The feeling I get from this whole insistance on both your parts is that...somehow...and I don't know enough about this stuff to say exactly how, but somehow, you are both looking at all this with some sort of tunnel vision. I don't remember 'Racer saying there were NO compression loads driving the car forward...just that the upper link is getting pulled while the lower basicly pushes the chassis down the road. But...there IS a problem. All of this is in constant change so its impossible to narrow it down...only as a snapshot of any particular moment. Now you gotta pick a moment to focus on! As for the pull on the top being equal to the push on the bottom...I think that leaves something completely out. Consider the "push" the housing receives from the axle thru the outer bearing. That is completely separate of the rotational torque created by the pinion/crown gear and I believe THAT is where the whole thing gets mentally trumped. MY thought is that both of you are more or less correct...at different instantanious moments in time. ...might as well try to explain the thought process of a woman as it relates to man! Hahaha
That's what I've been thinking too Hackerbilt. If you're driving your 6 cylinder Chevelle down the highway at 65mph and mash the pedal down, do the upper links go into tension? Probably not. When you get the green on a 1,000hp drag car, one that would bend leaf springs in the front, would a top link not be in tension?
Kerry, I will gladly answer your questions directly. 1) First off, I NEVER said there was no compression (or thrust) force driving the car forward. I have built dozens of back braces for drag housings, and I know exactly why they are necessary. The lower links are always in compression as the car acclelerates, regardless of suspesnion design. If you go back to posts #118 and 125, I pointed out that under hard acceleration, the upper link(s) attaching the axle housing to the chassis are under tension; to restrain the reverse rotation force from pinion climb, which is exactly what EVERY video I have since posted shows. In post 152, I pointed out that the upper links of a drag race 4 link point downhill, contrary to the drawing (which is a dirt track birdcage ,by the way!) you posted in #138. You then brought up in #153 that the drag race 4 link lower links (NOT upper as I had previously said) are usually horizontal (they are), but condescendingly agreed to "assume the 4 links are pointed downhill", which was never what I had written. That was the basis of my posting the picture in #196. As far as the topic of floaters or birdcages goes, I brought that up in response to Blowby's comment about one of the earlier videos I posted. AND I agreed with you in #166, that in the case of a housing with birdcages, ALL of the birdcage links are in compression and providing thrust force anti-squat. The birdcages don't matter to the necessity of restraining the basic force applied by EVERY axle housing, and you (should) know it. Period. In all of your replies I cannot remember seeing where you address the point that, in ALL the videos, the top of the housing is visibly moving to the REAR under acceleration... Nowhere have you addressed the idea that the basic force from pinion climb (which you acknowledge is there) is the SAME whether the outer links are on birdcages or welded to the axle. You propose in post 192 that somehow the force that is pulling against the top torque link in the videos magically reverses to compression if we simply weld the birdcage (outer) brackets solidly to the housing??? How? 2) Impossible to say not knowing the link angles. But assuming they are parallel and horizontal, as installed in a traditional hot rod, the axle centerline would move up into the chassis, as the projected instant center is below the neutral anti-squat line. I have never denied in any way that there is thrust force present at the rear axle, and LOTS of it in some cases. SOMETHING has to anchor the rotation force of the axle shafts (just as you pointed out that the engine mounts anchor the rotation force of the crankshaft) which carry engine torque multiplied by whatever overall gear ratio we have at any particular time. So, what keeps the axle housing from rotating to the rear as a (Newtonian) reaction to the torque being applied forward through the tire? Last time I checked, there were only links attaching the housing to the chassis... I would also like to apologize to the other readers...for not being able to ignore what is staring me right in the face. Heim joints do not get stretched by compressive forces.
I've changed a bunch of them but, like most racers, I never stopped to photograph them. Just threw them in the trash. Here's one: And his caption: "Here you can see the broken Heim joint. Notice that the ball is still on the shaft for the upper control arm mount on the rear end. The Heim joint opened up and stretched due to the torque that was applied to it when it failed." This was the forward facing top link on a 3 link rear suspension with NO birdcages... If you go to this link: http://jayauto.wordpress.com/page/3/ and scroll exactly 1/2 way down the page, you will see the same photo. I've got no reason to make this stuff up... I'm just trying to make sure people have a simple understanding of what they are building. You'll find that common theme in every tech thread I've posted on.
