I have not been able to find a diagram that exactly matches the way my 4 Link is installed. This is the closest one I could find and wanted to make sure I am calculating it correctly. The red lines were added by me, did I do those correctly?
Sorry, I was just stating that I like your work...looks good! And bumping your questions to the top of the list, hoping someone that knows the answers can see it and advise...cause I dunno Good luck!
Firstly the lower part of the drawing is incorrect. There is no such thing as a suspension "roll axis" . Apart from bumps etc the suspension/ axle is at a fixed height. It is the BODY that rolls on an axis not the suspension. The Roll axis is a theoretical straight line drawn between the front RC and rear RC .Once you know this ,you can picture how the body rolls [it is seldom horizontal] [think of a dinghy with the nose up leaning into a corner etc ] The Roll Centre on these drawings is "A" where the upper arms triangulate to an imaginary pivot point. The lower arms have nothing to do with the R/C [they can Arc up/down AND also Arc sideways like a parallelogram during Roll] Because the upper Arms are not a true Triangle, but a trapezoid shape instead ,the actual R/C is further forward at the point of intersection "A". This makes the R/C height dynamic, meaning it moves up/down with a cantilever effect when ride height changes [suspension movement] so the RCH will go up/down MORE than the amount of suspension travel If the upper Arms were a true Triangle [The pivot point at "A"] it doesn't have the cantilever effect but still will change with ride height. This ^^^ is why Colin Chapman of Lotus used a rear facing "True A frame" with the pivot point mounted to the diff head on his Lotus Cortina. In Chapmans situation the RCH never changed. Now it gets a tad more complicated with dynamic RCH's as above. With the RC behind the rear axle, the cantilever effect raises the RC as suspension compresses [the opposite dynamics to being in front of the rear axle] If the RC is below the CGH any rear squat from corner exit acceleration will raise the RC closer to the CGH [effectively stiffening the rear roll stiffness] This causes throttle exit oversteer. Having the RC in front of the rear stiffens the rear on corner entry [loose on corner entry] and softens the rear on corner exit [pushing on corner exit] ^^^ this is preferable with drivers that like "Thottle exit oversteer" I'll just re-hash this in simpler terms! Bodyroll is an "Overturning Moment" of the CGH rotating around the RC [the further apart these are , the more bodyroll forces there are. as long as the RC is below the CGH]
1. That Diagram is out of the Milliken and Milliken Race Car Vehicle Dynamics textbook. Obviously that doesn't mean it is right, but if it is wrong, they have been teaching it wrong for years. 2. "Throttle exit oversteer", in other words drifting???
With race car dynamics, the school of thought is constantly changing. So it doesn't matter whether you have 30 years experience or only 1 year experience, if the last 29 years have been outdated. Here is a drawing of the Lotus Cortina rear suspension I mentioned earlier. The Roll centre is at a fixed height under the diff. [circled red] So the body rotates [or rolls] around this pivot. Here is another drawing from a Lotus presentation to Ford which is more clear. This shows the rear Roll Centre Also from Lotus, I'll add this to show body roll. The wheels and axle are at a fixed height, and only the body roll [people sort of forget this] Now IF you refer back to the 1st drawing, and turned the A frame around 180. It would acheive exactly the same results except the RCH will change with dive / squat. I always refer back to Colin Chapman and Lotus because he was an innovator AND his cars were dead simple to explain the basics on. 2: Throttle Oversteer? Hmmm! I once heard a racer use the term "walking the car on the sidewalls" [referring to the tyres rutting over the bumps giving feedback through the seat/steering] In the pre-aerodynamic downforce era, the cars were set up to what was known as the "classic understeer" set up . If you drive on a geometrically perfect apex [modern F1] the car would plow the front wheels. But back in the era, they would "trail brake" on corner entry. This moved the braking distances into the "oh f*** zone. On a race car the brakes are not used to slow the car down [racers don't want to go slower] ,Brakes are used to transfer weight onto the front wheels to allow "turn in" [corner entry] A neutral set up car will be loose on corner entry with the trail braking technique. On corner exit, the weight gets transferred to the rear causing corner exit understeer. So an aggressive driver can induce "throttle oversteer" [flooring it] to get it to accelerate out of the corner. Both the front and rear will drift out together so the driver needs the car to be as straight as possible [this is the classic "late apex" racing line] Here is a geometrically perfect apex Here is the classic "late apex" racing line Understand these early GP drivers wern't "backing the cars in" like a dirt sprint car. They were very smooth with surgeon like feel in their fingertips, and the seat of their pants. The most difficult part to master is the "trail braking" technique. You feel the front wheels/brakes in your fingertips and you have to tell your brain to lift off the brakes as you turn in. All good fun
Funny thing is, I actually completely understand that and have never raced a day in my life. I live in WV, nothing but mountains and curves. I learned really quick that you could get on the gas a lot quicker and it would actually help turn the car on exit. The feeling is hard to describe but it often leaves the passenger trying to pump the brakes and grabbing for the dash....never new it had a term...late apex.
The Jalopy Journal
