fathead

You can go either way, my TC4 likes highish rollcentres & soft springs, but my friends Cyclone likes low rollcentres & stiff springs, it's a different approach to the same question, traction.
My TC4 was too twitchy and hard to drive consistently with low R.C. & stiff springs.

GymBeam

iTrader: (4)

high and low roll center is all relative. maybe whats high for one car is low for another. high roll center may give more traction on loose surface, but only to a certain point, too much and it will actually give less traction.

same as camber, too much camber will give less traction.

SlamMan

This is what I've been struggling with. How is a low roll center with stiff springs different from a high roll center with softer springs?

fathead

Ok here are my thoughts.
What we are aiming for is a car that has the fastest laptimes/racetime.
For that we need traction and driveablility.
Taking my TC4 (on foams/asphalt) I got close to identical times using low RC/stiff springs & high RC/soft springs.

Low RC/stiff springs - ultimate laptimes were slightly faster (maybe a 10th or 2) but my overall race time was slower because the car was harder to drive consistently, it felt like it was on a knife edge all the time.

High RC/soft springs - overall race time was faster as the car was easier to drive close to the edge consistently but ultimate laptime was slightly slower, the car feels "softer" it still reacts to inputs well but is a lot more forgiving which is helpful when going through traffic or dicing for position.

Low RC/stiff springs would be better on a super smooth surface, as the hard springs don't like bumps.
High RC/soft springs better on a slightly bumpy surface as the soft springs can take the bumps.

As an example say we want x amount of traction say 5 now to get that 5 we add springs (Y) + RC(Z) so Y + Z = 5
We can do either 1 + 4 = 5 or 4 + 1 = 5 or some combination in between.

Best thing to do is try both configurations (and one in between) and see which setup you can drive well & nets you the best race time.

PeteB

iTrader: (-1)

hello people,

I have found this a difficult concept to get a hold of aswell, mostly because different sources seem to say different things, It is hard to figure out which one is right when it seems that there are direct contradictions in all the sources I have searched in so far.

quoting from the JRXS type R manual "a high roll center...will keep the car from rolling...and yeild more traction" (p25)

The Hudy setup guide says "a higher RC will give less chassis roll and a lower RC will give more chassis roll." (p32)

the Hudy setup guide says "more chassis roll equals more grip" (p32)

and "A lower roll centre will generally produce more grip" (p32)


so if a higher RC = less grip, why does the JRXS manual say "on low bite outdoor tracks with rubber tires, a high RC will yeild the best results." (p25)

On p32 of the Hudy setup book they say that "the point where line C intersects the centerline is the RC"

but on p34 the 2 diagrams clearly show the RC as being on line B and on line C respectivly.

and then:
"Lower front RC...better on high grip tracks" (p33)
"Higher front RC ... use in high grip conditions to avoid traction rolling"(p33)
"Lower rear RC...use in low traction conditions to increase traction",(p33)
"Higher rear RC... use in high grip conditions to avoid traction rolling." (p33)


I hope that I am just reading it wrong, and that someone will be able to show me the error in my thinking, or maybe Im just thinking about it waaaay too much, in the end Im just trying to get a consensus as to what the correct answer actually is...

and here are a few more questions, just for good measure...

What difference does it make when having the upper arms parallel to the lower arm compared to being on an angle

and, what difference does it make by lengthening or shortening the upper arm (keeping camber the same of course)

adamge

iTrader: (1)

Pete, roll center's relation to traction is definitely not explained by 'lower roll center --> more chassis roll --> more grip.' There is a major gap in the physics to reach that final conclusion. I haven't seen that gap bridged anywhere, and I've read like four textbooks on vehicle dynamics and a ton of internet sites and threads.

I attempted to get into it here, but it's just musings based on the physics and lacks evidence and study that would spotlight the key:
http://forum.teamxray.com/viewtopic.php?p=45190

Parallel arms put the the roll center right at the road surface, i.e. lower than we use. It's not particularly special beyond that.

Upper arm length controls camber gain as the suspension compresses. A short upper arm will force the top of the hub to stay be pulled in a lot when the suspension compresses. Pulled in = more camber.

I think about it by imaging two circles: one with an axis at the lower hinge pin and radius passing through the outer-lower hinge pin (hub pin), the other with an axis at the upper arm's inner ball pivot and radius passing through the outer-upper ball pivot. A short upper arm makes that upper circle small, so the upper part of the hub is pulled in quickly relative to the lower arm's big circle.

Welcome to the land of trying to connect the direct effects of our setup changes to the effect on the car on a race track. There are a lot of fault lines.

At the last race I was at, I was quite happy with my setup. The car wasn't doing anything stupid (my first setup priority), and was pretty easy to drive (third priority). But, a few heats in, I was seeing that in the middle of a turn the front end seemed to be heavily compressed. I raised the front roll center a touch (smallest change possible). The result was good: more steering, more corner speed (second priority), and the car looked flatter mid-corner. Cool!

-Adam

adamge

iTrader: (1)

Not roll-center-related, but I'm thinking about it after the last two posts:

I think camber gain is more important on foams than rubbers. Rubber tires are round and deform to strange shapes under load (as evidenced by those damn inner wear lines on my RP30s), while foams are very flat and stiff. In fact, being slightly out on camber just causes the tire to wear in a slight cone. With both tires, you want maximum contact patch all the time. But with foams, being off on static or dynamic camber will lead to running on a tiny section of the inside or outside of the foam either mid-corner or in a straight line.

John Malin

Google "Milliken Research Associates" get their book call racecar vehicle dynamics. It will explain everything in great detail. Hard reading but worth the effort. Second really good reference source would be "Tune to Win" by a fellow named Caroll Smith. awesome book for chassis tuners. even has a model you can make in the back of the book for seeing what changes when you make adjustments.

adamge

iTrader: (1)

John, I've read both. The Milliken one is very good. It was a library book, so I had to take pics of the really good sections (example attached). The Smith one is pretty good. Note that Smith's other book, Prepare to Win, is of no use to us.

thunderbt3

iTrader: (14)

I'm a Formula SAE Alumni and I've also read both and actually have met Doug Milliken. I would say Milliken's book is awesome but it maybe a bit too much on the engineering side and maybe tough to read to some.

Smith's books are pretty good reads, he does make alot of references to companies, so much that it seems like hes plugging for them in his books .

Skiddins

So, with rubbers on asphalt are we better getting more roll and getting the grip from the edges of the tyre, or keeping them as flat as possible?

Skiddins

afr0sch

iTrader: (95)

eh...more rubber on the road = more traction.

Island Losi

iTrader: (3)

there was a post a few weeks back with some handling guide links. one of them was a short list for the effects of different changes u make to the car. it also went on to mention roll relative to roll centres during different parts of cornering. dose anyone know this tread or a link to the websitel. think it had a printable copy link on the site and it was all on one page.

adamge

iTrader: (1)

This?

Skiddins

I've got a copy of that, I think a few of the dexcriptions are not correct.

It mentions using low roll centres on high grip tracks etc.

Skiddins