3 and 4 link rear suspension questions

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Jer73
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Re: 3 and 4 link rear suspension questions

Post by Jer73 » Sat Feb 04, 2017 5:20 pm

Brian P wrote:Your supports for your upper arm look quite far apart (at both ends). Have you calculated the tension force on that upper arm under maximum acceleration and calculated the stresses on the crossbar that the upper arm is secured to?
No I have not done an analysis of the possible forces on any of the links or mounting. So far I have been building it to what I think is husky enough. I have thought about making spacers that utilize 2 bolts per rod end to secure the upper link instead of just one bolt as I do fear that it will have issues with flexing the mounting bolt. The mounts for the upper link on both the chassis and axle have a 5 inch spread and will be using 3/4in grade 8 fine thread bolts. If you could walk me through or present an example of how I would go about doing an analysis that would be greatly appreciated Brian.
j-c-c wrote:Might be a little late in the game for your progress, but this link applies I believe, and is a respect build:

http://www.lateral-g.net/forums/showthread.php4?t=52748
Very nice build looks like quite a bit of $$$ in that one :mrgreen:

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Re: 3 and 4 link rear suspension questions

Post by Brian P » Sat Feb 04, 2017 5:33 pm

Let's start with establishing how much pull force there could be on that upper link.

Engine torque output x transmission 1st gear ratio x torque converter multiplication (if automatic) x rear axle ratio / distance vertically between the lower and upper link attachment points on the axle.

Use consistent units and report back with what that number is.

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Re: 3 and 4 link rear suspension questions

Post by MadBill » Sat Feb 04, 2017 5:44 pm

Don't forget that distributed between the upper arm and the lower ones there are also compressive loads created by the forward reaction from the tire thrust that is accelerating the vehicle. On the upper, this cancels out a significant % of the tensile torque reaction but on the lowers it is additive. 'Fortunately', there are normally two lowers to divvy it up...
Felix, qui potuit rerum cognscere causas.

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Re: 3 and 4 link rear suspension questions

Post by Jer73 » Sat Feb 04, 2017 6:31 pm

Lets say the peak engine out put is 350 lb ft, it is just a mild 306 small block ford, transmission first gear is 3.35, rear axle ratio is 4.11 distance between the upper and lower links at their farthest configuration would be 21 1/2 inches apart, upper link being 14 inches from axle center line and lower links being 7 1/2 inches from axle center line. The closest configuration would be 14 inches apart with the upper link being 8 inches from axle center line and the lower link being 6 inches from axle center line.

350lbft x 3.35 x 4.11 / (21.5in / 12) =
350lbft x 3.35 x 4.11 / 1.791ft =
1172.5 x 4.11 / 1.791ft =
4818.975 / 1.791 = 2690.661 ft lbs
At its farthest configuration

350lbft x 3.35 x 4.11 / (14in / 12) =
1172.5 x 4.11 / 1.666ft =
4818.975 / 1.666ft = 2892.542 ft lbs
At its closest configuration.

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Re: 3 and 4 link rear suspension questions

Post by Brian P » Sat Feb 04, 2017 8:06 pm

Typo. 14" = 1.166 ft. The tension on the upper link is 4132 lbs. The forward thrust does offset some of this but it's erring on the safe side to disregard that.

So now you have 4132 lbs applied at the center of a "beam" (the pivot bolt for the upper link) which is 5 inches long and is the cross-section of a 3/4" bolt. The threads in that bolt don't help you but I am going to ASSume that you are not using a full-threaded bolt and that the most critical part of it through the rod-end is 3/4" diameter without threads on it.

The end reactions are 2066 lbs each and the moment is 2.5 x 2066 = 5165 in.lb. (Half the length of the beam x the magnitude of the end load)

The second moment of the area Ixx = 0.016 in^4 and the outer fiber is 0.375 inch from the centerline of the section

Stress = M y / I = 5165 x 0.375 / 0.016 = 121,000 psi

Ouch.

The side plates on either side of the rod end should be brought in as close as possible around the rod end so that the clamp bolt is as short as possible.

