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Lateral weight transfer??????/

Posted by LexusFman 
Ferdinand
Ferdinand Trauttmansdorff
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Re: Lateral weight transfer??????/
July 13, 2015 08:42PM
Quote
LexusFman
I've been getting conflicting messages about weight transfer. The drivers seem to say you have to shift the weight of a race car in order to be fast, and from what I've seen of drivers at maximum attack, it holds true. But the engineers seem to think weight transfer reduces grip. [...] why don't all race cars just run super stiff swaybars to control lateral roll...
It's important to understand that weight transfer is not caused by a vehicle's lateral roll. It's totally the other way around. Lateral roll happens because weight is being transferred side to side.

Weight transfer happens every time there is any change in velocity, accelerating/decelerating or any change in direction. It's purely a function of the acceleration acting through the height of the vehicle's centre of gravity relative to the distance between where the wheels touch the ground.

The vehicle accelerates, weight transfers to the rear. Decelerates, weight transfers forward. Turn right, weight shifts left. Turn left, weight shifts right.

The only way to decrease the amount of weight transfer experienced in any particular vehicle manoeuvre is to either redesign the vehicle to lower the centre of gravity and/or widen the vehicle's stance (wheelbase/trackwidth), or simply drive less aggressively. A tall narrow vehicle is more prone to tipping than a low wide vehicle.

How (and to which wheels) that weight shift is transferred determines how the vehicle will react to that weight transfer. The more aggressive you are with your control inputs, the more suddenly the weight is transferred. Smooth inputs result in more gradual weight transfers.

Everything after that in suspension design is a question of how to keep all four wheels properly aligned and in optimum contact with the road surface. Twice the weight on a tire does not produce twice the grip. It is some significant proportion less than that. A car with all four tires supporting equal weight will have better grip and corner better than a car tipped up onto only two wheels.

A car with extremely stiff suspension will react nimbly to steering inputs on a billiard table smooth race track. But running one wheel of that car over a rock will lift other wheels in the air, thereby suddenly reducing overall grip.

The bigger the rocks, the more suspension travel is required. Ideally you want everything as soft as possible, without ever bottoming out the suspension.
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john vanlandingham
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Re: Lateral weight transfer??????/
July 13, 2015 09:01PM
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Ferdinand
Quote
LexusFman
I've been getting conflicting messages about weight transfer. The drivers seem to say you have to shift the weight of a race car in order to be fast, and from what I've seen of drivers at maximum attack, it holds true. But the engineers seem to think weight transfer reduces grip. [...] why don't all race cars just run super stiff swaybars to control lateral roll...
It's important to understand that weight transfer is not caused by a vehicle's lateral roll. It's totally the other way around. Lateral roll happens because weight is being transferred side to side.

Weight transfer happens every time there is any change in velocity, accelerating/decelerating or any change in direction. It's purely a function of the acceleration acting through the height of the vehicle's centre of gravity relative to the distance between where the wheels touch the ground.

The vehicle accelerates, weight transfers to the rear. Decelerates, weight transfers forward. Turn right, weight shifts left. Turn left, weight shifts right.

The only way to decrease the amount of weight transfer experienced in any particular vehicle manoeuvre is to either redesign the vehicle to lower the centre of gravity and/or widen the vehicle's stance (wheelbase/trackwidth), or simply drive less aggressively. A tall narrow vehicle is more prone to tipping than a low wide vehicle.

How (and to which wheels) that weight shift is transferred determines how the vehicle will react to that weight transfer. The more aggressive you are with your control inputs, the more suddenly the weight is transferred. Smooth inputs result in more gradual weight transfers.

Everything after that in suspension design is a question of how to keep all four wheels properly aligned and in optimum contact with the road surface. Twice the weight on a tire does not produce twice the grip. It is some significant proportion less than that. A car with all four tires supporting equal weight will have better grip and corner better than a car tipped up onto only two wheels.

A car with extremely stiff suspension will react nimbly to steering inputs on a billiard table smooth race track. But running one wheel of that car over a rock will lift other wheels in the air, thereby suddenly reducing overall grip.

The bigger the rocks, the more suspension travel is required. Ideally you want everything as soft as possible, without ever bottoming out the suspension.

Without getting too much into shit way over the poor kids head and not in the least useful to him, one could mention reducing --or increasing if you're stupid---the effect of wight transfer when looking at center of mass in relationship to roll center..


