted by mark chandler
Curve reduces the stress point, you should also weld in the middle of the tubing not top or bottom.


Welding a brace/gusset to the middle of the tube makes the stress even worse, it really should be almost flush with the top/bottom of the tubes, the middle of the tube is the weakest, least stiff part as far as a gusset is concerned. That's why when you see gussets on rollcages they're generally two pieces rolled around and welded either side.



A roll cage is an entirely different load case, if look back 10+ years to the early days of as per book Locost building you will find horror stories of bent wishbones, after a bit of discussion among the group a consensus was reached that welding a plate with nicely tapered ends along the neutral axis of the wishbonebtubes was a valid solution.
Experience has shown that it works as stories of catastrophic failures in wishbones are pretty rare these days although there are a lot more cars on the road..

garyt - 27/4/14 at 02:58 PM

Hi if your after the plates.. I just copied the drawings to craig at plazcut waiting for costing to do the mx5 wishbone plates if anyones interested will post it when I get a reply
Gaz

PhillipM - 27/4/14 at 08:48 PM

quote:
Originally posted by Slimy38

But conversely, if the gusset is on the top (which would be the most likely due to it being a suspension wishbone), you've got the weight of the car (and the suspension) trying to do the same thing in a vertical plane. And suspension forces are guaranteed whereas an impact is (hopefully!) unlikely.

I do see your point, but for now I'll stick with the Haynes manual design.


Yes, but if you're putting a vertical load through a flat, horizontal plate and expecting it to be stiff, you've got more issues than welding it to the wishbone...
The very reason those talons ones you pictured have had to fit a horizontal bar is to spread the load to the top and bottom of the tube to stop the gusset kinking/cracking the main tubes in the centre.

[Edited on 28/4/14 by PhillipM]

Samuele - 3/6/16 at 10:24 AM

s