A rule about 4 bar rear suspension is to keep the lower bars level to the ground. I’m building a chassis that won’t accommodate that idea. The car will have an extreme rake (frame rails 5 degrees down in front). So, what ill effects does breaking the rule cause? I made a mock fixture to play around with angles and positions in the car. First photo has attach bracket perpendicular to ground, second, (my preference on this car) perpendicular to the frame. I swing it through the theoretical travel and the pinion travel appears to be similar, so will either plan work? I know everyone will say, “don’t do it”, but I’m trying to achieve a certain look here, while still being driveable. Fwiw, this is not a drag car, more of a driveable show rod. Thanks.
I'm not aware of any rule of thumb that says a 4 bar must be parallel on a rear end. I'd think the front suspension would be a different story. If memory serves me, I think the older Corvettes had links that weren't parallel. I believe they were also different lengths. Someone correct me if I'm wrong. I think the main consideration would be the change in pinion angle when disimilar lengths are used. Edit addition: Go to this site and there are a ton of examples you can look thru and maybe find what you want. https://www.google.com/search?q=4+l...gBEIkE&biw=1620&bih=1018#imgrc=sGme31a1s_PK8M This one looked promising:
Can you draw a sketch of the car? side view, show the frame rail, the ground, the tires, and your proposed locations for the 4 bar. To scale would be good, too. Or close. Looking at parts laying on the floor, doesn't tell me much.
Since the car is a bunch of parts scattered about, I drew this: I’m not wanting the bars hanging down off the axle, I want them tucked up parallel to the frame. They will be visible. Equal length btw.
Having them parallel to start with puts them at the end of travel, and the travel arch going up/down is pretty even and minor. When you have them angled up toward the rear, they are already on the short throw and going to get shorter as the suspension compresses and longer when it decompressed. Whatever real world disadvantages that brings with it are probably not that important for the kind of car you are building.
they're not going to move very far, as i expect the car will sit as low as possible, and not have much travel. The sketch is very helpful.
Seems the pinion angle in the sketch will change pretty constantly. That could put some extra load on the u-joints. The warning would seem to be sure the slip yoke can't pull out of the transmission, or bottom out.
Thanks guys. The pinion will move about 5/8" total travel fore and aft through the proposed suspension range. I figure the suspension will only have about 2 3/4" compression from ride height, and only a bit more at full droop, which probably helps keep me out of trouble. All this is coming about because I'm using some really big, OT rear tires, and really short fronts.
Careful with the really small fronts. I saw a couple REALLY nice rods at an event recently that had front tires that I thought looked too small. They can look wierd if out of proportion/scale with the rest of rod.
Drag race oriented for sure and maybe more information than you want or need, but here's a good tutorial on 4-link rear suspension. http://dragrace.academy/rear-suspension-series-part-6-4-link-basics/
How long are the bars? Could put the top bar to the rear, making watts link, axle position should have little four/aft movement but will rotate some.
1. Be VERY aware of your pinion angle at ride height. Measure twice (or three) times, weld once. OR...cut up one of your bars (upper pair or lower pair) and make them adjustable so you car adjust the pinion angle if you don't...get it right. 2. Because the axle centerline will be moving toward the top of its theoretical circle "faster" because of your desired bar orientation, BE SURE to give the drive shaft yoke a little more movement capability (clearance) than normal, so it doesn't crash into the trans., during a hard bump in the road. 3. See #2. Because the axle centerline has started into the rotation of its theoretical circle because of your desired design, MAKE SURE...you give everything - exhaust, anti-sway bar, etc. extra room in front of the axle so that there is no contact. Hint - Make a cartoon / template (moveable parts) on paper. Move the axle up and down a (scaled) three or four inches and measure how far the axle centerline moves forward. Also watch what the pinion angle does, and design around it. Mike
Having the bars parallel has the instant centre at an infinite distance. By lifting the lower bar at the front you get the instant centre at a true theoretical centre [the point of acceleration] Having the front point of acceleration lower than the rear axle centreline tries to cause the rear wheels to lift [in theory but not in reality because of gravity] and/or the frame to squat. The solution is .........stiffer rear suspension in the rear. Now the issue with a street car will be roll steer, but in this case "rear roll understeer" which is more desirable than "rear roll oversteer" Body roll on a corner will cause the outside wheelbase to shorten which tries to self correct the cornering [this requires more steering input] But the car will have good rear "side bite" on corner exit, so you can step on the gas earlier. In a straight line on a bumpy uneven surface the car will feel like it is tram-lining through all the bumps and grooves but it is quite stable and controllable. If the front point of acceleration higher than the rear axle centreline it would have "rear roll oversteer" Body roll on a corner will cause the outside wheelbase to lengthen which tries to turn the car in deeper. This would require "opposite lock" driver correction. And the car would feel quite "nervous" and twitchy in a straight line bumpy uneven surface. Rear roll understeer is far more desirable than rear roll oversteer The O/P would be OK doing what he plans [and trying to keep the bars as long as possible] as long as he is prepared to try different rear springs to tune it. ----------------------------------------------------------------------------------------------------------------------- Our O/T racecar [Mustang FR500C] has a lot of rear roll understeer, but on a short track it really sticks to the track mid corner. But on the front straight at Pukekohe Park Raceway which is really bumpy the car is darting and weaving all over the place , yet you can take your hands off the wheel at 235 kph [145 mph] because the car is self correcting.