Yup, I was going to say something, were it not for Second Law of Thermodynamics, and the need for a nap.
And just so there is no "confusion" about the picture in #218, that is a front-load quick change axle. You can see the driveshaft exiting the gear case at the left of the photo. You can just see the edge of the rear mounted fuel cell can (red) at the lower right of the photo. But, I'm sure I'm just "manipulating" things again....
Exactly right. And I'm only referring to the loads placed on the links under max acceleration. And that's all I've ever said on the subject...
Paul, It is possible to get the uppers under tension, but to do this you must reduce the axle thrust in proportion to pinion torque reaction. The way to do this is to have a rear end ratio 1:1 or higher and/or a very large wheel radius.[ this is something not practiced in automotive engineering ] A 6 cylinder Chevelle and a drag car work the same way. The more pinion torque reaction also equals the more axle thrust due to mechanical gearing. These guys can re-invent the lever all they want, or separate the forces so they are visible on a video. But there is no way the pinion torque reaction will equal the huge thrusts needed to accelerate a heavy vehicle at over 1G [especially with 4:1+ ratios ] The torque reaction is at the pinion not the axle [ the axle housing supports the pinion which confuses them into thinking it's axle torque reaction ] Like I mentioned earlier I learnt a very valuable lesson here. but I'm pretty much done with all this,
Without any comment so far about the obvious rearward movement of the top of the housing (and pull force on the top link) in every video I've posted to try and help explain my point... And I guarantee those cars do not have less than 1:1 axle ratio...not that it would make any difference.
THAT"S EXACTLY WHAT I'VE BEEN SAYING THE WHOLE TIME! No "gear reduction", no "parasitic loss", no nothing. Go back and read for yourself. The housing itself resists the torque applied to the tires, equally and oppositely through the pinion shaft and bearings. And the links hold the housing in place relative to the chassis... wow.
So...at the car show today I saw two old rods that had four links with the top links at the axle level, mounted at the front of the axle tube, presumably due to floor board space restrictions. I asked one guy about it, he said it worked fine.
I do that, on some cars, for floor clearance. None of them are high-torque applications, though. All triangulated 4-links, too. Every system is one compromise, or another.
Heres some info for others to read. http://www.raceglides.com.au/files/fourlink-ladderbar-tips.pdf Paragraph 4 sums it up! Most confusion surrounding the rear suspension linkage (ladder bar/four-link) is in the action/reaction torque around the rear axle and its housing; the pinion climbing the ring gear, and the fact that the axle housing wants to rotate in the opposite direction of the rear wheels. With a ladder bar solidly attached to the rear end housing, and pivot the front end in a bracket somewhere on the chassis, the counter-rotating rear end housing appears to be imparting and upward force on the chassis at the ladder bars pivot point lifting the chassis vertically at that point on acceleration. However, that would only happen if you tied the rear wheels solidly to the ground, and with enough engine torque the front of the car would lift and be rotated backwards. But thats not really what is happening when a drag car is launched. The tyre is trying to push the car forward. It is pushing from the point where it contacts the ground. The centre of gravity of the car, however, [SIZE=+0][SIZE=+0]is higher than this point. Inertia wants to keep the car where it is, rather than letting it move forward[/SIZE].[/SIZE]
Ray and Kerry, someone posted a link to this on another forum today, and it reminded me of you guys. I can't say if one of you is analogous to Proudhon and the other to Bastiat, however, nor if so which is to whom.
There's no debate here mate! He already won when he "got me" with this supporting argument, how can the laws of physics ever compete with 20 years of observations. I should've given up then
From post 184.... Dawie, I guess I'm Bastiat... I've been called something like that many times before...
Agreed, 100%...as far as it goes. Except, again, the effect of the "counter-rotating" rear end is just magically written out of the picture...! I've seen it bend leaf springs, break heims, and pick the front end straight up in the air with a pair of big sticky slicks on the back end. And this still doesn't address the supermodified in post 152, with a center of gravity BELOW axle centerline height and the rear of the chassis being lifted higher than the front. If, my "observations" violate the laws of physics... Then, I must have missed the "alternative" explanation for all these things somewhere in the discussion. Kindly repost just that part...
