Week by week we get ever closer to racing season here in the midwest. Some parts of the country are already racing.
For us, we have at least one solid month before we see any track activity and, with the early season rainouts that take place, we might not see the track for a solid two months.
Which in some ways is good because it seems we have at least four months of work to do before we go racing.
But, as it always seems, if we had to go racing tomorrow, we could get everything done.
Dirt Late Model Wheel Loads
In my last article I mentioned that one thing that is getting particularly common is using bump springs and bump stops on the right front.
This is usually in conjunction with a soft right front spring and / or a stacked right front spring.
Let’s talk about a stacked right front spring first.
Generally there is a lot of misunderstanding on how a stacked spring deal works.
I have some experience with this, but please forgive me if someone reading this has much more experience with it and sees some areas of inaccuracies.
We have been on again, off again, with stacked spring for as far back as 2006, but doubt in weather it is good has always kept us from really digging in deep with setting these up.
First, stacking two springs on top of each other will create a final spring rate softer than the two.
Take two four hundred pound springs and stack them on top of each other and you will get a final spring rate of two hundred pounds per inch.
I get a bunch of arguments telling me that the final spring rate is always more.
Stacking Springs Explained
To visualize how this works I have a pretty good way of explaining it so you can understand.
First you take a four hundred pound per inch spring and set it on a bench. (Let’s also pretend springs don’t weigh anything. Even though they do it will just complicate things)
Take a four hundred pound weight and set it on top of that spring. The spring will compress one inch.
Now, take that weight and spring that is compressed one inch and set it on top of another four hundred pound spring.
This new spring will also compress one inch because of the four hundred pound weight sitting on top of the original spring.
So, now you have two four hundred pound springs each compressed one inch for a total of two inches of compression.
Since we only rate springs per one inch of compression, we need to divide that entire stack in half to get that number down to one inch of compression.
Doing this you get 200 lbs. / in.
The formula is: Final rate (x)= (1st spring x 2nd spring) / (1st spring + 2nd spring).
Hyperco Spring Rate Calculator
The easiest way I’ve found is to just download the rate calculator spread sheet from Hyperco’s website. This will do spring rates from roughly one hundred to one thousand.
The really neat part of this technology is when you can adjust spring rate per ride height you want your car to run at.
You can have a one hundred pound per inch spring to a certain point then “lock out” one of the two springs and instantly increase spring rate to something much higher at a particular ride height.
You can run a one hundred per inch spring on the right front, then when it gets to the desired ride height, “lock out” the top spring and have the final spring rate that of the bottom of the two stacked springs.
Now let’s start thinking about this in terms of ride height and wheel loads.
We know a shock can be a timing device to tell the weight how fast to transfer. Well, a soft spring can help do the same thing without having to add all the rebound to the shock to hold the weight up there.
If you run a traditional 300 lb. / in. spring on the right front. And you transfer enough weight to compress that spring three inches. You will have a wheel load of nine hundred pounds plus what ever your wheel load was statically.
Let’s just say you ended at 1,400 lbs.
If you want to hold that car at that attitude you will need a shock with a substantial amount of low speed rebound because you will have a spring trying to pick the car up at 300 lb. / in.
Now, stack a spring with a total spring rate of 100 lb. / in. and lock out the top spring one inch from your desired ride height. After you get outside of that one inch window, you only have 100 lbs. / in. pushing up on the right front.
This will require substantially less rebound in the right front shock to hold the car at the desired ride height.
The question is what are your spring rates? This is one area which you will need to experiment with. You need enough spring rate for the track condition to keep your tire contact patch connected.
Some people are using a dive spring of 1,000 lb. / in. inside of their 2 ½” coil spring. Others are using a bump stop in place of the dive spring.
The good thing about a bump stop is it does have a little built in dampening because it is made out of foam rubber.
Some people think that a dive spring will help plant the left rear as the the car rolls onto the right front.
Realistically the right front will always only accept the amount of weight transfer.
The right front will stop compressing when the transferred weight equals the amount wheel load supporting it.
That may be a bit confusing, so let me break it down a little.
These numbers may not even be close, but I’m going to use them for this example any how.
Lets say at ride height our spring load equals 300 lbs. If we are running a 300 lb. / in. spring and we transfer an additional 600 lbs. on to that spring, it will compress an additional 2 inches.
Our final compressed spring load will be 900 lbs. If we want to lower the right front an additional inch, we would need to install a two hundred pound spring instead of the three hundred.
The final wheel load will always be only equal to 900 lbs.
Til next time,