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Old 08-29-2006, 06:30 PM   #1
bwco
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Default force / weight / Load when cornering?

I'm kinda curious, if any of you rally guys/guru can school me on this topic...

I wanna know what's the force or (you call it load) is there when you're sliding/drifting a 4WD Subaru around a corner? I know there's a lot of consideration that must be considered into the final answer - including the weight of vehicle, suspension tolorence, speed of vehicle...etc.

but in general, is there any force/load difference between FWD, RWD, 4WD vehicles if given all the variables the same?

I ask it here because Subaru community is the most knowledgable/ helpful amount the ones i solicit.
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Old 08-30-2006, 05:23 AM   #2
Necromancer
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Yeah, that's about right.

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Old 08-30-2006, 09:22 PM   #3
bwco
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sorry for my poorly organized sentence.

I was thinking more like, what's the quantity/unit of force ? Like 2000 lb of load ? or 400N of Force ?

I'm just thinking about F=MA(in relation to the center point of the corner). Or should I say (during hard cornering) the suspension compressed 2 inch and the springs are rated at 200 ftlb/in . Then, that means there's like 400 ft lb of load on the suspension? Or if there's some really simply formulas to consider?


So, if the weight transfer of FWD is different from RWD or 4WD, is there more weight biased towards the RWD cars then on 4WD or FWD?

Or simply say I'm stupid !
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Old 08-30-2006, 10:51 PM   #4
GaryC
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Tire loads are the only things that matter as far as traction is concerned. Weight transfer (longitudinal or lateral) during conrering can be calculated from the car's accererations(both lateral and longitudinal), suspension geometry(roll center height , track & wheelbase), and mass properties(location of the center of gravity & inertia). On top of tire forces there's also aerodynamic forces effecting tire loads, those are a bit difficult to determine.

Here's a book you want to read if you'd like to learn about the subject: http://www.amazon.ca/Race-Vehicle-Dy...UTF8&s=gateway
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Old 08-30-2006, 11:00 PM   #5
Necromancer
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Yeah, that's about right.

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I *think* I see what you're getting at ... when you apply power, where does the weight go? - FWD vs. RWD vs. 4WD in a drift condition? Dang, heck if I know. What do you need to do with this info? You trying to spec out some custom spring configuration or something?

Think about it in terms of the thrust vector. In a RWD car, the thrust is always pointed towards the front of the car, so there is only front-rear weight transfer when you're on the power.

In FWD, the thrust is vectored by the steering, so you get some side-to-side component, too.

In 4WD, you simply sum the vectors, weighted by the torque split. So if your car is 35F:65R,

rear axle torque output = total torque * 0.65
front axle torque output = total torque * 0.35

The dyno gives you the total torque.

Then you just add it all up. The rear component is always pointed towards the nose of the car, while the front is pointed where your steering is. That gives you the weight transfer vector due to the thrust of the engine. Add that to the cornering acceleration force and a bunch of other minor stuff (aero, yaw, etc.) and there you go.

This assumes locked (or at least not badly slipping) front, rear, and centre diffs -- if one corner wheel is spinning, all bets are off.
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Old 08-31-2006, 03:52 AM   #6
GaryC
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Quote:
Think about it in terms of the thrust vector. In a RWD car, the thrust is always pointed towards the front of the car, so there is only front-rear weight transfer when you're on the power.

In FWD, the thrust is vectored by the steering, so you get some side-to-side component, too.
That's only true when the car is in steady state, which never really do happen in racing conditions.

In transient, the trust points to steering angle- slip angle, relative to the chassis, for the front wheels. Note also that the steering angle of the left front wheel would be different from the right front wheels(depend on the % of (anti-)ackermann geometry built in), so are the slip angles. In either front or rear wheels (depend on the type of drivetrain and the diff. set up) the magnitude of thrust vector would be different for the left and right driving wheels- meaning a yaw couple is created. Typically on an AWD car the car de-yaws when throttle is applied.




Quote:
That gives you the weight transfer vector due to the thrust of the engine. Add that to the cornering acceleration force and a bunch of other minor stuff (aero, yaw, etc.) and there you go.
Thrust and breaking forces contributes to the majority of longitudinal weight transfer but not as much in lateral weight transfer. The majority of lateral weight transfer comes from the centrifugal body force acting on the cg of the car.
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Old 08-31-2006, 04:14 AM   #7
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OK GaryC, you win. I went down with the punch at "(anti-)ackermann geometry".

Admittedly I was giving a waaaaay simplistic view, but (if I interpreted correctly) the OP simply wants to know the difference between AWD, FWD, and RWD in terms of weight transfer. In that case the cornering loads and everything are equal between the three conditions (assuming the same yaw position, speed, tires and such), and the relevant difference is simply the net thrust angle, no?

How does the yaw force affect the weight transfer, by the way? In my half-asleep state I can only think it lies in the plane parallel to the ground, which means there should be no effect, but is that right???
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Old 08-31-2006, 04:41 AM   #8
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Quote:
OK GaryC, you win. I went down with the punch at "(anti-)ackermann geometry".
Sorry I wasn't trying to shut you up or "to win". I've enjoyed studying vehicle dynamics and thought it's appropriate that I involve in the discussion.

Quote:
OP simply wants to know the difference between AWD, FWD, and RWD in terms of weight transfer. In that case the cornering loads and everything are equal between the three conditions (assuming the same yaw position, speed, tires and such), and the relevant difference is simply the net thrust angle, no?
You are right, the majority of the forces acting on the car depend more on the mass and aerodynamic properties of the car, not the drivetrain. In fact I don't quite understand the OP's question and therefore I recommanded my favourite book. I was try to point out that it could be more invoving to get an accurate estimate, assuming the OP wants to find out the weight transfer in corners (or when drifting).

Quote:
How does the yaw force affect the weight transfer, by the way? In my half-asleep state I can only think it lies in the plane parallel to the ground, which means there should be no effect, but is that right???
You are right. The yaw rate has no effect on WT. It is easier to think of yaw as a result of unbalanced moment (due to force vectors) around the cg of the car.

I'll try to use more similies so that I don't sound like I'm looking for trouble next time
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Old 08-31-2006, 02:06 PM   #9
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It's all good bro ... this is a good discussion. Are you a chassis engineer or something??
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Old 08-31-2006, 04:52 PM   #10
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Quote:
Originally Posted by Necromancer View Post
It's all good bro ... this is a good discussion. Are you a chassis engineer or something??
Still a wanna-be I'm still in school but I've been dealing with some chassis/suspension design.

I'm excited when people talk about vectors
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Old 08-31-2006, 06:23 PM   #11
Necromancer
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Cool. I get more excited when people talk about elliptical curves than vectors, but hey, everybody has their thing, right?
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Old 08-31-2006, 08:09 PM   #12
GaryC
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Quote:
Originally Posted by Necromancer View Post
Cool. I get more excited when people talk about elliptical curves than vectors, but hey, everybody has their thing, right?
I'm sure.

On a side note, I hope the OP has got the answer he wants?
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