[Diy_efi] ABS retrofit, many messages replied

[Eric Byrd]

Thank you...I was not aware that variable bias valves existed.  Great
idea.

On Thu, 5 Dec 2002 00:45:43 -0800 Perry Harrington <pedward at apsoft.com>
writes:
> This message is mainly regarding Mos' message and Eric Byrd's.
> 
> A brake bias valve is designed such that *when* a wheel locks up 
> while braking,
> it's the front wheel.
> 
> The reason for this is that it promotes directional stability.  If 
> you lock up
> the front before the rear, the car will travel in the same direction 
> as
> it was when the brakes locked.  This is understeer.
> 
> If the rear brakes lock before the front, the car loses directional 
> stability
> and swaps ends.  This is very prominent on vehicles with a large 
> weight transfer
> to the front under braking, or vehicles with a large disproportion 
> in weight
> distribution.
> 
> A stock late 80's Mustang gives a greate example of the first.  The 
> rear suspension
> on those cars was very softly sprung, with a lot of braking force in 
> the front.
> If you were to chance hitting the brakes abruptly (emergency) at 
> high speed, the back
> end of the car will nearly lift off the ground.  It is neccessary 
> then to modulate
> the brakes in order to prevent the car from swapping ends do to a 
> large weight transfer.
> 
> A very good example of the second scenario is an 80's Dodge D50 or 
> Mitsubishi pickup.
> These trucks were very light and had almost no weight over the rear 
> axle.  This
> manifested itself in several manners, rear brake lockup and lack of 
> traction on steep
> surfaces.  This vehicle would always lock up the rear brakes in a 
> brisk stop.  It
> was very disconcerting as the front brakes were not great.
> 
> To address single axle ABS, the driven wheels are almost always the 
> ones with ABS.
> On FWD cars they put ABS on the front because the weight 
> distribution sucks so bad
> and only is worse under braking.  They effectively have to limit the 
> applied
> braking force in order to compensate for the small, cheap, crappy 
> tires often
> installed on such vehicles.
> 
> On RWD vechicles, ABS is almost exclusively found on the rear axle 
> of pickup trucks just
> for the reason I listed above.  Pickups are designed to carry 
> weights that are a
> significant fraction of their GVWR, thus they make the back end 
> stiffly sprung and
> light in order to cheaply carry loads.  They offset the design by 
> installing ABS on
> the rear axle only.
> 
> In inclimate weather this is no peace of mind, as the understeer 
> phenomenon is very
> present.
> 
> As to brake biasing, the vehicle's typical power is distributed 
> 70/30, front biased.
> The more bias you add to the rear, the more it is likely to lock up.
> 
> Thus if you biased the vehicle 70/30, an ideal vehicle with no 
> weight transfer would
> have both axles lock up.  If the same vehicle was biased 75/25 the 
> front would lockup
> and the rear would not.
> 
> Since there are few ideal vehicles (if at all), you must limit the 
> pressure to the
> rear brakes in order to prevent their lockup, or locking up before 
> the fronts.  In
> low friction coeffient situations you will likely lock both axles, 
> but with normal
> to high friction coefficients, the front will lock only.
> 
> VW has been installing a variable bias valve on their semi trailing 
> rear axle FWD
> vehicles for many years.  These cars have single, very large stamped 
> steel "swaybar"
> type trailing arm assembly with MacPherson struts.  There is a bias 
> valve attached
> to the body of the vehicle with the actuator rod connected to the 
> trailing arm.
> When the car experiences a large weight transfer the bias valve 
> progressively limits
> braking force to the rear axle, effectively preventing lockup.
> 
> As for the mechanics of weight transfer, here's a very simple model:
> 
> A tire has a given grip which is a function of it's friction 
> coefficient and weight
> applied to the tire.
> 
> Let's describe 2 tires:
> 
> The race tire has a coefficient of 1, meaning that 100% of the 
> weight applied to the tire
> can be transferred to grip.
> 
> The street tire has a coefficient of .85, meaning that 85% of the 
> weight applied to the
> tire can be transferred to grip.
> 
> This model only applies up to the maximum rating of the tire, let's 
> give the race tire
> a weight rating of 1500lbs and the street tire a rating of 1200lbs.
> 
> What this means is that the tire can see a vertical loading force of 
> 1500 and 1200 lbs
> respectively before overloading.
> 
> Our model car weighs 3000lbs.
> 
> If we take the ideal car that has a 50/50 weight distribution, you 
> will see that the