You know, this kind of reminds me of the story from Mark Donohue's book; about what happened when he was developing the 917 turbo engine with the Porsche engineers. Donohue was trying to get the engineers to realize that he needed the engine to run at low rpm with no boost so the car would come out of slow corners. The Porsche guys only wanted to see a bigger peak HP number to make their bosses happy. Mark kept arguing with them about how to tune the engine, but the Porsche engineers thought it was fine. Then he noticed that the engine dyno graphs started at 5000 rpm and went up from there. When he asked the engineers why, they said something to the effect that "the engine does not run right below that rpm". They had just eliminated everything they didn't want to see....
Something about Disraeli...and Gladstone, in the HAMB house of Lords? Folks, and builders, tend to focus on the events that sharpen their vision, esp in memory of failures. Not speaking from personal experience of course! Kerry, in post 229, you might be applying 'static' conditions, to a wheel-spinning dynamic situation...and the claimed 'lack' of lift force. The anti-rotation torque does exist and exhibit. Somewhere. In IRS stuff, the geometry employed will deliver either a rise...or a squat. I don't see any real disagreement when all forces are considered.
Actually, Kerry did agree that a lift force exists (post 202, if nowhere else). But somehow, that lift force cannot be coming from the top links of a 4 link being in tension, even if the lower links are horizontal and have 2 degrees of freedom (bushings at both ends)...? I still haven't heard where it DOES come from, if not there....
For those of you who haven't run out of popcorn and moved on, here is a quick diagram to illustrate the point in contention. The yellow arrows represent "forward" forces applied to the ring gear (torque) and the ground (thrust). The thrust only becomes thrust because the torque of the tire is anchored to the ground through the tire contact patch. The blue arrows represent the equal and opposite "reverse" force generated in the housing (red). The pinion lift arrow is a force normal to a line (dashed line) running through the axle centerline (green) and the R&P tooth contact area. This force is transferred through the pinion support bearings and into the housing itself, causing the housing to want to rotate to the rear. The black arrows represent the suspension links and bushings, with the arrow directions matching the forces applied to the housing. The magnitude of these forces are relative to the length of each set of brackets from the axle centerline.
Ex, doesn't the car itself create a resistance against the forward thrust of the lower link, thereby trying to rotate the axle housing forward at the top and eliminating at least some of the tension on the upper link?
all right all ready... have some fun skip the 1st half... http://www.youtube.com/watch?v=jbvip1Ot6jQ oh joy rapture i'v got a brain...
The chassis creates resistance against both directions of motion. The axle housing reaction is created about the axle centerline. Any amount of torque passing through the ring gear and into the axles will create the opposite force in the housing itself. Since we are gentlemen, let's settle this with a gentlemen's bet... Mythbusters time! I will give a check for $100 to anyone who can provide video proof of the following: If Kerry and Al are correct, it should be possible to remove the upper links from a (O/T) GM A body, G body, or Fox body Mustang; then tie a rope through the mounts and to the rear of the frame. If I am wrong, it should be possible to drive the car this way, as the rope will always be in tension under power (forward facing links in compression, right???). Clamp a GO-Pro under there and let's settle this once and for all. I picked those models because the springs are at or ahead of the axle centerline...just so there is no "confusion" about the springs rotating the axle back. I hope Kerry and Al will step up and match the bet, at least to help pay for the damage...
Hi exwestracer. Nice picture. I think that's a nice portrayal of the forces necessary to restrain the axle rotation (which are similar to what happens when a farm tractor's tires are frozen to the ground). Something similar happens laterally when accelerating with a non IRS as shown in fig 5 here. http://www.web-cars.com/corvette/1963_SAE-paper.php?page=4 there are additional forces when the vehicle is accelerating due to : - thrust at tire contact patch at the ground acting thru the instant center of the various suspension links - the CG acting lazy applied up at the CG height above ground. Re suspension binding that must be accomodated by some combination of bushing or control arm twist: I think the desirable situation is shown at 2:50 to 3:02 here - http://www.youtube.com/watch?v=upquH0f-LQA Stock GM 4 link rear does it with easy to twist relatively flexible channel control arms and big squishy low stress bushings.
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