This is like Science class 101..." there is no such thing as a vacuum sucking air into a void...it's positive air pressure pushing itself into the void." I hated my Science Teacher...
When I was an Autoshop Teacher, my dilemma was always how to get the kids interested in the physics of how things work. The way I approached it was to make it simple to things they could relate to. You know, the electron flow can be compared to water flowing in a pipe yada, yada, yada................... Then when they were grasping the concept, I would have a day when I told them about all the lies I had just taught them. They got a laugh or two from this and eventually got to questioning everything I told them the first time. You guys are pretty hung up on exactly how the thrust is applied and what is doing what at a given instant a very dynamic situation. I have the same problem ExWest in understanding that if the net force on the upper link in a 4 bar system is compression, why do all the heim joints show failure in a tension mode never in compression. Personally, I couldn't care less if anyone is 'right' here. Just tell me what is happening to the vehicle when it is moving and what changes might make it behave differently and the compromises involved. I'm done, and NOT subscribed.
Originally Posted by Kerrynzl "There is no way that driveshaft torque can be greater than axle torque in a typical rear end. Pinion torque reaction is a product of driveshaft torque [ it tries to climb the crownwheel ] creating lift. Axle torque reaction is a product of driveshaft torque multiplied by gear reduction [ it tries to propel the axle forward against traction ]" Originally posted by exwestracer: "RIGHT! And if we put 1000lb/ft through the axle shaft, it has to come from the crownwheel (ring gear), correct? Even if the torque is generated at the pinion, it has to be anchored somewhere. Think about a big industrial gear reduction box in a factory somewhere. Does is still work if the housing isn't bolted down to something? Will a transmission transfer power to the driveshaft if there are no bolts holding it to the bellhousing? That "anchor" must resist the full output torque, regardless of gear reduction ratio. So, what anchors the torque applied to the axle shafts by the crownwheel? It's not the driveshaft.." Interesting discussion even though its a bit off from the topic. So the pinion tries to climb the ring gear, thus rotating the housing down. Due to gear reduction the torque is greater at the wheel and is rotating the housing up (pinion goes up) surpassing the reaction generated by the pinion axle. Did I understand correctly? Jukka
Actually pinion climb is what rotates the driveshaft yoke UP. Since the yoke (and the pinion it's attached to) are held into the housing by the pinion bearings, this pushes the front of the housing up as well. The "debate" we are having concerns how much upward force is present at the pinion shaft (reverse rotation of the housing), and the NET effect that force has on the suspension links that hold the axle housing into the chassis.
Back in the '80s I had a slew of Chevelles and El Caminos. They would wheel hop like crazy. Was that the upper arm going from tension to compression? There were several aftermarket cures, Lakewood ladder bars that clamped around the housing and lower control arm mount, effectively eliminating that pivot point. Then came some bars that raised the upper rear mount and clamped to the pumpkin, effectively eliminating that pivot point I think, though I never tried those.
Ok, thanks. Just wanted to make sure I´m understanding this correct. I would be interested to see the experiment you talked about earlier, replacing the upper links with chain or cable. I believe it would be possible to drive when accelerating heavily. Jukka
The links ran uphill to the frame at stock (or higher..) ride height. This caused the axle to "pull" (I think?!) itself up into the chassis under acceleration. The riser brackets put the arm angles more downhill to the frame, causing the tires to plant better.
Only if the gear ratio is 1:1 or higher would that work. In an IRS car when the linkage [ A-arms ] break the wheels drive forward. You have to look at where the force is applied. In a pinion gear it is where the teeth mesh [ trying to force the crownwheel down ] so there is an opposite force at the centreline of the pinion trying to go up. The reason it rotates is because it is constant around a crownwheel using an axle as a pivot. When you get to the tyre contact patch the torque is forcing rearward , so there is an opposite force thrusting forward at the centreline of the axle. This is linear because the ground is level and the radius of the wheel is constant.[ in theory ] I think some people here are confusing the axle centreline with the axle housing. For this debate the axle housing is a fixture that supports the pinion. The axle thrust from the wheels is transferred into the vehicle via the axle / carrier bearings. Axle thrust does cause lift , but only via suspension geometry. Axle thrust can accelerate a T/F car at over 6G's [ there are huge forces here compared to the lesser torque reaction at the pinion ] If the pinion torque reaction was anywhere near this, there wouldn't be enough gravity to keep these things stuck to the planet. http://www.raceglides.com.au/files/fourlink-ladderbar-tips.pdf
On an IRS system with the differential housing bolted to the chassis, there is NO pinion climb reaction force transferred into the suspension links. Only forward thrust is present, as with a solid axle "birdcage" system. To be clear, during acceleration, there is forward thrust force present in all links of a 4 link system. The point of contention is whether, under HARD acceleration, the pinion climb reaction exceeds the forward thrust on the top links ONLY; putting them in tension. This would apply to a street or drag race setup, where the suspension brackets are welded directly to the housing.