> race tire will sustain 1G on 2 tires and the street tires will 
> sustain .68G on 2 tires.
> 
> This may confuse you at first, but there is more to come.
> 
> First, let's explain the ideal numbers:
> 
> Car weight			3000lbs
> Weight per tire at rest:	750lbs
> Maximum load per 
> tire:		1500lbs(race)	1200lbs(street)
> Friction coefficient:		1.00(race)	0.85(street)
> Percentage of static grip:	50%(race)	73.5%(street)
> 
> When the car is loading the tires you must take into account the 
> weight rating and
> the friction coeffient as multipliers.  Your weight rating is the 
> maximum vertical
> load you can place on that tire, the friction coefficient is how 
> much grip the tire
> will generate at maximum load.
> 
> Grip is distributed in 2 axis, lateral and forward.  You can quickly 
> determine how
> much of a grip budget you have per tire by just doing the friction 
> coefficient math
> first, thus the race tire has a budget of 1500lbs and the street 
> tire has a budget
> of 1020lbs.  This is the useful grip you can expect from the tire at 
> maximum load.
> 
> Thus with the race tire you have 750lbs lateral thrust and 750lbs 
> forward thrust
> available.  With the street tire you have 510lbs of lateral thrust 
> and 510lbs of
> forward thrust.
> 
> You can trade off these values all you want, but you cannot exceed 
> your budget.
> 
> Weight transfer directly affects your budget, because weight gives 
> you grip.  This
> is where the friction coefficient comes in.
> 
> Say you are driving forward, but you only have 1700lbs on the rear 
> tires, you will
> only have 1*1700 or .85*1700 lbs of forward grip available.  This 
> means that even
> though your tires CAN generate more grip they aren't because there 
> is not enough
> weight transfer to cause them to generate maximum grip.
> 
> Take drag racing as example:  The ultimate ideal drag racer has 100% 
> weight transfer
> to the rear tires.  In our example that means 3000lbs on the rear 
> axle.  With
> 100% weight transfer the race tires in our example will generate 1G 
> of forward
> thrust. The street tires will generate .68G of forward thrust before 
> slipping, but
> 100% weight transfer will never occur because this would exceed your 
> budget.
> 
> Now you must take into account that the friction coefficients of 
> these tires are
> based on a surface with a coefficient of 1.  On drag strips they can 
> modify the
> surface to increase the coefficient and thus the coefficient of the 
> tire.  If you
> doubled the coefficient of the surface, you double the coefficient 
> of the tire.
> 
> This plays in both directions.  If you halve the surface 
> coefficient, you halve the
> tire coefficient.
> 
> This is where rain, snow, and ice come in.  This is also where 
> weight transfer comes
> in and why bias matters so much.
> 
> If your weight transfer were exactly the same for all surface 
> coefficients, your
> brake bias would be the same for all surfaces.  Unfortunately there 
> is a problem.
> Your weight transfer is determined by the friction coefficient of 
> the surface.
> 
> If you apply your brakes on a surface coefficient half of normal, 
> your weight transfer
> will be less and thus generated grip of the tires.  Your tires need 
> weight transfer
> to develop grip, otherwise they will only have the grip they do at 
> static weight.
> 
> This is why your brakes lock up when it's slippery outside.  Your 
> brakes are generating
> the same force, but the tires aren't and thus there is less weight 
> transfer and less
> grip.
> 
> The whole stumper in all this is:
> 
> Tire manufacturers as a general rule do not release friction 
> coefficient information.  Thus
> you do not know how much better a tire is.
> 
> You also cannot rely on treadwear ratings as they are not standard 
> across manufacturers, only
> within model lines.
> 
> What you can do is make some educated assumptions.  Race tires 
> usually have a FC of around 1,
> the more expensive, the higher the FC.  Street tires are similar.  
> The cheaper the tire, the
> less the FC.  There are many factors in tire construction which 
> influence the FC, so price
> is a good starting point for estimating FC.
> 
> I hope this makes sense to everyone.  You can find similar material 
> in books on race cars,
> with nice pretty graphs.
> 
> --Perry
> 
> -- 
> Perry Harrington			Data Acquisition & 
> Instrumentation, Inc	
> perry at dainst dot com					 
> http://www.dainst.com/
> 
> Those who would give up essential liberty to purchase a little 
> temporary safety
> deserve neither liberty or safety. Nor, are they likely to end up 
> with either.
>                              -- Benjamin Franklin
> 
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