I'm helping a friend setup a 4-link on a Model-A rod he's building. I know we need to get the angles of the links right to keep the pinion angle correct throughout suspension travel - and I seem to recall there's some math/geometry involved - but I don't remember the details & we want to get it right. I searched the forum & the archive but didn't find many details. Thanks!
There are many opinions on this but I'll start the ball rolling by saying with a 4 link you ideally should have a U joint angle of 1-2 deg. on both ends of the drive shaft at ride height.
i set the pinion angle between 1 degree down and 0 degree's for street use. from what i can remember the two u-joint angles should add up to no more than 5 degree's total, but it's been a while
Old friend told me that 0 for the street will work very well . I have never had a problem doing it that way. $0.02
what type 4-bar is it? triangulated or parallel? with an equal length parallel rear 4-bar there shouldn't be any change during suspension travel as for pinion angle , this has been discussed on here many times before and i usually stay out of it ...too many different opinions. as for how i do it , i always end up with the pinion pointing up a few degrees to match the down angle of the tranny. you want both the front and rear u-joints at the same angle
In truth the pinion angle is a traction device, it wants to be pointed downward a bit so that when you stomp it the pinion will point straight twards the tailshaft and that'll drive the rear tire into the pavement. The other guys are righton about the actual angle and there is a fair amount to play with. The pinion angle is the difference in angle of driveshaft/pinion, it has nothing to do with being level (you can't put an inclinometer on the yoke while the car is in your garage - won't work). 4 link bars seem to work best when parallel to the ground, at least that is the simplist way. Theres' quite a few other ways but you can get into rear steer where the rear end actually moves forward or rearward when you go asymetrical so it is best if both bars are parallel to ground and each other, the bars should be same length. You'll just have to mess with a panhard bar or diagonal link to keep rear housing located. Piece of cake. 36-3Window is right too, there is a lot of room for interpretation and there isn't anything wrong with his advice. If you were on a forum where the NHRA stock class guys hung out you would read volumes about this subject, they have to run stock suspension stuff and they live or die with pinion angle.
I've never really understood pinion angle as a traction device. If you change your pinion angle much, wouldn't that screw with the u-joints, since you can't change the transmission angle to match? On a street car the pinion angle should be the same as transmission (36 3 window post). If you race the car the pinion angle should be a few degrees down from the street set up to make up for the deflection and loading of the suspension. How much down depends on how much horsepower the car has and how it is loaded. This is from Chris Alston, Jerry Bickle, and a couple of other chassis guys. Larry T
Yes, the 4 link arm lengths, angles and positions could be arranged so that the pinion angle is both minimal and remains constant during suspension travel. However, this isn't at all what the car needs to launch properly. Parallel and equal length arms will keep the pinion's static angle constant viz. horizontal, but its angle to the shaft and transmission will change whenever the suspension moves.
Thanks everyone! Yes, this will primarily be a street car, and the links will be parallel when viewed from above. It sounds like we might want the lower link angled slightly upward from the axle to the frame to give the car a bit of anti-squat on launch.
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