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Re: 3 and 4 link rear suspension questions

Post by Jer73 » Sat Feb 04, 2017 11:05 pm

Brian P wrote:Typo. 14" = 1.166 ft. The tension on the upper link is 4132 lbs. The forward thrust does offset some of this but it's erring on the safe side to disregard that.

So now you have 4132 lbs applied at the center of a "beam" (the pivot bolt for the upper link) which is 5 inches long and is the cross-section of a 3/4" bolt. The threads in that bolt don't help you but I am going to ASSume that you are not using a full-threaded bolt and that the most critical part of it through the rod-end is 3/4" diameter without threads on it.

The end reactions are 2066 lbs each and the moment is 2.5 x 2066 = 5165 in.lb. (Half the length of the beam x the magnitude of the end load)

The second moment of the area Ixx = 0.016 in^4 and the outer fiber is 0.375 inch from the centerline of the section

Stress = M y / I = 5165 x 0.375 / 0.016 = 121,000 psi

Ouch.

The side plates on either side of the rod end should be brought in as close as possible around the rod end so that the clamp bolt is as short as possible.
I did the math on the farthest configuration and got 78827.977 psi. Which is still a no go as that also exceeds the shear strength of the hardware being used.

I ASSumed that since some modifieds are running a similar suspension like this https://youtu.be/_5kc-qUrS6g I would be okay with the brackets and hardware. Dirt cars like modifieds don't usually see 100% traction they don't weigh 3200lbs either... :oops: I am going to need some time and coffee to see if I can work around what I have if not, it is time to break out the cut off wheel and start over. Or just hook up the 4 link bars and go from there. Again thank you for your input Brian this could have been a potentially hazardous oversight.

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Re: 3 and 4 link rear suspension questions

Post by MadBill » Sat Feb 04, 2017 11:46 pm

When I was messing with 3 links a few years back, I found an on-line calculator for A-S %, loads in the links, etc. AIR, the tire thrust forces reduced the net single upper link tensile load to about the same as in the two lowers.
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Re: 3 and 4 link rear suspension questions

Post by Brian P » Sun Feb 05, 2017 12:48 pm

It will help. Really what's needed is to treat the whole axle assembly as a supported beam in side view.

We have 4818 lbs applied at one end of the beam (the tire contacts).

If we assume 28" tall tires (a guess) then the lower links are 8" above ground and the top link is 14" above that. Imagine a teeter-totter with the pivot being the lower link, one end being the tire contact patch, the other end being the upper link.

The tension on the upper link taking the drive force into account with the links in the closest configuration is 4818 x (8 / 14) = 2753 lbs.

So it's about two-thirds of way too much, but it's still too much.

Bear in mind that the pivot of that teeter-totter - the lower links - has to handle (in compression) the total of the tire contact thrust AND the tension on the upper link, so that's 2753 + 4818 = 7571 lbs split (unequally) between the lower links. The split will be unequal because the upper link isn't in the middle, but it's close enough to the middle for this purpose - and the tire contact patch loads, which are the bulk of it, are split equally.

Going to a 4-link won't make this go away!

Making the side plates on either end of the relevant link-end joints closer together, e.g. so that the side plates immediately adjoin the rod-end itself, so that we are not dealing with 5 inch long bolts in bending, will fix this. But it requires a bunch of cutting and welding.

Installing a tubular solid bushing on either side of the rod-end, filling the gap out to the side plate, of significantly bigger diameter than the bolt itself, and with a good thick washer on the outside so that the clamping load gets distributed better, would help very substantially by no longer relying on just the bolt itself to handle the bending load. And this way you don't have to cut and re-weld everything.

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Re: 3 and 4 link rear suspension questions

Post by Brian P » Sun Feb 05, 2017 12:53 pm

On looking at the photo again, it looks like the lower links have the side plates much closer together - those shouldn't be a problem. It's just the upper link that has them too far apart.

Go with the spacer idea to spread out the bending loads. It will let you fine-tune the amount of driveshaft torque cancellation by using different spacer lengths (so that you can move the link side to side).

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