Any suspension jocks care to take a stab?
I have a few questions in my quest to make a great handling car:
First:
Why do people bitch about the roll center moving? Whats the big deal.
Second:
I figure the biggest challenge to overcome is setting up the car with a correct combination of camber change and wheel rates so you can make the dynamic camber in the corners a good number, say you shoot for 0* or -.5* to account for tire rollover (assume you can pull constant g's in all the corners)

I'll tell you a little about my setup (front only) I'm going to have 100lb/in wheel rate on each side with no anti-roll. This gives me a natural freq of about 2.1Hz (damn stiff). Ive got about 140lb of weight transfer at 1g so that translates to 1.4in jounce and about 2.4* roll (65in track)

I made this setup:

Edit: all the data that was posted is out of date and apparently unimportant. It has been deleted.

This will give me ~ -.5 camber gain in the steady state corner at 1G not counting the little extra from caster/camber interaction.
Edit: Ok, I guess you get at least a degree from camber/caster interaction, I didn't realize it was that much. Anyway if I factor that in, do you think I'm in the right arena?

Does this seem right to you, am I missing something obvious?

Thanks in advance for the input!


Modified by buzz1167 at 11:21 PM 8/21/2006

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by buzz1167 &raquo;</TD></TR><TR><TD CLASS="quote">First:
Why do people bitch about the roll center moving? Whats the big deal.</TD></TR></TABLE>

The big deal is, the location of the roll center will determine how much body roll you have for a given weight transfer. Additionally, total grip can be effected by the height of the roll center. If its too far above the ground, you won't have much body roll, or you'll even lean into the corner if its above the center of gravity. If its too far beow the ground, you'll get much more body roll than expected for a given spring rate.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by buzz1167 &raquo;</TD></TR><TR><TD CLASS="quote">I'll tell you a little about my setup (front only) I'm going to have 100lb/in wheel rate on each side with no anti-roll. This gives me a natural freq of about 2.1Hz (damn stiff).</TD></TR></TABLE>

I'm not sure how to calculate frequency from wheel rate, but 100 lbs/in doesn't sound stiff at all. I'm aprox 196 lbs/in in the front of my Integra, and then add in stock swaybar, and its still way too soft...

Well, thats about the extent of my knowledge. I'll let those who know more take it from here.

 

Lol, the front of my car weighs ~400lbs. Entire thing is going to be about 1100 (unsprung). Anyway I had forgotten about that whole roll center thing, ****. Distance from the CG determines roll...... Now I gotta make sure it isn't stupid when it rolls over.

I'm looking for how that total grip is affected by the roll center... No-one seems to know. Is it just becuase it affects roll and that affects grip or what.

Also: Anyone ever used Anti-Sqat and Anti-Dive suspension before?

Thanks,

 

What car weighs 1100 lbs? Is this all in your imagination?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by suspendedHatch &raquo;</TD></TR><TR><TD CLASS="quote">What car weighs 1100 lbs? Is this all in your imagination?</TD></TR></TABLE>

No one said it was production... Its in my garage being built. I'm trying to find the best suspension arm lengths and pickup points.

 

Bump up the volume...

Anyone got some better ideas for best dynamic camber during cornering?

 

What exactly are you asking? I see you looking for something diff each post...

Roll center and grip
Control arm lengths and pickup points
"best" dynamic camber

When your roll center drops down, that end of the car will be less resistant to roll, grip will be increased. When it raises up, more resistant to rol, less gripl. In an ideal world, your roll center never migrates.

Without having any idea of what you're building, how can anyone suggest how to design the suspension?

I don't know what the "best" dynamic camber could be, since by definition, dynamic camber is always changing. If you're looking for a static camber setting to account for camber gain during cornering, then that will depend more on many things.

 

pick up one of those formula racing books, for like f2 f3 whatever series they have...unless you are not building your chassis like this.

