[Torbjorn968]

Getting racers edge A arms after all warning words on this forum. Does the longer pins just solve the Roll center issues of a low car or does it just let the bearings work on better angles?



//TL

Partial Post: Please Login or Register to read the full post.

[benspeeder]

Anybody???

Inquiring minds want to know :-)

Partial Post: Please Login or Register to read the full post.

[Rich Sandor]

All the longer pins do is prevent binding against the rims of the balljoint socket, it's simply geometry correction.

To truely solve the roll centre issues of the lowered car, I think you'd need to raise the suspension mounting points.
(which is obvisouly not practical for 99.99% of people)

Partial Post: Please Login or Register to read the full post.

[Torbjorn968]

<!--quoteo(post=70527:date=Apr 18 2009, 06:49 AM:name=Rich Sandor)-->QUOTE (Rich Sandor @ Apr 18 2009, 06:49 AM) <{POST_SNAPBACK}><!--quotec-->All the longer pins do is prevent binding against the rims of the balljoint socket, it's simply geometry correction.

To truely solve the roll centre issues of the lowered car, I think you'd need to raise the suspension mounting points.
(which is obvisouly not practical for 99.99% of people)<!--QuoteEnd--><!--QuoteEEnd-->

I discussed this with a race engineer for one of the top World Touring Car teams. His opinion was that there was not much of a roll center issue and that there was an advantage of having an upward angle of the A arms. The closer the arms are to be 90 degrees to the shocks the smaller the camber changes will be when the suspension is working.

He gave the example of a high roll center as a starting point. If the roll center would coincide with the center of gravity there would be no suspension acting in mid corner bumps on the outer wheel. For the suspension to work there needs to be roll (the outer wheel being depressed is roll in relation to the roadsurface if not necessarily in reference to the horizon). Roll implies having a rollcenter lower than the Center of Gravity. To him it was just an issue of how low is a good thing and that the surface of the ground was not a magic point.

With very stiff suspension he said that a bit more body roll due to a low roll center (even below the ground) was not much of an issue. He would rather have the low center of gravity.

For the front he quoted 610 - 620 mm above the ground for the wheel arch as a good target.

Me? I am a Libra so I will just keep the new higher ride height for a while then maybee change when it is time for a new suspension setup.

Partial Post: Please Login or Register to read the full post.

[flash]

generalities are the things that get people into trouble - there may not be so many issues with modern cars, but with this antiquated suspension design (it dates back to the super beetle and rabbit) and very soft lower chassis construction, roll center is a huge factor

i would love to have the same conversation with that engineer - i suspect the answers would be different - it is not simply roll center alone, though this is a large part of what is wrong with this car when lowered, and perhaps he was referring to other larger shortcomings in design that he felt were even more significant

having played with ride height a LOT on this car, with springs ranging from 160lbs to 400lbs up front and rates to correctly correspond in the rear, i can tell you from experience with THIS car that anything below 630mm results in LOWER cornering speeds with standard control arm orientation

lower chassis reinforcement does help with this though - far too much camber change occurs without it

sway bars larger than M030 help a lot too

not overspringing the car helps too

lower center of gravity is good - too low is bad if it screws up the roll center - been there done that - it may "feel" like it's better, but if you actually stop and measure your entry and exit speeds, and gather some real g-force data you will find that this is not the case

that being said, getting back to the original question, altering the roll center to maintain geometrically nominal orientation does help you go faster - however, as you move a point out on this effective lever, as would be the case with longer pins, you must also reinforce it, as that particular point is already known to break in race application

as stated above, the correct way of doing it is to move the inboard control arm mounting points - it's not all that hard, and eventually i will get to it (i already have a production design for it), but will possibly change the class in which you race

Partial Post: Please Login or Register to read the full post.

[Rich Sandor]

Moving your a-arm mounting holes will DEFINATELY change your race class. (assuming anyone who knows anything about it actually notices it, and choose to file a dispute) Welding new chassis or frame reinforcements or modifying factory suspension mounting points bumps you into the same race classes as full-tube frame race cars.

As far as longer balljoint pins go - we are in the process of testing a beefed up pin with a conical sleeve - daigo on rennlist has some photos of his design, which is being tested on a couple of cars. (including mine) I think this will be the definitive (and race class legal) fix for the problem.

I now have an excuse to bounce off all the curbs at the track; "no it's not cuz i suck at teh driving; i'm testing new balljoints! lolz"

Partial Post: Please Login or Register to read the full post.

[Rich Sandor]

<!--quoteo(post=73786:date=Jun 12 2009, 10:47 AM:name=Rich Sandor)-->QUOTE (Rich Sandor @ Jun 12 2009, 10:47 AM) <{POST_SNAPBACK}><!--quotec-->Moving your a-arm mounting holes will DEFINATELY change your race class. (assuming anyone who knows anything about it actually notices it, and choose to file a dispute) Welding new chassis or frame reinforcements or modifying factory suspension mounting points bumps you into the same race classes as full-tube frame race cars.

