It's All About The Little Differences...

msquared

Member
Greetings everyone. This is a rare post from yours truly Msquared.

I was filling a manual order for JC Specality, and as usual I was reading some pages from my chassis manual. It got me to thinking, especially about staggers, casters and cambers. I remember a topic about front stagger and some people saying that it does not matter. After all, when you calculate the height difference of 1.25" to 1.50" of stagger, the overall tire height and change to chassis height is very small. A lot will say front stagger is used to set cross. True but drop spindles are a much better option. However, that very slight change in height due to a stagger change has big consequences. It changes a lot of things related to the front-end especially the front roll-center, which is negative on the front and highly positive on the rear. Just look at it this way, put 1.75" of front stagger in and RF caster or 10 and LF caster of 6, run the kart. Then flip the caster numbers and reset your camber and other percentages. The handling and corner speed will be different. Same for rear stagger. 1.5" will be different than 1.75". That tiny bit of increase will increase your turning power, meaning the kart will turn in a tighter arc. Do not confuse front stagger and rear stagger. They are different in that rear stagger deals with tires that are connected by a solid axle. Front stagger is dealing with heights and rates of spindle jacking.

Lets now explore the build of the driver. Jr. Restricted, a boy is say 5'3" @ 100 lbs, another boy of the same class is 5'11" @ 145 lbs. The setup and tire prep is going to be different for both of them but I see it all the time, the setup and tire prep is the same for both boys! WRONG!!!! Super Heavy is the same way. I see drivers in that class running air pressure of say 5 lbs when in fact they should be running air pressure of 8 lbs or more! Why run on sponges? I had a super heavy guy running on much higher air pressure and less tire prep and he was running stock medium times!

The lesson is, DO NOT DO what everyone else is doing, do what works for you. Think outside the box. Look at your driver compared to the others. If they are 5'5" and your driver is 6'0", why in the World would you run the same setup and tire prep? Even if your drivers are the same height but carry their weight differently, the setup will be different.

Think about it people.

Mike McCarty
 

msquared

Member
Lets talk about tires. Each tire has a spring rate and that spring rate changes with air pressure. Ever had a set of tires that at the same air pressure and diameter caused erratic handling of the kart? In other words, the kart say pushed? Been there and done that! When looking at the tires spring rate the right side tires had way different spring rates. Hence, why rim width on the LR with the same tire size and air pressure will change handling. When you have say a RR tire that is much stiffer than the RF, the kart will push. This is because it changes the way the kart dynamically transfers weight. With the stiffer RR, the kart will transfer weight more and faster to the RR leaving the RF underloaded causing the push due to lack of grip on the RF because less weight is transferred there. It is all about the balance.

Mike McCarty
 

cking

Member
This spring rate post leaves me a little confused. Wouldn't having a tire with softer spring rate on right front allow kart to roll onto right front tire easier/harder? Also I would think softer RF would transfer weight back to the RR slower/softer?
Just trying to understand this. Haven't messed with spring rates much but something interested in trying in future.
 

Cope1

Member
Different beast but in cars the heavy spring gets the weight. I am guessing that the tire with the heavy spring (sidewall stiffness) also gets the weight hence the reason for using the same sidewall stiffness to distribute weight as evenly as possible. Correct?
 

flattop1

Dawg 89
Thats my understanding the stiff tire gets the weight .
The soft tire absorbs that weight during compression .
 

Cope1

Member
I have stuck the durometer on the sidewall just to see how hard they were versus the tread. Not sure why I even did it other than curiosity. If I remember correctly with an air pressure change the durometer changed more points on the sidewall than the tread did or maybe I just dreamt that.
 

cking

Member
So would a stiffer LR do the opposite of a stiff RR? Stiffer LR would load the RF harder helping turn?
A LR tire on a 8.75" rim should be stiffer than that same LR on a 8.5" tire correct?
 

flattop1

Dawg 89
Correct the wider wheel would be stiffer .
I do not think a stiff lfr is going to affect transfer to the right front .
It would only affect that corner .
 

msquared

Member
So Cope, if that air pressure change changes the sidewall like that it made the tires spring rate stiffer. Therefore more weight transfer to that tire. The RS tires are more critical than the LS tires. Flattop is correct because weight will transfer from the LR to the RR faster and it will affect weight transfer to the RR. So lets think about all this. The rear of the kart is very, very stiff especially considering the rear axle. This means that it has a high roll stiffness. The front is much softer and has a very low roll stiffness. The waist of the kart is also pretty flexible but has become stiffer through the years. Other factors are the VCG and the CG which are located more towards the rear of the kart. In a perfect World, if a kart weighed 400 lbs, each tire should carry 100 lbs each, even when cornering. But we have cornering forces at play. Cross is used to heavily statically pre-load the RF and LR tires and leftside is used to pre-load the LS tires heavily. So, when we corner weight will be more evenly split between the RS tires and the LS tires will carry their proportionate weight based on the contact patch size maximizing overall grip. When you put a soft spring rate LR tire on, all other things being equal, less weight transfers at the rear and more transfers at the front but total weight transfer remains the same. This should give everyone something to think about. It is all about weight transfer management.

Msquared
 
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