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

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?

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]

Thx guys- ur advice is most welcome

Bigger they shall be!

chris

kb58:

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It's not that simple...

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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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I don't think there is a "correct" size because there is no "standard" pothole.

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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.

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 maņana, meaning not today.

Pete

Are most people using mild steel for wishbones or chromoly?

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Originally posted by scuzzer
Are most people using mild steel for wishbones or chromoly?

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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]

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..

oops, forgot about cornering.
cant go in a straigt line forever!


Russ.

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:

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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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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

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?

Oh dear, if this had been posted in running gear i might've read it before using 3/4inch 16gauge ERW for my tubes.....

JoelP:

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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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Yeah, just the lower front bones...

Sod it, just have to take some pritt stick when i get out...!

pritt stick?? thats a bit posh init?
cat you just rub the pieces together with some spit or sumfink?

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