Roll center location? :iono:

[DEnd]

Originally Posted by GC86

It takes most of a day, two people, some special measuring tools, and a reasonably expensive program to get the numbers from an FT86 geometry into a computer.

I can do it with string, nails, a level, a plumb bob and a tape measure. It won't be 100% perfect, but I know my accuracy is with 1/16th of an inch or 1/10th of a degree or better.

I'd love to use better tools, but I can get workable measurements without them. The biggest bitch is figuring out how accurate your measurements are.

[GC86]

Originally Posted by DEnd

The biggest bitch is figuring out how accurate your measurements are.

Thats what takes the extra person and the extra time.

[fika84]

Originally Posted by DEnd

I can do it with string, nails, a level, a plumb bob and a tape measure. It won't be 100% perfect, but I know my accuracy is with 1/16th of an inch or 1/10th of a degree or better.

I'd love to use better tools, but I can get workable measurements without them. The biggest bitch is figuring out how accurate your measurements are.

The good ole fashioned way.. Did you ever do this by chance???

[Gunman]

Morse Measurement has the K&C data, if you want to buy it:
http://www.morsemeasurements.com/pro...-car-database/

[Ubersuber]

Originally Posted by GC86

It takes most of a day, two people, some special measuring tools, and a reasonably expensive program to get the numbers from an FT86 geometry into a computer.

We checked the hand written notes, and the front king pin inclination (kpi) is pretty much 14.5 degrees, and the strut is 0.546 meters long at static ride height and zero degrees.

So that will get you started.

That is a fairly low kpi, lower than previous Subarus.

Not sure why. It seems to have to do with cost and engine width more than any geometry "trick". I have been noticing that in a few places on this car suspension.

Ground Control

PS. In that cool illustration, the strut on the left is 7 degrees, and the strut on the right is 29 degrees at static ride height.

I'm pretty sure the strut angle was selected at least in part to get the hoodline down.

As for lower arm length using struts the main limiter is torque loading from drive forces or braking forces. The longer the lower arm the more torque will try to push it out of alignment on its bushings. For front struts the main limiter on length is still space occupied by the drivetrain though. Boxer engines are widest low down which is a significant constraint, especially when you want to put the engine as low as possible.

The strut was first invented to save money. Colin Chapman proved its versatilty in the late 50's early 60's and built the very successful Elite with struts at the rear (the rear struts are sometimes called Chapman struts for this reason). If Porsche likes them for the Boxster and even uses them at the front of the 911 there is no real limitation to using struts.

On fwd cars with rear struts the lateral links are often very long to keep camber rise under control when different loads are carried, there are usually no physical constraints on rear control arm length with fwd.

Obviously the longer the lateral links are the less camber change you get with suspension travel. Most road cars use lower arms with a little downward slope at static ride height, the initial compression induces a small degree of negative camber movement which then begins to reduce as suspension compression rises further. This pattern provides good turn in and final understeer. Fit stiffer springs, and especially lower ride height, and you will begin at a more negative camber and the transition to less negative camber may not even be reached.

The rear of these cars begins at negative and goes more negative in compression. Of course, this is relative to the body. Actual tire angle at the road is another matter. All road cars need more suspension travel at the rear than at the front. Our cars might have benefitted from being built only as two seaters, for example.

Car bodies roll around their roll axis, that's what the term :"body roll" means. If the CG is lower than the roll axis then the body rolls inwards in cornering.

The roll axis is dynamic in most road cars.

There are some who don't think roll axis is particularly important. Since you can't easily change it, at least in a car not designed for competition, I tend to agree.

[mike156]

You just tried to talk down to a person that makes a living based on knowing how to make a car handle well. Ground Control has a strong following and is well respected in numerous communities. You are kind of the laughingstock around here...

You are out of your league.
And seriously, did you copy and paste that from wikipedia?

[7thgear]

I didn't even read his post... but my hand just reached for the pencil and paper...

[wparsons]

Here we go again... can the mods just get rid of this moron again?

What was the point of your whole post? Are you so bored with your life that you feel the need to try to "educate" the masses with mostly incorrect information?

Originally Posted by Ubersuber

I'm pretty sure the strut angle was selected at least in part to get the hoodline down.

If they just wanted the hood line lower, they could have run shorter struts up front.

Originally Posted by Ubersuber

On fwd cars with rear struts the lateral links are often very long to keep camber rise under control when different loads are carried, there are usually no physical constraints on rear control arm length with fwd.

Obviously the longer the lateral links are the less camber change you get with suspension travel.

That's not entirely true.

With struts, longer control arms can affect the camber curve, but they are by no means the only piece that matters. With multilink or double wishbone, it's the difference in length between upper and lower control arms that affects the camber curve.

Another benefit to longer control arms is less change in track width for the same amount of suspension travel. With short arms their angle changes more for a given amount of travel, which means the wheel moves in/out more through the suspension travel.