As far as longer balljoint pins go - we are in the process of testing a beefed up pin with a conical sleeve - daigo on rennlist has some photos of his design, which is being tested on a couple of cars. (including mine) I think this will be the definitive (and race class legal) fix for the problem.

I now have an excuse to bounce off all the curbs at the track; "no it's not cuz i suck at teh driving; i'm testing new balljoints! lolz"<!--QuoteEnd--><!--QuoteEEnd-->

WTF. How do I edit my posts to add stuff? I can't find the edit button anywhere. Doh.

Anyways I was going to say that there IS a rennlister (some crasy swedish or finn guy with the white 951 with red & blue vinyl) that has completely re-done the front A-arm mounting holes. Very very trick setup, albiet highly experimental.

Partial Post: Please Login or Register to read the full post.

[Torbjorn968]

<!--quoteo(post=72972:date=Jun 2 2009, 01:09 AM:name=flash)-->QUOTE (flash @ Jun 2 2009, 01:09 AM) <{POST_SNAPBACK}><!--quotec-->generalities are the things that get people into trouble - there may not be so many issues with modern cars, but with this antiquated suspension design (it dates back to the super beetle and rabbit) and very soft lower chassis construction, roll center is a huge factor

having played with ride height a LOT on this car, with springs ranging from 160lbs to 400lbs up front and rates to correctly correspond in the rear, i can tell you from experience with THIS car that anything below 630mm results in LOWER cornering speeds with standard control arm orientation<!--QuoteEnd--><!--QuoteEEnd-->

Good morning again Flash!

My car is still high :-)

The engineer in question knows the 944/968 very well owning a few and naming it his favourite track day car. He also works for a Swedish car mag and was the person behind the chasis on firefish (swedish 968 forum member) car. That car was a project car for the magazine in question. So it is not a matter of generalization but rather of a different view of things. I only get access to discussions in snippets but basically it is: keep the car as low as you can, spring it hard and run it with lots of camber.

Why is a low roll center a problem? Could it be bumpy US tracks (I hear they are) requiring "soft" suspension and thus creating excessive roll when lowering?

How does the rest of you US racers run your cars?

//T

Partial Post: Please Login or Register to read the full post.

[flash]

well, all i can say is that he would finish behind somebody who set it up right - this car does not respond well to that - that is very old school thinking, and while our suspension is antiquated, it does some very odd things when you do that - you end up scrubbing a lot of speed in corners that you could be carrying

as for why a low roll center is a problem, i would have to start at the beginning of physics to really answer that one, but it's all about keeping the center of gravity of the car below the roll center (if you can) - if the roll center get too low relative to the center of gravity, too much of the cars weight results in centrifugal force and induces more body roll

basically we have a two fold problem - the macpherson strut suspension already has a low roll center - the rear however, has a high one - this makes for a very strange roll axis - this is where the real problem lies, and where real cornering speed can be affected - in a perfect world we would lower the rear a lot and leave the front alone - we can't really do that though, which is why the car works better with the butt a touch low, and sprung soft - under load it dips in the rear, which brings the roll center artificially lower when it is needed

as for camber, reinforce the very weak suspension mounting points, and you will very quickly find that you don't need nearly as much of that - people add camber on these cars to correct for frame flex and compression more than anything else - i am now pulling much better Gs with less camber than i was with a bunch of it - this is because i now brought the inside tire back down and flattened it out and put more of the load of the car on it in a turn - it had the same effect as if i added 3-4 inches of tire width to the outside tires

it's all about lateral weight transfer

you can calculate this if you like - some formulas to get you going are:

centrifugal force = weight x (speed)2 / 14.97 x radius

lateral weight transfer = centrifugal force x CG height / track width

then, after you draw out your suspension and determine your roll center height, where F and R are the distances from the CG:

centrifugal force on front roll center = sprung weight CF x R / wheelbase

centrifugal force on rear roll center = sprung weight CF x F / wheelbase

front weight transfer due to roll center height = Front CF x front RC / front track width

rear weight transfer due to roll center height = rear CF x rear RC / rear track width

then, if you really want to get nutty, you can start calculating the effects of the unsprung weight, but that's for another day

as for how guys set up here, some do as you do - this is most often due to rules limitations - however, the ones that go fastest do it like i am trying to tell you

it has been a fun car to work the design issues out on - it has not responded to the traditional techniques, and has been a very interesting balancing act of multiple forces and lots of geometry

Partial Post: Please Login or Register to read the full post.

[Torbjorn968]

<!--quoteo(post=73907:date=Jun 14 2009, 07:11 PM:name=flash)-->QUOTE (flash @ Jun 14 2009, 07:11 PM) <{POST_SNAPBACK}><!--quotec-->as for how guys set up here, some do as you do - this is most often due to rules limitations - however, the ones that go fastest do it like i am trying to tell you<!--QuoteEnd--><!--QuoteEEnd-->

Would make a decent road car as well. Not the case now....

//T

Partial Post: Please Login or Register to read the full post.