LocostUSA.com

i would think that the forces in the link arms will go into tension when the suspension is in jounce with no power applied to the driving wheels.

under acceleration the lower arm will be in compression and the upper in tention.

if the arms are in parallel, there will be no lifting of the chassis.

if they converge to the chassis, then they will tend to lift at an imaginary convegent point causing lift at that point, if that point is forward of the center of mass the back will squat and if arft of the center of mass the rear will lift

i believe someone has discussed using different lengths on the arms and also not having them parallel to the chassis as viewed from above and was something to do with roll.

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

Lower arm in compression makes sense. If the upper arm is in tension, it can't be much. The net of the upper and lower arms has to be several hundred pounds of compression. If the overall force on the chassis is 500 lbs. per side, and the upper links carry 200 lbs. of tension, then the lower links have to carry 700 lbs. of compression. OTOH if the upper links are in compression, even carrying just 100 lbs., the lower links only need to carry 400.

I looked at Chuck's worksheet link, and it's obviously not a simple calculation. It would've taken me hours to even approximate the 3 or 4 dozen measurements and weights you have to fill in.

Sometimes single shear is what works or makes sense. It's nice when double shear mountings work out, but important examples of single shear would be wheel lug studs, flywheel mounting bolts and the bolts that hold airplane propellors on.

Lonnie, that was just a quick thought from the picture as you say. I don't really know anything about live axle locating links in particular…

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

Definitely, but what is it that makes single shear work better or make more sense in this case?

Thanks,
Rob

_________________
* vsusp
* 5.0 442E build

Aw come on, it isn't that bad. X,Y,Z positions for 4 link ends and some basic car info is all it takes. It took me about 5 minutes to understand it and punch in some rough numbers. Pretty quick if you make some gross assumptions and are only interested in force on the links. With some very rough assumptions, like both links parallel to the ground, links are parallel to the centerline of the car as viewed from the top, a 1400Lb car with 700 on the front wheels. And some basic track and wheelbase numbers; The upper link sees +1750 lbs force, the lower link sees -2450 lbs force. But I'm not sure which direction + and - force is.

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

If that's right, + must be tension and - compression. But those are huge numbers. 1750 lbs is about 8-9K psi on the tubes. Only a quarter of the yield strength of the links, but still, hard to believe.

[quoteThe upper link sees +1750 lbs force, the lower link sees -2450 lbs force. But I'm not sure which direction + and - force is. ][/quote]

I think the pair see those forces. So the loads in individual tubes is half that.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

If that's true, you're looking at a net of 700 lbs., or half a G. Tires have got to be better than that.

The rear tires together would have 700 lbs. of weight on them. If they deliver 1g of traction that would be 700 lbs. of force…

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

No, that's not enough. What's in the links is the total force delivered to the chassis to accelerate the car. 700 lbs. is anemic.

The average accelleration of 1 G from 0-60MPH is 2.74 seconds. I read it on the internet so it must be true. That is about the most one can turn a tire (unless some exotic rubber compound or road suface is used like 1/4 mile dragsters.) without loosing traction.

I played with the calculator (now known as Chuck's Calculator, I guess ) The weight balance from front to rear makes no difference in the link forces. The difference between the forces seems to work out to 1/2 the total car weight no matter what numbers I enter. Even when I change the distance between the upper and lower link, while the forces do change, the difference does not.

If I make the assumption that the calculator assumes a 1 G force accelleration, then the total force to push the car at 1g is the car's total weight. In my test numbers, 1400 lbs total. And since the differences in link forces = 700lbs, then I must conclude that the numbers represent the forces "per side".

To do that, would require both rears tire to put down 2g's of thrust each, since each has only 350 lbs of weight (not including transfer weight) and each must provide 700# of thrust. This is a virtual impossiblity without load transfer. At least I think that is all true. Comments?

Updated 12:06PM

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

Thanks Chuck, that makes sense to me. Apparently your calculator is using 1G acceleration as the limiting force. That is asking a lot of the rear tires, but I'm sure drag slicks could handle it.

What's more intersting is the huge forces needed to keep the diff from rotating. Way more than the actual torque delivered by the axle. I used to worry the the welds between the link brackets and the axle were in tension. Sounds like they're actually more in shear, trying to keep the axle from rotating.

You are wise to be wary of the forces there, which is whatever the tractive force is at the tires, multiplied by the axle ratio, the made even worse by the typically short distance between the differential mounts (which is what Mazda's powerplant frame nicely handles.) I remember reading about differentials breaking free in several IRS cars where they seriously underestimated the forces involved.

_________________
Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

Thanks all for the comments, all appreciated

After a lot of testing and retesting, I found a setup for the rear links which doesn't bind: equal length parallel 31-1/4" links. Considering Cheapracer's research, I'm not sure why they don't bind, but I tested several times and they work fine. They are slightly inclined towards the front of the frame to clear the frame tube.





Next, I'll reinforce the brackets and weld up the links and rear end brackets completely.

I've started looking at the front suspension. So far, it looks like I'll need to make some changes to the frame if I'm going make the lower wishbones parallel with the ground, because the lower ball joints will be level with the frame I'm using the Mustang II 2" drop spindles with K772 ball joints top and bottom.

_________________
* vsusp
* 5.0 442E build

Came up with a nice front end geometry, minus steering for now. The lower ball joints are indeed low enough that having horizontal lower control arms will require attaching the pivot brackets to the sides of the lower frame tubes. So I'm making this part of the frame square. I don't see any clearance issues so far when using the TTL +4 nosecone from Kinetic.

The new front section will make the frame an inch and a half longer, and I'll probably use asymetric wishbones to shorten the wheelbase a little.

_________________
* vsusp
* 5.0 442E build