


Roll understeer
Do any of you guys know where on the web I might might be able to find some sort of calculator which will calcualte the amount of roll understeer that my car has based upon my 4 link mount positions? Also, what is deemed to be an acceptable level of roll understeer?
Thanks, John

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Forgot to mention this in my PM, John: The usual "rule of thumb" is to keep the lower links horizontal. This will force the instant center to be below the axle centerline and you'll always have roll understeer. You would then adjust the upper links for the amount of antisquat you desire.
http://www.racetec.cc/shope 




Triaged, I just looked at your 4link spreadsheet and I'm wondering if I'm understanding it correctly. It appears that the wheelbase is 108.5, the tire radius is 19.75, and the CG height is 34. When viewed from the side, the rear pivot for the lower link is 3 forward of the axle centerline and 19.75 up from the shop floor. The front pivot for the lower link is 47 forward of the axle centerline and 28 up from the shop floor. The rear pivot for the upper link is directly above the axle centerline and 28 up from the shop floor. The front pivot is 32 forward of the axle centerline and 32 up from the shop floor.
Now, if I am reading this correctly, it would appear that you are not using the SAE definition for antisquat. And, of course, there's no law against your doing that. But, since that is so very unusual, I would suggest that you warn your users that they should expect different results with most other spreadsheets available. It appears that you have taken the ratio of 800 to 1500, multiplied it by the wheelbase, and used the result as the horizontal distance forward to a point which will lie on the 100 percent antisquat line. The vertical distance to the point would be the center of gravity height. The second point...to define the line...would be at the rear tire patch. The SAE definition uses the rear tire patch point and another point which is at the intersection of a horizontal line through the center of gravity and a vertical line through the front tire patch. Using your geometry...as I understand it...yields an SAE antisquat value of 103 percent. http://www.racetec.cc/shope 


Thanks for the replies guys but what are you trying to tell em Triaged? The link that you posted doesnt really go anywhere that I can understand?
EDIT  sorry, wasnt looking at the correct link! Doh! Billy  I have used your IC location tool on your site many times now and have found it invaluable. Thank you. However, I can have many combinations of location points and was wondering how I can determine what each combination will give me in terms of roll understeer/ oversteer. From your post above (and your PM) I think I am correct in taking it that as long as the IC is below the axle centre line I will get roll understeer and the closer to the ground it gets the more there is? Thanks John 


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Well, beam axle roll steer involves that sort of steering, but at the rear of the car. So, to determine whether it's roll understeer or roll oversteer (or neither), all you have to do is figure out which wheel is moving forward and which wheel is moving rearward in a turn. Suppose the instant center is at the same height as the axle centerline. Assume, also, that it remains at that height as the car turns left. The right rear would go into jounce as the left rear goes into rebound, but both would move forward...relative to the chassis...by the same amount. Now, suppose the instant center is above the axle centerline. This time, as the car turns left, the right rear would move back as the left rear would move forward, resulting in roll oversteer. (Note that tire loading is the same in both cases, meaning that this is not the same as the oversteer which causes dynamic instability. Only the required steering input has changed.) Since the instant center is determined by the intersection of the link lines, a horizontal lower link will assure that the instant center will always be below the axle centerline and the car will always have roll understeer. With an IRS, wheel movement forward or back might not cause roll steer. It would depend on the geometry. http://www.racetec.cc/shope 


Many thanks Billy



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For understeer/oversteer I calculated the axle roll axis as the line between the intersection of the upper links and the intersection of the lower links. If the links are parallel they are assumed to intersect at infinity. It isn't too hard to do on graph paper. If the axle roll axis slopes down towards the front that is roll understeer. If the axle roll axis slopes up towards the front that is roll oversteer. Quite obviously if your lower links are parallel and flat you have neither roll understeer or oversteer. 


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If you don't have the book, my site repeats the Millikens' definition. Well, I might as well repeat it here. The 100% antisquat line intersects the rear tire patch and the intersection of two other lines, one a horizontal line through the center of gravity and the other a vertical line through the front tire patch. The slope of this line would be the CG height divided by the wheelbase. So, to get the percent antisquat of a given setup, you'd construct a line through the instant center and the rear tire patch and then multipy the ratio of the slope of this line to the 100% slope by 100. (Actually, the line should pass through a point below the rear tire patch at a distance equal to the tire radius times the ratio of "m" over "M," where "M" is the total weight of the car less the rear unsprung weight and "m" is the rear unsprung weight.) Quote:
When I look at your example, for instance, and see the instant center above the axle centerline, I see...during a left hand turn...the right rear moving back and the left rear moving forward, causing roll oversteer. Seems to make sense, but my "common sense" has led me wrong before. http://www.racetec.cc/shope 


The question really comes down to which CG to use. I will have to draw out and look at your m/M modification. I have RCVD. There is no mention of which CG to use (vehicle, sprung mass, other). It has been a while since I did all that work. I will have to lay out some free body diagrams and sort it all out again. I calculated the antisquat just as Milliken and Milliken (ICz/ICx)/(h/l)...now what h? I will get back to you on this.
As for the roll axis thing I think of it this way. What would happen if the lower link chassis mount and the upper link axle mount were at the same height off the ground. Now take the lowers and triangulate them all the way so that there is a single point at the chassis mount (making it an Aarm or Ylink). Now do the same thing with the uppers at the axle. There will be quite a bit of upward slope on the lower link but the axle would not steer with body roll. It would however shift the axle housing to the outboard side in a turn. If the links are not fully triangulated then you just have another Instant Center but in the top view. So the roll axis only is applicable for small angles of roll. Last edited by Triaged; 09232009 at 09:01 PM. 


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http://www.racetec.cc/shope 


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(Vehicle_CG_Height*Vehicle_MassRear_Unsprung_Mass*Tire_Rolling_Radius)/(Vehicle_MassRear_Unsprung_Mass) Quote:



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A free body diagram indicates that either the CG height of the entire car should be used...if the rear axle assembly weight is ignored...or the CG height of everything but the rear axle assembly if it is not. http://www.racetec.cc/shope 


excel follows the order of operations so it works out as [(h*Mtot)(m*r)]/(Mtotm)



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http://www.racetec.cc/shope 

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