What Pinion Angle are you running?

[Tom Cole]

I use a smart tool (digital angle finder) exactly as you described.  It also works great for setting caster and camber.

[gramps05]

Something to keep in mind, when adjusting the third link you are rotating the rear end, this also will change the wheel base length and can also effect the frame heights in the rear of the car.

[DARK AGE 53]

Something to keep in mind, when adjusting the third link you are rotating the rear end, this also will change the wheel base length and can also effect the frame heights in the rear of the car.

I didn't find any difference in wheel base length after I adjusted the angle,  it seems the rear trailing arms would prevent that from happening. JMO

[gramps05]

When you adjust the third link it rotates the rear end on the lower trailing arms attach points which are two inches lower than the axle tubes and are stationary so the wheel base will change + or - depending on your pinion angle. Also when the rear end is rotated the lower shock mounts move higher or lower when measured from the ground which will effect your frame height. Granted the change will not be great and you may need a tape measure to see it but if your running close to the limits and make an adjustment at the track you could end up DQed in tech. Something else, if you change your wheel base length guess what it does to your pinion angle?

[Tom Cole]

My Guess is that if you increase the wheelbase and don't touch the third link, your pinion angle will increase and if you decrease your wheelbase it will decrease your pinion angle.

[pegalis]

Heh, bringing a topic back from the dead

Rotating the pinion up or down is really just a reference point.  By reducing the length of the upper links, you are affecting what is known as Antisquat.  * hey, i see you snickering over there.. *  This is a very important term in suspension dynamics.  antisquat or AS, is when the rotational force of the axle is distributed to the suspension.  When you take off in a legends and have really good traction, the ass end sags down a little, the nose picks up and you take off like a starwars ship jumping to hyperspace .  The ass end squats down because all that rotational force has to find the best path to travel. This is into your suspension, then your frame THEN to the road. 
The pinion wants to rotate down with that twisting motion, with pulls on the links, with makes the car squat. 
The trick is to turn this into traction. 
A lower pinion means shorter upper links. Less squatting. More power to the ground.  This can cause hopping, due to the axle pushing itself to the ground.  Best setup for low traction like dirt tracks. 
Higher pinion, longer upper links, less power to the ground, less likely to hop under throttle.  But, there is less bite.   This is better for pavement where you have gobs of traction.

A good all around figure is about 80% Antisquat.  Our cars, are about 83% to 95% according to what I can see.
 

[MegaMang]

I sniggered really quietly.  You shouldn't have heard me.  You spying on me?  This 'puter have spy stuff on it?

Good explanation.
Mike

[MegaMang]

One question Pegalis...

How do you translate % of AS to degrees of pinion angle?

Thanks,
Mike

[justfreaky]

Mike,
 Your going to love this!

"The angle of the upper link in a 3 point system has a very important influence on the performance of the chassis under braking and acceleration. If the upper link is mounted  at a downhill angle towards the front of the car, this will promote more anti-squat under acceleration. This means the rearward pitching moment of the car under acceleration is reacted by the mechanical leverage of the suspension arms and thus causes additional  downforce to be placed on the rear tires. However, the drawback of this is the opposite and equal reaction under braking. A large amount of  anti-squat (downhill angle of the upper link) will cause rear end lightness and and rear end hop under braking The more downhill angle of the upper link, the more severe the problem will be.
 An uphill angle of the upper link will promote good firm traction of the rear wheels under braking. But, it has a big drawback  under acceleration - It promotes pro-squat, which lifts the rear wheels up under acceleration and  diminishes traction.
 The answer is a compromise.................."

Source: "Dwarf Car Technology" by Steve Smith


 I could get out several of the other books on suspension theory and design; But I think that explains it pretty well.
Simple physics; "For every action, there is an opposite and equal reaction."


 Tell You What I'm Gonna Do - 

I'm gonna buy you the book and send it to you, in exchange for that right front spindle.

Sound Fair?



Steve

[MegaMang]

That's a beauty and I love the trade idea.  Not to forget though that Inex Legends don't allow you to change the angle of the upper link.  One mounting point, not variable mounting points.

Shortening and lengthening the upper link will change the pinion angle, but how does +3 degree angle translate into Percent of AS?

Mike

[justfreaky]

My Guess is that if you increase the wheelbase and don't touch the third link, your pinion angle will increase and if you decrease your wheelbase it will decrease your pinion angle.

Read all the posts in this thread. There is some Great info and insite!

True;
 My suspension will have multiple mounting points. NOT Legal for the Legends Cars.
Think about how changes to the  lower links, or upper link, change the angle. Even if only slightly. 


Steve


PS.

 I'll get on that book for you.

[pegalis]

Ah,  but you are moving the link mount when you change the pinion angle.  Drop the pinion, and you have just moved the upper link pivot point on the axle forward, and the lower backward.  Reverse for upping the angle.
On a long WB car, this 1/2" of pivot location adjustment would do little.  But on a short WB car, with short suspension links, its a world of difference. 
The shorter the suspension links, the less adjustment is needed to see results. 
A good example:   Take a drinking straw.  Loosely grip one end with your thumb and index finer, about 1" from the end.  Now pivot the straw from the long end with your other hand, and note the distance the short end moves in relation to the long end. For every 1" of movement on the long end, there is only 1/8" on the short end. Now, do the same thing, only pivot the straw from about 1" away from your other fingers.   now, its suddenly 1/8" to 1/8" relation.
Long links, bigger adjustments.  Short links, little adjustment.

Mind you, the percentage you can get from our suspensions is only + or - 6 or 7%.   But that 6 or 7% can be the few needed seconds on the track you need to win!
 

As for calculating the % of AS on our chassis, I have fabricated enough custom 3 and 4 link suspensions to get a good feel for it just by looking at the link locations in relation the the axle and frame.
If you want, ill dig up the numbers and calculations and get some exact figures for you.   

[MegaMang]

That would be great Pegalis.  Just trying to wrap my mind around the geometry of the linkages so I can think through what advantages I can get from adjustments.  It's one thing to read what everything does and remembering it, but another thing entirely to truly understanding it.  If I can understand it properly, I can make changes much easier to effect the results I want.

Mike