Calculating suspension geometry - Working BACKWARD from Roll Center

The Technical Manual I have lists an "anti dive" value for the rear, in addition to an antisquat value. Could that anti dive value actually be the anti lift value?

Will do as soon as I get the rear to my liking

 

the anti dive could coincide by chance with the anti lift value, but if it happens to be the same, it's pretty much by luck and not design. You have two instant centres looking at the car from x-z plane, if you can see how you have two ICs, hopefully you'll see that if theta front is the same as theta rear, and front:rear braking percentage is 50-50, it's a pure coincidence. I quoted your earlier message below because my impression from that was you found all the stuff for the front via CAD. If you repeat the same exercise for the rear, you'll get your anti lift and anti squat.

 

Is there such a thing as antidive for the rear suspension? Thats what I was getting at. Perhaps whoever wrote the handbook that this data comes from was using antidive (listed under REAR) as a synonym for antilift. As I understand it, antidive only exists for the front?

 

if the manual is from the chassis manufacturer, the chances of them getting this stuff wrong is pretty small.

i have more than a hand-ful in my office; and i don't think i've seen someone write anti-dive for the rear.

Let me check for you tomorrow. But as far as I remember (and I most def would've remembered something that obvious, as the next thing I'd need to do is to email them to clarify before I continue with my workflow), I've never seen that before.

If the manual was from a 3rd party (ie whomever you contacted as your source of research due the needs for your mods, then who knows)

 

I've taken a screenshot. The manual was actually plucked from a public website that a kind soul pointed me towards. I have a feeling like it was not supposed to be in a public place but there it was.

The first column is for the front suspension, second column for the rear.

View attachment 20978

 

Could be translation error? Could be they assume you know anti-lift happens to the rear under braking? Is negative anti-lift (pro-lift), anti-dive? Draft out possibilities and see what looks correct?

 

the anti lift is def not related to anything to the front. From the anti squat alone I am fairly certain there is no rear anti lift. The "anti" lift is negative; which means it's pro lift.

Wheelbase is somewhere in the region of 2681.37mm?

 

2700 +/- 10mm listed officially

 

I went ahead and built the rear suspension with the assumption that the "antidive" value listed for the rear was actually anti lift. Ran into another problem with the spreadsheet.

There seems to be some confusion and disagreement as to the origin plane for the height on the susp page of the isi spreadsheet. I had originally thought it was simply the "floor" and used the tyre radius at the front and rear for my wheel and spindle heights. After talking with some of the other guys im working with, we're not exactly sure what that plane is supposed to be. Some of us think its the bottom edge of the car body, some think its a line bisecting the front and rear ride heights, and some think its the origin of the 3D model of the car as built in 3dsmax.

Can anyone clarify just exactly what the origin for heights in the spreadsheet is supposed to be?

 

In the spreadsheet 0 height represents the bottom of the car. It's calculated by subtracting the ride height (input by you) from a loaded tire's radius (a couple of cells below static radius on the tire page) tire at the given axle. Technically, since you can have different ride heights at each axle, it represents the lowest part of the chassis at the given wheels vertical centerline.

In rF1 there were mods that used the wheels center as a 0 point for the .pm file. But even then I believe the sim moves the wheels according to the process in the first paragraph.

TK

 

Just to add to this. The HDV parameter under the Suspension part "CorrectedInnerSuspHeightAll" can be used to correct this. There is an explanation in Micheals blog how the values are correctly calaculated.
Anayways the spreadsheet calculates the values automatically and if done "correct" I think it should be 0 or very close.

 

Thanks for all the help folks! We're getting closer and closer but now I'm a little more confused on Antidive. I've spoken to some of the other guys Im working with on this and one of them is convinced that either the technical handbook I have has listed the antidive inversely or the ISI spreadsheet calculates it inversely. Its listed at 87% for the front, but he insists that anything over 25% is ridiculous and would cause abnormal behavior. Is there a possibility that its supposed to be 13% antidive and not 87?

 

There is always someone that is going to pipe up and emphatically state that what you are doing is somehow wrong. Perhaps they know something, perhaps they don't? Here is how I'd handle such a situation: In order to consider a change, bring me better data than what I have.

The data you have is great, some of the best I've ever run across. You'll need a second source to validate it. Because you have what you have, it might open the conversation with a team and they might be able to validate if what is printed is correct.

 

Well, of course I do not have data, and I do not have any need to convince anyone.

That was just my 2 cents to help improving the behaviour of the car. Clearly there is no need at all for any kind of help.

What I have is this:

From Herb Adam’s book ‘Chassis Engineering’: “It is possible to get some anti squat, but a value of about 25% is the practical limit”.