@mohr hp Add to the above. The longitudinal instant centre [for accelerating and braking] works the same as the "roll centre" for cornering. Road Racers think "three dimensionally" In Road Racing it is highly desirable to have the R/C as low as possible [sometimes below ground level] That way lateral acceleration is transferred via "overturning moment" to the outside tyres. The greater the distance the R/C is below the CGH the stiffer the springs need to be [roll stiffness] Overturning moment during cornering is called bodyroll Overturning moment during acceleration is called squat [of sometimes front lift] ,and during deceleration is dive.
Lots to consider here, thanks guys. The bars I have are threaded on one end for adjustability. They are 25" long . The pics on the original post are my mock up, which I have run through the proposed travel ranges. (see the sharpie marks on the table.) The pinion angle is "0". The rear end will move the driveshaft in and out of the trans about 5/8" total. On the springs, don't tell anyone, but I'm using Air springs, and adjustable shocks. Mimilan, thanks for the info on rear roll understeer, and squat/dive. I'm visualizing all these motions based on my experience with aircraft pitching dynamics. Makes complete sense. So when I hammer the gas it ought to bottom out rear the suspension, should help it hook up with a lot of weight transfer. I appreciate all your thoughts fellas!
Rear 4 bar (Normal duty): Up to 900mm long, tube OD 22mm, wall thickness 3mm, 5/8" UNF or 16mm fine (Metric) Rear 4 bar (Heavy duty): Up to 900mm long, tube OD 25mm, wall thickness 3mm, 3/4" UNF or 19mm fine (Metric) I was of the view that when set up correctly and to be geometrically correct, parallel 4-bars need to point up at the front or be horizontal to the ground, providing anti-squat with the correct amount of pinion angle at ride height. Pointing them down at the front will cause the car to squat under load which will in turn can cause the rear tyres to easily unload. With anti-squat built in and bars pointing up at the front up, the wheels have a tendency to turn the axle in the opposite direction, this motion causes the bars to push the car up and forward. With them pointing down the opposite occurs and the rear drops, not what you want. http://www.zigsstreetrods.com/Techtips/c07938.html
Is the frame going to be exposed or will you have fenders? Are you running a 4 bar on the front as well? I think the basic answer to your rear end question is simply that the arc that the rear end moves thru will allow more forward movement, and you will change the instant center so that traction may suffer. Whether it has an effect that makes a real difference for normal driving is probably moot.
Yes, exposed frame. IFS. I know this whole deal is not perfect, the thing I was most concerned with was whether the car would be ill handling or undriveable. Also, I don't see how pinion angle is a factor here; when I move my mock up bars through range of travel the pinion angle (0) stays the same. I'm learning by these responses though!
That set of videos is excellent. I watched them a few years ago and have them saved as a 'refer to' for a lot of rear suspension things. You should all watch the whole shebang in order from 1 to 6. I know it's race car related, but it translates to what we do, if you can get your head around it................GOOD STUFF.
I think it is the opposite you want. If the rear squats on launch, the axle is actually moving upwards and unloading the tires. When the rear raises on launch, the axle is moving downwards and planting the tires, increasing traction. As covered by @mgtstumpy in post #20...................
I guess that's possible. I'm thinking of the way Pro Stock cars leave. they virtually hit the ground in the rear. I know it's apples and oranges. Sounds like the shocks need to be stiff in full compression. I think I should look at these stepped variable-rate snubbers.
I always understood that the 4 bars should aim/point at a theoretical centre of balance point, chassis people in the day said that the camshaft centreline at the rear of the engine was a close starting point. The explanation they used was - where would you push on a fridge to move it forwards quickly without pushing it over in either direction.
Big difference between a well mannered, drivable car and a Pro Stock car. The Pro Stocker has way more power/torque and has rear tires that will stick that power to the ground, providing the rear end is set up properly. A Pro Stock car may be set up to have some squat off the line so it just lessens the massive grip of the tires enough to stop tire shake. Too much bite will cause this, and the race is over right there............ It's a fine line. Not enough traction and you get wheelspin. Too much bite and tire shake will be the enemy. That very, very small window in between the two............GO MAN GO. Having your bars angled down at the front will increase bite at acceleration (raise the rear of the car), but I doubt you will get into tire shake territory. Just don't go overboard with the downward angle. The further you go, the weirder things will get.............
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