Nice piccies out there demonstrating clearly even to a dimwit like me just why it is so stupid to "lower' cars just by chopping springs shorter..


But what I wanted to clarify is the last line...it should read "without too frequently clanging out the suspension"

If you don't occassionally clang it out, then you are losing out on useful travel..
Longer but skinnier bump stops is an aid..


Finally if the kid is still around---bath-tub..pretend the car is a bathtub about 1/3 or so full..
Drive.
Now its visually obvious where the water--the weight ---goes...

Your job is to put the weight where you want it..



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Ferdinand
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Re: Lateral weight transfer??????/
July 13, 2015 09:41PM
Quote
john vanlandingham
Finally if the kid is still around---bath-tub..pretend the car is a bathtub about 1/3 or so full..
Drive.
Now its visually obvious where the water--the weight ---goes...

Your job is to put the weight where you want it..

If you have a whole hour to kill, this is a classic video by Jackie Stewart. Otherwise, watch just the two minutes from 42:58-45:04 for his "ball in a dish" demonstration.



?t=2575
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Pete
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Re: Lateral weight transfer??????/
July 13, 2015 10:07PM
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Ferdinand
It's important to understand that weight transfer is not caused by a vehicle's lateral roll. It's totally the other way around. Lateral roll happens because weight is being transferred side to side.

Unless you have a swing-axle car and you go over a rise or are stupid enough to brake while in a corner, causing the rear end to go up and the roll couple to change signs so the roll center is ABOVE the center of gravity, so side loads don't cause body roll per se, they just cause the suspension to top out on the outside and leave the ground on the inside (because the CG is still above ground, unlike say a BOAT where the CG is below waterline, something something metacentric height who cares water is for drinking not travel) and the upshot is the suspension does a great big WTF and you find yourself on the cover of Unsafe At Any Speed.

And that is why nobody sane uses high and mobile roll centers anymore.



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Pete
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Re: Lateral weight transfer??????/
July 13, 2015 10:19PM
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Ferdinand
If you have a whole hour to kill, this is a classic video by Jackie Stewart. Otherwise, watch just the two minutes from 42:58-45:04 for his "ball in a dish" demonstration.

A video search for water cup drift brings up this documentary:





Notice carefully the weight transfer of Iketani's watering eyes rolling to the outside of the corner.

But, you say, that is a simulation and not real life. Then OBSERVE:







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fliz
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Re: Lateral weight transfer??????/
July 15, 2015 02:58PM
How about this argument for why rally cars don't have swaybars:
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hoche
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Re: Lateral weight transfer??????/
July 15, 2015 04:02PM
They do have anti-rollbars. At least, the WRC ones do.



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fliz
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Re: Lateral weight transfer??????/
July 15, 2015 04:55PM
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hoche
They do have anti-rollbars. At least, the WRC ones do.

Yeah...didn't think about that.

Was more a fun picture to show why you don't want a "big" swaybar.

I can't imagine with that much droop on the tire in the air that the swaybar is transferring a lot of weight.
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john vanlandingham
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Re: Lateral weight transfer??????/
July 15, 2015 08:24PM
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hoche
They do have anti-rollbars. At least, the WRC ones do.

And no doubt they have a truck load to fine tune things..And no doubt they test the reults of dozens of bars and different preloads on them and log the times on sections...And burn up scores of tires per bar checking..

Just like we all do, right?

I still look at the set-up sheets--actual set up sheets for a whole bunch of different events for Ford and on every one it says "Full soft or disconnected"..



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Re: Lateral weight transfer??????/
July 15, 2015 10:11PM
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john vanlandingham
Quote
hoche
They do have anti-rollbars. At least, the WRC ones do.

And no doubt they have a truck load to fine tune things..And no doubt they test the reults of dozens of bars and different preloads on them and log the times on sections...And burn up scores of tires per bar checking..

Yep.

Quote
john vanlandingham
Just like we all do, right?

Well, what we can do is evaluate an event and make a decision based on it rather than getting dogmatic about whether rallycars should have swaybars or not. For instance, I've been known to remove mine for Gorman (and go with slightly different toe settings), but the long bomber sections of Prescott get a little squirrelly and it's a pretty smooth event so I run with it on there. I can see how a jaunt around Pico at Doo Wops might've been better without, but I rather liked having it on for Smith/Brooklyn.

