8smokingbarrels
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| posted on 21/8/03 at 03:36 AM |
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Wishbone strength
Just a quickie guys- 
Got hold of some offcuts of seamless tubing- its 21mm OD, 2mm thick.
Im going to make the upper wishbones from this- they will be quite short (7-8" approx). Just want to check with u all, if u think its strong
enough given that car will have either Rover/ford/chevy V8 (been watching Bullitt again! )
Thx as usual
chris
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Rorty
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| posted on 22/8/03 at 03:05 AM |
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I normally use 25.4 X 2mm CDS/CDW for mine, and they're anything up to 600mm long. However, the weight/loads they see, are small.
I would have thought the material you mention would be on the light side, even though your wishbones will be short.
Do you have any data on the tube?
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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kb58
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| posted on 22/8/03 at 04:14 PM |
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It's not that simple because the static loads are not the whole picture. If the geometry says that at 1-g you are feeding 300lbs into the
A-arm, that's by no means the end of it. The really hard part is figuring the additional forces of shock loads - pot holes. This drove me
nuts because just what force does a "standard" pot hole feed into the suspension? I assumed a 10-g shock load worst case and went from
there. FWIW, for my mid-engine Mini, the rear lower links are 1.25" x 0.065", and all other links 1.00" x 0.065".
To get an idea of how strong your tubing is, find a mechanical engineering book and find the "Column strength" table.
Keep in mind that compared to the vehicle, A-arms weigh practically nothing. It's cheap insurance to make them slightly larger than you think
you'll need. The consequences of a failed A-arm are rather dire...
[Edited on 22/8/03 by kb58]
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8smokingbarrels
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| posted on 22/8/03 at 05:57 PM |
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Thx guys- ur advice is most welcome
Bigger they shall be!
chris
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Rorty
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| posted on 23/8/03 at 05:11 AM |
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kb58:
quote:
It's not that simple...
I know it's not, I was just trying to give the bloke a bit of guidance, and that's why I asked about the specs of his tube.
The majority of builders here wont have a clue about the mechanical properties of their materials, or how to go about calculating them. I'm
obviously not au fait with the car in question, but if it's close to a locost in principle, then I think the specs I suggested would be fair,
and what's more, easy to weld without blowing holes in it.
It's generally a very low key forum, most people just wanting to know what's worked for others.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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kb58
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| posted on 23/8/03 at 04:49 PM |
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That didn't come out right... I didn't mean you were over-simplifying, just that it isn't easy (for any designer) to come up with
the correct size. I don't think there is a "correct" size because there is no "standard" pothole.
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pbura
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| posted on 23/8/03 at 05:49 PM |
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quote:
I don't think there is a "correct" size because there is no "standard" pothole.
I called the local street department yesterday to ask if anyone there could tell me what sort of G forces a pothole or curb would exert on a moving
car. My interest is more in specs for springs and suspension travel, btw.
No one could tell me, and the traffic engineer suggested I consult a Mech E. Fair enough. Maybe someone here can step up to this question.
In Chassis Engineering, Herb Adams says that 1 G will be typically be generated by a shock on a smooth road, whatever that means. Divider
strip? Surface undulation?
Somewhere in a web page in suspensions for heavy equipment there was a reference to a 3 G road shock. This is perhaps the shock of running over a
Locost?
Well, in a typical Locost, a 1 G bump will compress the suspension a bit over 1". 3 Gs would be a bad 'un, well beyond bottoming out the
front suspension.
Not being argumentative here in any way, just sharing what little I've read about road shocks.
Pete B.
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kb58
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| posted on 24/8/03 at 01:58 AM |
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Yeah but I suspect the true shock load (not shock absorber), but the load instantaneously transmitted through the suspension pickup points is a whole
lot more.
Think of it this way. If you're going 60mph, or 88fps, in 1/88th of a second you go one foot. A "typical" pothole is a foot
wide... say. So if the pothole is, oh, 2" deep, that's a 2" step applied in 1/88th of a second, right? How many g's is
that?! We know that 1g = 32ft/sec, and we are moving 0.16/ 1/88sec. I think that works out to about 14g's.... Yeah the tire itself takes up a
far amount... but how much? I don't know.