Originally Posted by Ubersuber

All road cars need more suspension travel at the rear than at the front.

And why is that mighty suspension guru?

[Ubersuber]

Originally Posted by mike156

You just tried to talk down to a person that makes a living based on knowing how to make a car handle well. Ground Control has a strong following and is well respected in numerous communities. You are kind of the laughingstock around here...

You are out of your league.
And seriously, did you copy and paste that from wikipedia?

I don't think so.

The question was posed as to why Subaru chose the front struts they did and Subaru says to keep the hoodline low:

http://www.newswire.ca/en/story/1368...-adds-new-trim

This is a Subaru press release.

The geometry of McPherson struts was the topic and, apparently, nobody noticed that the second diagram shows a longer lower arm as well as a different strut angle. The two variables need to be considered together to understand how the camber changes.

The longer the suspension arms the less camber change you get for a given travel. Note the length of suspension arms on the average F1 car, just for example. Road cars have other problems to solve that don't permit long suspension arms with short end travel.

For the curious among us, rear spring travel must be longer than front spring travel for most road cars because of the variable loading problem presented by adding passengers and luggage. I'm pretty sure all road cars have longer rear suspension travel than in the front. Suspension travel adds to all the challenges of suspension design, so much so that the simplest thing you can do to "improve" handling is to reduce suspension travel to the minimum required, as for karts as an extreme example which as you know have minimal suspension travel (but not zero).

[mike156]

F1 uses long arms (and "wrong" roll centers) for the same reason they select any other part on the car...aerodynamics. But they also use nearly equal length arms anyway to basically eliminate any camber curve. They also use a third spring to control pitch independently. F1 is so far off from what any of us actualy do that bringing up F1 is completely pointless. Seriously...does your car generate 3x it's own weight in aerodynamic load??? Do you have $100 million to go racing this year??? No? Then don't even worry about what F1 is doing.

If you want to bring up high level racing, GT cup, DTM, GT350/GT500, V8 super cars...they are all still 1000x more car then what we have, but they are at least slightly based on road going cars.

The reason strut angle got brought up anyway I'm pretty sure was just to point out that the diagram was WAY outside of what is actually used. Seriously, 29* strut angle? I don't think any car EVER has used that steep of a strut angle?

EarlQhan has the "right" mentality here...but force based analysis is even more difficult then kinematic analysis. Kinematic analysis is useful, it's just not the end all be all for suspension design.

[unsurety]

ANYWAYS

When lowering 1-1.5", is a roll center adjustment kit beneficial?

[Ubersuber]

Originally Posted by unsurety

ANYWAYS

When lowering 1-1.5", is a roll center adjustment kit beneficial?

No.

[Ubersuber]

Originally Posted by mike156

F1 uses long arms (and "wrong" roll centers) for the same reason they select any other part on the car...aerodynamics. But they also use nearly equal length arms anyway to basically eliminate any camber curve. They also use a third spring to control pitch independently. F1 is so far off from what any of us actualy do that bringing up F1 is completely pointless. Seriously...does your car generate 3x it's own weight in aerodynamic load??? Do you have $100 million to go racing this year??? No? Then don't even worry about what F1 is doing.

If you want to bring up high level racing, GT cup, DTM, GT350/GT500, V8 super cars...they are all still 1000x more car then what we have, but they are at least slightly based on road going cars.

The reason strut angle got brought up anyway I'm pretty sure was just to point out that the diagram was WAY outside of what is actually used. Seriously, 29* strut angle? I don't think any car EVER has used that steep of a strut angle?

EarlQhan has the "right" mentality here...but force based analysis is even more difficult then kinematic analysis. Kinematic analysis is useful, it's just not the end all be all for suspension design.

If I understand you correctly you are suggesting that neither the pinnacle of motorsport nor the kart scene can give us useful information about modifying our cars.

You guys really crack me up sometimes.

[wparsons]

Originally Posted by unsurety

ANYWAYS

When lowering 1-1.5", is a roll center adjustment kit beneficial?

Don't listen to Ubersuber, if you're in the 1.5" lower range then you could absolutely benefit from a roll center adjustment kit, especially up front. The further below horizontal the front control arms are at rest the better part of the camber curve you'll be in as the suspension compresses.

Originally Posted by Ubersuber

No.

Wrong.

Originally Posted by Ubersuber

If I understand you correctly you are suggesting that neither the pinnacle of motorsport nor the kart scene can give us useful information about modifying our cars.

You guys really crack me up sometimes.

Did you seriously just bring up vehicles with zero suspension, solid rear axles (no diff at all) and suggest that how they're set up can help with setting up these cars? Just when I thought you couldn't suggest anything dumber...

Are you going to suggest that we should configure our suspension like the Nissan Deltawing next?

As for F1, any car not dealing with the packaging constraints of a typical passenger car offers very little in relevance when looking at suspension. Touring cars and whatever the ALMS series is called now GT3 cars are a MUCH more relevant source of information than F1, prototype cars, etc.