I thought you would have figured that out by now. Everything increases together proportionally. Increasing the driveshaft torque , increases torque reaction. It also increases axle torque ,and axle thrust at the same time. The only thing that doesn't increase is the mass of the vehicle, therefore it increases acceleration and weight transfer.
Did you see the photo in #276? No birdcages on that one. It's hard to ignore my eyes and years of working on this stuff. You keep pointing out that I'm mistaken (as are a LOT of other people), but other than saying it can't happen, what is the alternative explanation? SOMETHING wears those parts out. By the way, I'd be interested in hearing your thoughts on my questions in that post...
You were lucky! My science teacher hated me During my high school years when all my classmates were getting bored with physics , I was asking or presenting all these scenarios that were way out of the depth for my teacher.
[FONT= ][/FONT] A pull bar still extends on a NON-birdcage axle, Kerry. Explain to the guy with this stretched heim joint on a spring loaded pull bar how that is not possible... I didnt answer this earlier because I thought most race engineers would already know this. Beware of the correlation between Cause and Effect [ Hell, Im still trying to convince my missus that the benefits of her giving Oral has its positive effects on the Dow-Jones ] If torque [ reaction ] caused this breakage the part should never been put on the race car. Heim joints are a throwaway item that have a limited life expectancy , there is no such thing as a little bit broken so they need to be biffed out regularly. Yes they do wear out, Ive seen a few that have lots of end play [ this movement itself can cause stress fractures when clunking back and forth ] Ive also broken a few myself in road racing [ both uppers and lowers ], usually when I am downshifting I feel a wobble then when I get on the gas shes all over. Ive also broken one while getting on-and off the gas just warming up the tyres. The biggest cause of the breakages is ME, I am now just a hobby racer so I stretch my maintenance budget. Interesting photo though! Is that a driveshaft mounted quick-change?? A pull-bar on a non-birdcage rear end is something I havent seen since the late 70s when my dad was racing. They used to mount the lower links directly in front of the axle centreline ,so all the axle thrust is transferred at this point and the axle tube is free to rotate with torque reaction [ bird cages became popular over here with open tube rear ends ] If you go back to the Lotus Cortina rear end photo I posted earlier [ The one that started this heated debate ] youll notice that's exactly what lotus did by mounting the links directly in front of the axle ,except they mounted the triangulated 3rd link underneath.
No...I loved Science itself...just had a problem with the Teacher's attitude and expectation that we accept what he said as unquestionable fact. No real discussion, just accept and move on. As a result, I was NOT his favorite student. The point being, 100% acceptance isn't proof of anything or even desirable for that matter. In fact...the BEST thing possible in Science or the like is a core of capable disbelievers who wish to prove a theory wrong. But CAN'T. I lean towards exwestracer as being...I don't wanna say correct...perhaps more understandable and easier to visualize...BUT your input has given me quite a few reasons to question my thoughts AND a new way to look at some of this stuff! This thread has been awesome BECAUSE you both passionately/respectfully disagree!
I have not read through all 11 pages but Halfdozen has it absolutely right. The problem with urethane or poly whatever is that it has no torsional shear and limited compressibility. Herb Adams - suspension guru and author of the definitive book on this subject - Chassis Engineering - has the following somewhat restrained comment to say about it. "It looks nice on the shelf in the parts store". I had a discussion on this with Herb many years ago and his comments in private were not as restrained! I detest polyurethane which has become a buzzword and handle for a lot of people - manufacturers and suppliers who don't understand their subject matter to hang their hats on. IMO polyurethane has little place in any kind of suspension. It causes a harsh ride and has a tendency to bind....
I'm with you Weas. Poly parts are a bad choice. Polygraphite is even worse, yet so many just sing the praises. Gimme a new OEM style bushing any day. That "new car ride" comes back from the use of "new parts"! Who'da thunk it...
Ditto. The only place I have any use for poly is if I need to make a slight change in anti-roll (sway bar) stiffness. I use poly mounting bushings and spherical steel end links. Most of the last half of this thread has been a debate about something that has nothing to do with the OP question, anyway.
[/QUOTE] i know that this is an old thread, but this is the correct way to triangulate. if you make your bushing parallel then all you are doing is making a parallel 4 link with the apperance of triangulation. Atlest thats is what i have been taught by my buddy who builds race car chassis for a living. bushing however could be parrallel in a 3 link such as this Parallel
AMEN ! If we would just leave these skinny poly bushing ends out and use the cushy designs, we would even have nice riding suspensions too !
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