I am relatively new and had a question. How do you set and measure droop in the front end of a pancar? I see it mentioned in all of the setups given, but am not sure how to do it!

Thanks
Chuck

Droop is adjusted by the washers on the very top of the king pin.

It can be measured several different ways.
-Most common is just watching how much the springs sag when the car is set down.
-If you want to get more precise, you could put very stiff springs in, to remove sag, and measure chassis height for a reference point, then put the springs you plan to race with back in and re-measure chassis height to see how much droop you have.
-A tire durometer can be used by pushing on the bottom of the king pin, as soon as the spring compresses, note the durometer reading.
-A feeler gauge could be used between the steering block and lower A-arm also.

It seems that no one else is going to add to this so I will voice an opinion.

In my neck of the woods when we speak of pre-load, we are speaking of what was basicly described above; the amount of load you put on the front springs before the weight of the car comes into play. By keeping the chassis off of its wheels and shimming the king pin until there is no play between the spring and the upper A arm <but without compressing the spring> is called Zero Pre-Load. The more shims you add, the more pre-load is placed on the spring which makes the spring stiffer and react faster when the car is in a corner.

Now, here’s the confusing part. The more pre-load you have the more the chassis is angled down in the front. But, we call this chassis “rake”. If you try to get more chassis angle (rake) by preloading the springs, you are changing the tension of the springs which affects your front end “tune”. Chassis rake is a good tuning tool and affects how much weight transfers to the front end and how fast it transfers. But, this is better done with shims between the chassis and the lower A arm, this way it does not affect any of your front end tension.

When we speak of “droop” we are talking about the angle of the rear pod in relation to the chassis. This is controlled by the length of the center shock NOT the stiffness of the spring. You can see this in a couple of different ways. The best one that I know of is to turn the chassis up-side-down, lay a straight edge the full length from the front of the chassis to the rear of the pod. Now, measure the “gap” between the chassis and the straight edge. If there is none there and the straight edge is completely flat along the chassis and the pod then you have Zero droop. Lengthening the Ball Cup by unscrewing it slightly, will ADD droop. Shortening the length by screwing it in will SUBTRACK droop. <In the beginning, you may have to put a different ball cup on so that you will have the adjustability that you want. >

Ask a dozen people how to do this and you will get a dozen different ways; that was just one of them.

So, you see we are talking about three different things; Pre-load, Droop, and Rake and they are all tuned separately by different means.

-A feeler gauge could be used between the steering block and lower A-arm also.

I,ve found when measuring Front suspension droop that the Wire Style Spark Plug gapping tools work very well, much better than flat feeler gauges. They can be found at most decent parts houses and on tool trucks relatively cheaply. Look for the ones with a 1/8-1/4 90 degree bend at the wire end and sizes .005,.010,.015,.020, .025. Some of them now have nothing but the larger sizes .045-.080, which you wont need.....at least I,ve never ran more than .030 front droop anyway.

Might be a mistake to post this, but the spring does not get stiffer when you shim it, unless of coarse you have a progressive spring.

when you preload a spring and then attempt to compress it the same amount it does take more force to do so...so yes it does change the spring rate.

Might be a mistake to post this

Now you,ve done it!!! :p

when you preload a spring and then attempt to compress it the same amount it does take more force to do so...so yes it does change the spring rate.

your right it does get stiffer if it is a progressive spring, not if its a straight rate spring, you may want to do some research so you understand what your talking about!!

a 12lb spring will always be 12

a 12lb spring will always be 12

exactly katfish, unless its goes into coil bind, usually the chassis will drag before that happens.

I think you may want to go do some research first yourself. Example using a larger spring because its easier to measure...take a valve spring...At a compression height of 1 in exerts 300lbs of seat pressure. Now add sims to that spring same .200 shim...compress the assembly to one inch it reads 320 lbs of seat pressure....the spring is still the same but the preload changes the amount of force the spring applys. this is a real world example using a larger spring but the same holds true for our little springs. Springs arent meansured at just 12lb the are measure by lb/distance on how much pressure it take to compress a spring a given amount.

nick bell

Let me see if I can take a crack at this.

A sping, linear for this example, is rated at 300lbs. That means it takes 300lbs, to compress it 1 inch. So to compress it 2 inches it takes 600lbs. Now shim the spring to whatever you want. To continue to compress this sping to the 1inch mark a total of 300 lbs will have to be added to the spring to compress it this far. This shims will effectively be "weight". Let's say that on this 300lb spring 20lbs compressed the spring .020". If you add .020" of shims to this spring is is like adding 20lbs on that spring, precompressing it. To get it to go to 1 inch of compression it will still take 300lbs (total) to compress it to 1 inch, in this case 280lbs. The rate never changed, but like Lin stated it will be "quicker".

The spring rate did not change with the preload, the amount of pressure it is putting down did, but the "rate" of the spring will not change no matter how many shims you preload it with.

Now if you do the same with a progressive sping, that is totally different.

I think you guys are saying the same thing, only different.

In Nick's example the "rate" of the spring did not change, it is still a 300lb spring, with the amount of preload on it, the "pressure" changed. It had to, it was being compressed further. But the spring it self did not change characteristics just because some shims were added.

Maybe this helped, maybe not.

If a spring's rate is linear (most racing springs have linear rates), its rate is not affected by the load put onto the spring. For example, a linear rate spring rated at 500#/inch will compress 1" when a 500# weight is placed onto the spring. If another 500 pound weight is put onto the spring, the spring will compress another inch. At this point, the load on the spring has increased to 1000 pounds. The rate of the spring, however, remains constant at 500#/inch.

:wave:;);):freak:

Might be a mistake to post this

I knew this was gonna happen.

A number of people do not understand the differance between spring rate and spring preload or seat pressure which was used incorrectly in one example. Spring rates do not change with preload only seat pressure changes with preload.

[QUOTE=Fl Flash;2536384][B][COLOR="Blue"]I knew this was gonna happen.

I knew it also, but I was bored!! Sitting here watching the race, Love when people are so sure of themselves, they are willing to argue about something they really dont understand.:woohoo::wave::wave:

Hey what else are we going to do between all the red flags?

One thing we forgot to consider is the effect of the paint or powder coating has on the spring rate. :D

Depending on how much droop you run, a very handy tool to have in your pit box is a set of "L" shaped allen wreches. Pick up the smallest set you can find, they come really small. I use them as feeler gauges.

In the above examples people do need to remember that while the rate does not change, it will feel stiffer to get the spring moving because you have to get enough force to get it moving, in the 500# spring example compressed 2 inches it will take 1000# of force to start the spring moving. Which it has on it to get it to be 2 inches. In the valve spring example it will take 300# to unseat the unpreloaded valve but will take 320# to get the other valve to unseat while both have the same rate after they begin moving. The only time this will be a thing to consider is when running no droop and preloading springs, a softer preloaded spring react different than a stiffer unpreloaded spring will, the stiffer spring may actually begin moving easier but will take more force to keep it moving, where the softer one will take a greater force to start it moving but will move easier once it starts, makes the springs digressive to a point.