Rear roll center

[Matt49]

My hope is that it doesn't. But if it does, I hope I'm done turning. My feeling is the roll centers further to the left are assisting in keeping the LF down all the way around the track as opposed to the older setups where it was sometimes never on the track. To lift the left front, the roll center would have to be pretty far to the right assuming you aren't getting a lot of help by squatting the RR...something else we're seeing less and less of.

[save the racers]

I tried these theorys on a car.The problem is when you change r/c it will change other things.Camber curve ,bump steer etc in the front,motion ratio it the rear.The car responded in the rear like the spring mounting points set the r/c.If you are going to offset the rear r/c be ready to put springs in it that are outside the box to normal.You want to now where the r/c is mostly because you don't want it to be in an odd location causing the car not to respond in a predicable manor.

[let-r-eat]

I agree with your comments Matt49 but why do you think the old saying of "the stiff spring gets the weight" actually happens?

Wouldn't it happen because of the reasons you just described?

[7uptruckracer]

A LF oriented Roll Center puts more weight jacking into the LF the extra RF travel we are seeing would raise the dynamic RF upper control arm angle which will tend to migrate the RC right as well which would take off some LF loading BUT at the same time the RC is dynamically lower which will help load the front end.. So by moving it farther left you are in a way accounting for that migration. I do think MATT hit on the tendency for it to travel lower which a lower roll center will definitely make the front end steer and load in a more positive manner good point. Another thing that was hit on was the Camber gain. With the extra travel we are seeing you tend to see more dynamic gain and you don't see the RF bleed back as much height as we use to on the straights the RF stays pretty pinned. We use this on asphalt front ends a lot. You don't have a tire transitioning through a camber curve as you are trying to turn it you are already at optimal camber for corner entry at the moment you turn in. Also higher camber settings like matt has discussed in other threads with Camber Thrust theories. I think we are seeing the higher camber to 1) Load the tire more and 2) Like we use on asphalt to stretch our contact patch which i think helps fight a dirt tires tendency to deflect which will roll up under the tire and distort your contact patch You are seeing more LF rebound and softer compression so you aren't rolling the front ends as much as before either that helps with Roll Center Migration. Just my 2 cents. I don't pay attention to all the rear design i just tune how it was built for the power and package I have lol You can really make that act so many different ways it ends up being a mute point just know how to tune it for where you need to be

[FlatTire]

Track width hasn't been mentioned, but it also plays a role in how the lateral roll center migration effects chassis roll.

[Matt49]

Right on, FlatTire. How far the right rear contact patch is from the moment arm has a tremendous impact on the loading of that corner of the tire.

[save the racers]

7up, if you would, how much caster are you running?What are your thoughts on caster gain?

[billetbirdcage]

A LF oriented Roll Center puts more weight jacking into the LF the extra RF travel we are seeing would raise the dynamic RF upper control arm angle which will tend to migrate the RC right as well which would take off some LF loading ***BUT at the same time the RC is dynamically lower*** which will help load the front end..

Just want to point out this isn't always true:

Here are 3 brands of cars RC both static and with dive and roll typically seen (I know this from data acquisition)

Brand A: RC hieght 1.8" and 2.5" left - During roll and dive: RCH 1.3" and 29.1" right

Brand B: RC hieght 5.8" and 1.9" left - During roll and dive: RCH 6.0" and 8.5" right

Brand C RC hieght 2.8" and 0.8" left - During roll and dive: RCH 3.5" and 13.9" right

Only one has a lower RC during roll and dive, something to think about

[billetbirdcage]

These days, most folks agree that a softer RF spring helps the car turn on entry. This is contrary to the old school thinking "heavy spring gets the weight" that was applicable when the cars ran much flatter.
Thoughts???

I'm gonna play devils advocate here, lol.

I will argue that heavy spring gets the weight is still true even in this situation so the RF has less weight and therefore less traction then with a heavy spring in theory.

So why under many situations does the car turn better? Guess that depends on where you are talking turns better? On gas or Off gas.