Quote
john vanlandingham
I still look at the set-up sheets--actual set up sheets for a whole bunch of different events for Ford and on every one it says "Full soft or disconnected"..

Oddly enough, I was recently looking at the old Ford Escort prep manual, and it said that the stock front swaybar should be used. However, I think that was specifically for the ones that had to use the stock front suspension, where the swaybar is effectively a tension strut and is critical for locating the control arm. Can't remember the details offhand.



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john vanlandingham
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Re: Lateral weight transfer??????/
July 16, 2015 12:00AM
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hoche
Quote
john vanlandingham
Quote
hoche
They do have anti-rollbars. At least, the WRC ones do.

And no doubt they have a truck load to fine tune things..And no doubt they test the reults of dozens of bars and different preloads on them and log the times on sections...And burn up scores of tires per bar checking..

Yep.

Quote
john vanlandingham
Just like we all do, right?

Well, what we can do is evaluate an event and make a decision based on it rather than getting dogmatic about whether rallycars should have swaybars or not. For instance, I've been known to remove mine for Gorman (and go with slightly different toe settings), but the long bomber sections of Prescott get a little squirrelly and it's a pretty smooth event so I run with it on there. I can see how a jaunt around Pico at Doo Wops might've been better without, but I rather liked having it on for Smith/Brooklyn.

Quote
john vanlandingham
I still look at the set-up sheets--actual set up sheets for a whole bunch of different events for Ford and on every one it says "Full soft or disconnected"..

Oddly enough, I was recently looking at the old Ford Escort prep manual, and it said that the stock front swaybar should be used. However, I think that was specifically for the ones that had to use the stock front suspension, where the swaybar is effectively a tension strut and is critical for locating the control arm. Can't remember the details offhand.

Welll of course as always Horses for courses....

I am coing from the world of single brand "fan boi heroes" where there is such endless "debates" about one stock bar vs another as the magic bullet which will make their car suddenly "allow them titanic feats of daring-do and reveal that the are on par with Mario Andretti"..

re Escort meh. You're kinda right that for homologated classes they retained a stock bottom arm aka TCA and used the bar as the triangulating element...And same with Sierra/Xratty.. There its interesting that in Group N, where they had to retain OEM arm and knuckle---like everybody here has----on gravel they always used a 26mm bar which was the thinnest bar available 9and just happens to be what stock Xratty is...

I think its always a good thought exercise to stretch out an idea way past reasonable both ways to see what occurs and in this respect looking at the long travel modern Dubya Arsey cars they have such soft spring rates---I've inquired and got a file of Skoda rates----that it makes sense that they want some element of roll control because the cars are on like 150 lb/in springs (with a car weight of around 2730lbs and supposedly the best are going flat fast.. That;s under half the rate you'd see on a mid 90s Dubya Arsey car when travel was around 200mm/8 inches...

If you recall in the mid 90s they started making everything wider track and stiffer and by maybe 95 the cars , all of them, were getting "hard to turn in" on anything other than asphalt. I remember both Toyota and Ford at Indonesia in a panic fitting winter suspension on the Escorts and older ST165 stuff on the Toyota and pooof! the cars worked..
Kinda interesting..

Maybe the best answer is same as i say on spring rates "as little as possible" because if you err on the softer side, the driver has options to drive around it (I just did over 100miles on gravel this week-=end in a 245 volvo (wag-goon) with blown strut on driver's side, worn to limp=-ti-tude on pass side and wifey and the girls and a back full of campy gear---managed fine and averaged 40 to 50 mph and no complaints) but err on the stiff side and there's nothing you can do (except charge extra crazy hard).



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hoche
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Re: Lateral weight transfer??????/
July 16, 2015 02:42AM
I got no argument with that.

And in your earlier comment you reminded me something that's overlooked: most of us are running with used tires or are trying to get through an event with a mere single set of new ones, and that's probably a bigger detriment to general road grip than "I was leaned over sideways and hit a bump and my tirepatch got real small".

Anyway, I've got an event to put on so I'm out for a few days.



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p0inT92
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Re: Lateral weight transfer??????/
July 16, 2015 08:02AM
A german former-WRC guy Antony Warmbold, has blogpost about this very thing, about springs, and going from hard to softs over time in WRC: http://wrcbehindthestages.blogspot.dk/2012/02/springs-co.html



Edited 1 time(s). Last edit at 07/16/2015 08:04AM by p0inT92.
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