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Old 04-05-2018, 12:19 AM   #26
Senior Member
Join Date: Jan 2011
Location: Strichen Aberdeenshire
Posts: 1,485
Subframe: Watson
CC: 1800
Make: Rover
ECU: K maps 160 Race

I've done what Andrew has written above, my set up is MGF 240mm vented disc's with Mintex 1144 pads, super minifin rear drum's with 3/4 bore slave cylinders and a brand new stepped bore yellow band master cylinder. As I said above this is what Geoff @ Watsons Rally told me to fit the 3/4 bore slave's to the rear, I have also fitted a bias valve down by my hand brake, this is wound all the way in and I can still lock the rears before the fronts.
Bias valve was from Torques on Ebay;
This is a contradiction to what Fynnbar has said he was told to do by the same company, I have a pair of other new slave cylinders somewhere so I'm going to dig these out and measure the size of them and if there smaller then I'll fit them and see how I go with the rear's locking up under heavy breaking. If they no longer lock up I'll start opening up the bias valve until they do and then close it back in a bit.

"Instant Custard; I can’t get my head around why the weight transfer to the front causes the rear drums to lock up before the front, I can see how that causes the rear end trying to switch places with the front but I don’t really understand how it would cause the actual locking of the brakes, that surely has to do with the bias and the pressure put on the rear brakes"

It is the same principal, it is weight transfer, think about it when you accelerate hard you get wheel spin in a fwd car because the weight has transferred to the rear and made the front go light, same as in a rwd car you tend to see the back end squat down and get grip because the weight has transferred to the rear when you drop the clutch.

Weight transfer
From Wikipedia, the free encyclopedia
For other uses, see Weight transfer (disambiguation).

Camaro performing a wheelie during drag racing.

A motorcyclist performing a stoppie.

A Toyota MR2 leaning to the outside of a turn.
Weight transfer and load transfer are two expressions used somewhat confusingly to describe two distinct effects:[1] the change in load borne by different wheels of even perfectly rigid vehicles during acceleration, and the change in center of mass (CoM) location relative to the wheels because of suspension compliance or cargo shifting or sloshing. In the automobile industry, weight transfer customarily refers to the change in load borne by different wheels during acceleration.[2] This is more properly referred to as load transfer,[1][3] and that is the expression used in the motorcycle industry,[4][5] while weight transfer on motorcycles, to a lesser extent on automobiles, and cargo movement on either is due to a change in the CoM location relative to the wheels. This article uses this latter pair of definitions.

1 Load transfer
2 Cause
3 Center of mass
4 Traction
5 Rollover
6 See also
7 References
8 External links
Load transfer
In wheeled vehicles, load transfer is the measurable change of load borne by different wheels during acceleration (both longitudinal and lateral).[3] This includes braking, and deceleration (which is an acceleration at a negative rate).[6] No motion of the center of mass relative to the wheels is necessary, and so load transfer may be experienced by vehicles with no suspension at all. Load transfer is a crucial concept in understanding vehicle dynamics. The same is true in bikes, though only longitudinally.[4]

The major forces that accelerate a vehicle occur at the tires' contact patches. Since these forces are not directed through the vehicle's CoM, one or more moments are generated whose forces are the tires' traction forces at pavement level, the other one (equal but opposed) is the mass inertia located at the CoM and the moment arm is the distance from pavement surface to CoM. It is these moments that cause variation in the load distributed between the tires. Often this is interpreted by the casual observer as a pitching or rolling motion of the vehicles body. A perfectly rigid vehicle without suspension that would not exhibit pitching or rolling of the body still undergoes load transfer. However, the pitching and rolling of the body of a non-rigid vehicle adds some (small) weight transfer due to the (small) CoM horizontal displacement with respect to the wheel's axis suspension vertical travel and also due to deformation of the tires i.e. contact patch displacement relative to wheel.

Lowering the CoM towards the ground is one method of reducing load transfer. As a result load transfer is reduced in both the longitudinal and lateral directions. Another method of reducing load transfer is by increasing the wheel spacings. Increasing the vehicle's wheelbase (length) reduces longitudinal load transfer while increasing the vehicle's track (width) reduces lateral load transfer. Most high performance automobiles are designed to sit as low as possible and usually have an extended wheelbase and track.

One way to calculate the effect of load transfer, keeping in mind that this article uses "load transfer" to mean the phenomenon commonly referred to as "weight transfer" in the automotive world, is with the so-called "weight transfer equation":

{\displaystyle \Delta Weight_{front}=a{\frac {h}{b}}m} {\displaystyle \Delta Weight_{front}=a{\frac {h}{b}}m} or {\displaystyle \Delta Weight_{front}={\frac {a}{g}}{\frac {h}{b}}w} {\displaystyle \Delta Weight_{front}={\frac {a}{g}}{\frac {h}{b}}w}
where {\displaystyle \Delta Weight_{front}} \Delta Weight_{{front}} is the change in load borne by the front wheels, {\displaystyle a} a is the longitudinal acceleration, {\displaystyle g} g is the acceleration of gravity, {\displaystyle h} h is the center of mass height, {\displaystyle b} b is the wheelbase, {\displaystyle m} m is the total vehicle mass, and {\displaystyle w} w is the total vehicle weight.[7][8]

Weight transfer involves the actual (relatively small) movement of the vehicle CoM relative to the wheel axes due to displacement of the chassis as the suspension complies, or of cargo or liquids within the vehicle, which results in a redistribution of the total vehicle load between the individual tires.

Center of mass
Weight transfer occurs as the vehicle's CoM shifts during automotive maneuvers. Acceleration causes the sprung mass to rotate about a geometric axis resulting in relocation of the CoM. Front-back weight transfer is proportional to the change in the longitudinal location of the CoM to the vehicle's wheelbase, and side-to-side weight transfer (summed over front and rear) is proportional to the ratio of the change in the CoM's lateral location to the vehicle's track.

Liquids, such as fuel, readily flow within their containers, causing changes in the vehicle's CoM. As fuel is consumed, not only does the position of the CoM change, but the total weight of the vehicle is also reduced.

By way of example, when a vehicle accelerates, a weight transfer toward the rear wheels can occur. An outside observer might witness this as the vehicle visibly leans to the back, or squats. Conversely, under braking, weight transfer toward the front of the car can occur. Under hard braking it might be clearly visible even from inside the vehicle as the nose dives toward the ground (most of this will be due to load transfer). Similarly, during changes in direction (lateral acceleration), weight transfer to the outside of the direction of the turn can occur.

Sorry for the long winded reply!
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