From Orlando Ríos's 'La suspensión en automóviles de competición' book:


The books I have used as reference usually state those values. Some of them even state that minimal antidive should be used in racercars.

 

I'm assuming the OP has more numbers than the mere screenshot he posted. Because I asked about the wheelbase by working backwards off the stuff he provided using known formulae, and the number is off by 20mm +/- from the one you gave. I mean who knows, my formula could be off, but considering the formula was from a very respected figure in vehicle dynamics, I'd probably question my number crunching ability next lol.

So now the question is, how valid is the data, and also does it even matter? The OP, at this stage, is merely building a model of a car for a game afterall, and not building a model for a team that is actually racing (my assumption).

To answer the OP's question, probability of 13% anti-dive is a lot higher than 87% anti-dive, and even more so if this is a GT car. If one has the basic concept of how anti-dive was calculated (or can visualize it via the angle of the control arms in relation to another), that number for a GT car application, should have alarms going off. (and a 390mm CGH has me to believe this is a GT car instead of a open wheel / LMP variant)

 

The Antis are calculated as follows (I believe these are also what you used):
F. A-Dive: .60*(tan 11.8)* (2700/390)* 100
R. A-Lift: .40*(tan 10.2)* (2700/390)* 100
R. A-Squat: (tan 0.5)* (2700/390)* 100
Rounded to the nearest %, the numbers check out.

Could my 2700mm also be rounded?
Certainly, but there precision my number working forward than using the percentages and working backwards. I've also found it quoted in multiple sources.

Yes he has more data. Enough to locate the Instant Centers for the Antis.
How valid is it? VERY

Having drafted out the locations myself, 87% is correct.

 

I appreciate all the help from everyone. Im glad Lestrat mentioned the possibility of the values being inverted. All of these concepts are sort of new to me, so having so many different ideas on how to approach this is good for investigating and figuring out exactly what we should be doing.

Thank you lordpantsington for checking the numbers and doing your own calculations. I had done that myself with some formulas I found online, but you seem to have a much better handle on these concepts than I do, and I absolutely DO NOT trust my own math!

 

Had a thought. What sort of information would I need to locate the exact position of the instant centers, or at least how far back they should go along the car? I noticed that I can raise the antidive by both raising the balljoints height AND moving them forward. How can I know that I dont have them too high or too far forward?

 

You would need the precise location of the origin points used as reference to locate the body side suspension pick up points. Luckily you have data to give you a reasonable estimate of where they are.

On page 7 of the Homologation document you get nice pics of the car from the underside. Note that the ride height location measurement points it is showing is not the origin for the suspension. Bring those photos into your CAD program. Use the overhang dimensions listed on page 6, as well as the wheelbase and trackwidths from page 42 of the handbook, to place them at a scale. Use the graphic on page 72 of the Homolg.doc to locate the suspension origin point (fore/aft and left/right). From there you only need to move the suspension origin up and down.

You know that the Anti-Dive angle starts at the center of the wheel at ground level and angles up at a known angle. The two inner upper points make a line, the two lower points make another line, extend them so they both meet and that point is your IC. This IC sits on the Anti-Dive angled line. Adjust the origin for the suspension up and down (which moves all of the body side pick up points with it) until the IC sits on the Anti-Dive line.

The same procedure can be done at the rear. But there is another reference you can use to better scale the rear pic. On page 9 of H.doc the origin for the engine location is specified. The transversal measure to the engine centerline is 411mm, which is also the centerline of the car. If you look at page 76 you'll see that the origin for the engine is the same as for the suspension. This means you'll only need to located the suspension origin fore/aft using the scaled pic. As you did with the front draw the Anti-Dive line from the known angle, connect the pickup points to make lines, connect lines to find IC, move everything (up/down) to place IC on the A-D line.

You know that you are in the ballpark only because of those pics showing the origin for the suspension points. If you didn't have those you'd be working with two unknowns and wouldn't be able to solve. When I did this out of curiosity for myself, I placed the origin for the front at the design ride height (55mm front, 75mm rear). It was close enough to the line that I'm confident the Anti-dive percentages are correct, even if they conflict with rule of thumb.

This is a problem you are going to run into again when locating the IC for the roll centers. In order to proceed with that, you are going to need to place the ball joints on the upright. Sure you've got those nice pics of the uprights, but almost nothing to scale them with. I poked around a bit and it looks like the spacing for the brake caliper bolts is really the only thing that is obtainable. looks like 210mm front, 130mm rear.

 

I gave that method a shot. Running into this issue:



Now I havent yet played with the upright-side points yet but If I move them to look like the image, they'll surely be too far to the rear? Im not sure what Im missing.

Im using AutoCAD to map out the reference point. Should I be using a 3D CAD program?