 

Ok, PIC, thanks thats mostly what I was looking for.
I am being confusing because people keep asking me questions, I don't mean to be. I am trying to design the suspension for a car that I need to be able to drive around town but AutoX will be its main function. Its going to be very light but I have a lot of choices for pickup points right now because most of it isn't finished. If I wait till I'm more finished with the design (building) then maybe I figure something out and have to comprimise more than I needed to.
I Am in college, so this project needs to get done before my school takes all my time away and it just sits in the garage.

I can't think of a better way to describe what I'm looking for other than dynamic camber. What I mean is that when the car is in a turn, what should I be shooting for as the final camber angle (with the roll, caster,jounce,etc. all taken into account). Should I be hoping for 0*, or do sporty tires like Falkens with 40 aspect ratio roll over enough to justify making it -1* in a corner or am I looking at it all wrong?

Here's what I was hoping to do... (and what I wanted advice on)
Set the suspension so that when cornering at say, 1g, and in a wide corner, say 10deg wheel turn, that the tires would stand up 0*. But for 20-30* wheel turn they would be more like -1* (due to caster influence). Would you say that this would be an appropriate setup?

I'm looking into keeping the roll center stable, but I don't understand why lower makes more grip. Doesnt it just increase the moment and make the car roll more, wouldn't that just make the suspension harder to tune? Are you saying that you should lower the RC and throw on a big rollbar to compensate?

I guess this question comes from the idea that I have that the rollbar is just a bandaid to make the ride better and not roll; I always thought that stiff springs with no rollbar was the best way to go for a racecar. Although race teams do use them, I figured it was becuase of the crazy downforce and the resulting insane g forces.

Thanks for your help,

 

i let my mechanic worry about this lol

 

You cannot shoot for a "final" camber amount, since your suspension will react differently in different corners. Thats why tires work well within a range, and some prefer more camber than others. However, ANY tire will have the most grip when maximum contact patch is achieved - meaning, the best camber is always 0 basically.

As far as I know, there is no mechanism or design out there that will allow a corner to have no camber at all times - so you must sacrifice some straightline stability by running a given amoung of static camber to aid during cornering.

Lowering the roll center on one end will lower the amount of roll resisted by that end. In other words, that end is more prone to roll, and the tires at that end will be more evenly loaded - more grip.

The way I understand it, a high roll center is like a big ARB, in that it will transfer load very quickly from one side to another, thereby reducing grip. In which case, I suppose you could make up for a very low roll center with a big bar, but you're working in circles at that point - just don't make your roll center too low to begin with. IMO, if given the choice, roll centers should be placed (relative to the sprung mass CG, and to each other) to compensate for undertseer/oversteer issues you're experiencing due to limitations of other parts of the suspension design. It sounds like you have a lot of flexibility in determining arm lengths, pickup points and things like that, so this shouldn't be a problem for you (or could it be?? Its nice when someone else has done most of the work already, and you work within THEIR limitations - Thanks Honda!)

The ARB's are not band-aids and shouldn't be treated as such. They are tuning devices. You are the third person I've come across that believes the ideal suspension has no ARB's at all. Odd... springs and ARB's have somewhat different functions and cannot take each others' places in a suspension system. edit: And I'm not saying thats wrong, not at all. I believe differently. But enough people have expressed the contrary that I'm wondering where the reasoning comes from, I'm not seeing it.

 

Ok well, that sort of make sense. If the car is allowed to roll then the weight can be transferred slower through roll damping and inertia, otherwise it's very quickly changing loads. I've heard of formula teams experimenting with low and underground roll centers but no-one has proved that they have the "right", best, answer.
The deal is that I would of course like to be able to stop in a jiffy so I want the wheels the stand up pretty well in jounce. I know that there will be different radius corners and therfore more or less caster affecting the camber, I guess my problem is more of a tire choice problem now that I think of it. If i don't know where the specific tires like to run then it's sort of pointless. I'm leaning towards making mounts that are very adjustable (a lot!)