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pbura
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| posted on 24/8/03 at 03:40 AM |
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Hey Kurt,
Here's an illustration of a bike hitting a speed bump I found on a Google:
http://mcasco.com/qa_bsb.html
Interesting that the tire slows the rate of the collision, else the force of impact would be infinite. To do this right, one really needs to know the
tire's compression/rebound rates.
As far as the mass that would be accelerated against the wishbones, I guess that would be the mass of the wheel assembly. Then there's the
weight of the car pushing in the opposite direction, and the coilover wouild absorb part of the impact.........
This stuff strains my brain beyond belief. I'll look at it again mañana, meaning not today.
Pete
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scuzzer
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| posted on 29/8/03 at 09:17 PM |
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Are most people using mild steel for wishbones or chromoly?
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Alan B
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| posted on 29/8/03 at 09:24 PM |
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quote:
Originally posted by scuzzer
Are most people using mild steel for wishbones or chromoly?
Mild steel here.
Fronts are 1" 0.090 wall
Rear links are 1" 0.065 wall
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suparuss
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| posted on 29/8/03 at 11:49 PM |
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if the shocker is connected pretty much right over the bottom ball joint, wont all forces generated by hitting a bump or pot hole go sraight up the
shock and into the chassis? the major strain on the wish bones are generated by braking and acceleration, obviously there will be some strain from
bumps but i wouldnt have thought it would be too much, providing that the suspension is doing its job.
Russ.
[Edited on 29/8/03 by suparuss]
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Alan B
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| posted on 30/8/03 at 01:36 AM |
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I'd agree with that, but add the side loads due to cornering too.
Ideally the the wishbones should see no bending loads and act as pure strut or tension members......pretty much the same criteria for ideal spaceframe
members too..
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suparuss
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| posted on 30/8/03 at 10:10 AM |
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oops, forgot about cornering.
cant go in a straigt line forever! 
Russ.
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kb58
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| posted on 30/8/03 at 07:16 PM |
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True. I guess I got off topic, in that the force I was referring to was the forces at the shock attachment point. You're right, if the shock
doesn't count on the a-arm (laterally) the forces into the shock have nothing to do with the a-arm.
quote:
Originally posted by suparuss
if the shocker is connected pretty much right over the bottom ball joint, wont all forces generated by hitting a bump or pot hole go sraight up the
shock and into the chassis? the major strain on the wish bones are generated by braking and acceleration, obviously there will be some strain from
bumps but i wouldnt have thought it would be too much, providing that the suspension is doing its job.
Russ.
[Edited on 29/8/03 by suparuss]
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8smokingbarrels
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| posted on 31/8/03 at 06:24 PM |
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Just checking..
I assume the wishbone sizes people have been mentioning refer to OD and not ID.
I got the hold of some 1" ID but it looks rather big to me- can someone confirm
Thx
chris
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Rorty
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| posted on 1/9/03 at 03:56 AM |
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Tube, be it square, oval or round, is always measured about its exterior. Pipe is measured by its bore (flow rate or capacity), as it's primary
use is for gas or fluid transportation.
1" ID may be OK, but possibly overkill. What's its wall thickness, and is it seamless?
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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JoelP
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| posted on 1/9/03 at 09:38 AM |
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Oh dear, if this had been posted in running gear i might've read it before using 3/4inch 16gauge ERW for my tubes.....
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Rorty
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| posted on 2/9/03 at 04:46 AM |
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JoelP:
quote:
Oh dear, if this had been posted in running gear i might've read it before using 3/4inch 16gauge ERW for my tubes.....
Which tubes are you talking about Joel, wishbones? 
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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JoelP
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| posted on 2/9/03 at 08:21 AM |
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Yeah, just the lower front bones...
Sod it, just have to take some pritt stick when i get out...!
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suparuss
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| posted on 2/9/03 at 03:59 PM |
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pritt stick?? thats a bit posh init?
cat you just rub the pieces together with some spit or sumfink?
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8smokingbarrels
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| posted on 7/9/03 at 05:14 PM |
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Ah but me thinks Joel is perhaps intending to use the pritt stick for his own use. Being as high as a kite stops u worrying about silly things like
wishbone thickness in the 1st place!
Personnally I find evostick a much quicker alternative!
chris
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