Lets start with off Gas, as Matt said if the old system is true softening the RF should take weight off the RF and apply some of that to the RR and LF. So if the RF has less traction how can it turn better? My argument is it doesn't turn better because of more RF traction, but a function of other things going on like:

1. Not upsetting the car and causing excessive hike down
2. Added Roll steer
3. Much smoother entry and transitions for the driver therefore less driver errors
4. Very possible that the extra travel on the RF finally gets into the progressive curve of camber gain and finally gets to the optimal camber needed for fuller contact patch

Like I said playing devils advocate here and not picking on anyone or there ideas as I find matt49, mastersbuilt_racer, and other always give good advise.

[billetbirdcage]

I just spent/wasted 2 hours reading my notes, various books and a couple old RaceWise note books on rear roll centers. After reading that and these posts by guys I respect I officaly refuse to even think about where this make believe point is from here forward. I now think it doesn't do anything but add a piece of pretty much useless poo that can't be proven to me in a definitve way to do anything positive in tire loading adjustment.JMHO

If you're looking for a basic guideline, here is my opinion so take it for what it's worth

Use the 2nd diagram I posted. It starts out more in the center of the 2 mounting points of the j-bar if the j-bar is flat. As the j-bar has rake introduced, it starts moving right on that line and the more rake the more it moves right.

That is very basic and simple and I believe that shows you what is happening in general and can allow you to plan around that simple analogy

[Matt49]

I'm gonna play devils advocate here, lol.

I will argue that heavy spring gets the weight is still true even in this situation so the RF has less weight and therefore less traction then with a heavy spring in theory.

So why under many situations does the car turn better? Guess that depends on where you are talking turns better? On gas or Off gas.

Lets start with off Gas, as Matt said if the old system is true softening the RF should take weight off the RF and apply some of that to the RR and LF. So if the RF has less traction how can it turn better? My argument is it doesn't turn better because of more RF traction, but a function of other things going on like:

1. Not upsetting the car and causing excessive hike down
2. Added Roll steer
3. Much smoother entry and transitions for the driver therefore less driver errors
4. Very possible that the extra travel on the RF finally gets into the progressive curve of camber gain and finally gets to the optimal camber needed for fuller contact patch

Like I said playing devils advocate here and not picking on anyone or there ideas as I find matt49, mastersbuilt_racer, and other always give good advise.

Don't get me wrong...I'm not saying the "heavy spring gets the weight" theory is out the window. I'm saying that other things are influencing the cars ability to turn that outweigh (no pun intended) the effect of the softer spring. You have outlined a few of those as did I and I think there is probably even some overlap. I think we're thinking the same things :-)

The same discussion can be had about LR spring stiffness as it applies to "heavy spring gets the weight" theory.

[7uptruckracer]

I'd rather not say what caster I run or gain, its a really really really big topic in asphalt right now and I think overlooked on dirt. I'll instead say that you can use caster and gain to effect your camber gain and or loss as you turn the wheel you also have to think how your jacking weight into the car and on dirt you turn both directions so be careful. The trend in asphalt which mind you are more front end dominate is to always turn left. Its common to run a 12 and 3 degree spindle or in that area So you get weight jacking onto the LF when you turn to help you cut you can also use your caster split as well as a high caster setting to help dewedge the car and help turn in but you run extremely high cross (Bite) so as you straight the wheel out to come off the corner and power down you have good drive and traction. I think if you can find a way to apply it to dirt and pair it with the right shock package you can find something there

[kaij15]

7uptruckracer, Are you measuring swing caster or true caster? That is a big topic with stuff I do, old school comparison vs what it actually is. I agree with you and how much caster is over-looked on dirt. A great example is looking at a Road Course car, and the amount of caster that is ran (typically maxx). Not only is it putting "feel" in the wheel, but maintaining load in the rear tire. Also keeping camber in the front tires throughout wheel angle changes. On a side note, this has been the most productive post I have seen on here in a long LONG time. Thank you all for the comments, and providing direction. To take some of the comments on "heavy spring getting the weight" I think the thought is still correct and can be implied, just depending on what type of track you are running. From experience, stop and go tracks, this applies 80% of the time. Momentum tracks, the rear springs are all about potential energy to provide grip. I would like to hear peoples thoughts, more indepth if you will.... also the heavy spring getting the weight....what about bi-linear RF's LF's (both stacked and BS's) and LR's.....Does this tie the theory of heavy springs, with a momentum type track? Scatter brained when I am writing this, excited to hear people's take on these.