But for conversation I tell you why "I" think ARB's are bandaids. Keep in mid this only holds for cars with 4 springs/dampers independantly controlling their respective corner (just about all of them).
Lets look at the action of a roll bar... Assume a right turn for this explanation:
As we turn right, the left wheel jounces while the right wheel goes into rebound. If allowed to move unchecked (no ARB), the left wheel will end up with a loading = [1/2 weight of the car + weight-transfer load] and the right wheel with the balance (conjugate). A roll bar takes some of the rebound action of the right wheel away and transfers load to the left tire. This means that the right spring see's load but some (or all) of that load is moved to the outside tire. Now this disallowment of the inside tire to rebound allows for less roll, but it creates more force on the outside tire, therefore the load on the outside tire w/ARB = [1/2 weight of car + weight transfer + rollbar loading]. Since tires have more grip with less loading it seems evident that this would always be a last resort.

This brings on all sorts of "your stupid" retorts: Well F1 cars use them and they aren't wrong. --- Well, F1 cars take corners at 5g's... The car would plum bottom out with the clearances they need to acheive downforce if they didn't.

Lots of other factors have to be taken into account also, The ARB is a good solution to car that needs a smooth ride but needs to not roll at the same time. Otherwise, you should have probably gotten stiffer springs to stop it from rolling in my opinion. I believe in using the ARB as a tuning device as long as there is not another way to acheive the same result.

ex: a FWD understeers like crazy. Solution: Get rid of Front rollbar. Change spring rates to achieve decent camber ranges in corner. Stiffen rear rollbar to induce traction loss.
Maybe not as good a solution as redesigning the suspension for the 'racy' setup, but definitely cheaper and easier.

I look forward to critique:



Modified by buzz1167 at 11:24 PM 8/21/2006

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by buzz1167 &raquo;</TD></TR><TR><TD CLASS="quote"> Since tires have more grip with less loading it seems evident that this would always be a last resort.</TD></TR></TABLE>

Grip will increase with load up to a certain point, after which grip will begin to increase less with more load, eventually grip will be lost as load increases, which is just load sensitivity.

I think you've more or less answered your ARB questions on your own. They have their place in any suspension. Its important to keep in mind that springs and bars will never totally do the job of one another.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by PIC Performance &raquo;</TD></TR><TR><TD CLASS="quote"> Odd... springs and ARB's have somewhat different functions and cannot take each others' places in a suspension system. edit: And I'm not saying thats wrong, not at all. I believe differently. But enough people have expressed the contrary that I'm wondering where the reasoning comes from, I'm not seeing it. </TD></TR></TABLE>

i agree 100% sway bars and springs effect the car differently. But sometimes when in a pinch we just use whatever we can to get the car to do what we want it to.

An example. SS drivers often run SUPER SUPER high pressures in the rear, to help the car rotate. Now there rules say they can't change springs, shock, or swaybars. (somtimes you can add a sway bar, but the rules must say you can). Also they often run some funky toe settings.

Running that high of a rear pressure, SHOULDN'T be done if you can mess with damper adjustments, spring rates, and sway bars. Those 3 things should be your major suspension adjuster.

Then you Get into alignment. This is really were it gets weird in my book. There are lots of different things you can do with camber and toe settings to get the car to do what you want. Some people take camber out of the rear to help the car rotate, some people toe out the rear wheels to help the car rotate, and some people toe out the front to help the car 'turn in better'.

The adjustments listed above are the most common, i don't agree with one of them, but the rest i like in small moderation.


Im not sure if you ever watch NASCAR, but if you do. Durning practice or qualifing and sometimes the race. They will make small changes in air pressure to get the car to do what they want. Or they will 'add a rubber to the right rear', or they will add some or take away some rear sway bar.

They don't make these adjustment by just picking one out of the hat. The crew cheif listens to the drive, is the car 'loose on entry' or 'tight on entry' or 'tight in the middle' or 'loose in the middle' or 'tight on exit' or 'loose on exit'. Once the crew cheif gets word from the driver he knows what changes will effect the problem and makes the adjustment.

If you watch enough, you can get an idea of what effects what. Note that often times more then one change can be made, some times even they don't make the right one.