[7uptruckracer]

As far as I know true caster, I check alot of different things and compare all the numbers to see what's built into the car, why it's built into the car and see how it fits in my package, There are a lot of things that can skew numbers so you have to look at it as a package deal.

[Dirtmod13]

Don't get me wrong...I'm not saying the "heavy spring gets the weight" theory is out the window. I'm saying that other things are influencing the cars ability to turn that outweigh (no pun intended) the effect of the softer spring. You have outlined a few of those as did I and I think there is probably even some overlap. I think we're thinking the same things :-)The same discussion can be had about LR spring stiffness as it applies to "heavy spring gets the weight" theory.

I have been wondering about this too. Seems like the softer rf spring is allowing that corner to roll over more which is compressing that spring. Wouldn't the actual rate be quite higher there because of that travel?500lb spring compressed 1" =500 lb rate350 lb spring compressed 2"= 700500 compressed 2" = 1,000400 compressed 2.5"=1,000So is it that the car is actually gaining wedge by dropping LR to get the RF to squat and roll more?

[Matt49]

I have been wondering about this too. Seems like the softer rf spring is allowing that corner to roll over more which is compressing that spring. Wouldn't the actual rate be quite higher there because of that travel?500lb spring compressed 1" =500 lb rate350 lb spring compressed 2"= 700500 compressed 2" = 1,000400 compressed 2.5"=1,000So is it that the car is actually gaining wedge by dropping LR to get the RF to squat and roll more?

Spring "rate" doesn't change due to travel unless you are coil binding.
If a 500 pound spring is compressing 1 inch then a 350 pound spring would compress 1.43 inches but the load on the wheel is the same.
I think you're mixing up cause and effect here. Weight transfer is what causes the spring to compress. The load on the spring isn't different if you change the spring rate.
If you transfer 1000 pounds to a 500 pound spring, it will compress 2 inches and the additional load is 1000 pounds.
If you transfer 1000 pounds to a 250 pound spring, it will compress 4 inches and the additional load is still 1000 pounds.

[Dirtmod13]

Spring "rate" doesn't change due to travel unless you are coil binding. If a 500 pound spring is compressing 1 inch then a 350 pound spring would compress 1.43 inches but the load on the wheel is the same.I think you're mixing up cause and effect here. Weight transfer is what causes the spring to compress. The load on the spring isn't different if you change the spring rate. If you transfer 1000 pounds to a 500 pound spring, it will compress 2 inches and the additional load is 1000 pounds. If you transfer 1000 pounds to a 250 pound spring, it will compress 4 inches and the additional load is still 1000 pounds.

I understand your point. What I'm asking is with the softer spring allowing more travel, wouldn't even more weight transfer because of the amount of roll thus making the softer spring compress even more than the initial higher rated spring. We know through your example that weight will compress a spring to a certain point. But with an even softer spring allowing additional roll, doesn't even more weight transfer due the the moment center/ thrust angles.

[MasterSbilt_Racer]

I understand your point. What I'm asking is with the softer spring allowing more travel, wouldn't even more weight transfer because of the amount of roll thus making the softer spring compress even more than the initial higher rated spring. We know through your example that weight will compress a spring to a certain point. But with an even softer spring allowing additional roll, doesn't even more weight transfer due the the moment center/ thrust angles.

If more roll gets the CG higher, then more weight can transfer. That is what it comes down to. But if there isn't significantly more total weight transfer, the softer spring just gives up weight to the other one on that side.

[Matt49]

^^What he said...
My example was a crude one based on a single spring accepting weight. In reality, you are transferring weight from one PAIR of springs to another PAIR of springs and then the "heavy spring gets the weight" (we've gotta come up with a shorter way of saying that) theory would actually mean the softer spring would compress LESS than in my examples as you go softer relative the the OTHER spring in the pair that is receiving weight transfer. But what you're saying also makes sense if you are considering other dynamic changes (e.g. lower dynamic roll centers, higher VCG, etc.).
So maybe it's a wash and the RF spring rate doesn't matter ;-) Of course I jest but there are cars out there winning races on anything from a conventional 225 to a conventional 425 RF and an endless list of combinations of bumps stops, dual-stage, etc. are also winning. It's more about the overall package than any magic at one corner of the car.

[hucktyson]

Matt , spring rate increases in a non lineal fashion before coil bind. You would be surprised how close all of those rf combinations are to each other in terms of coil load at full travel