Brake Balance for Plus 2s

[gerrym]

Debating value of Proportioning Valve for rear brakes, plus determining what the ideal booster ratio should be.

Attached spreadsheet (thanks race tech for most of this) details weight transfer and then calculates front and rear braking torques for a given pedal pressure against targets for an overall g force deceleration.

Most of the input data came from the Lotus Manual. Some came from direct measurements such as the pedal ratio and diameter of the rear pistons (spare I had lying around).

For the weight of the plus 2 I've taken the workshop manual value and added 200lb for driver and 100lb for fuel.
One of the largest unknowns is the vertical COG value. I resorted to an eyeball approach and comparison against the Elise (value for this car is included). Any better values as souces much appreciated.

I consistently found that deceleration is limited by rear wheel lockup (in the dry) and so have added the Tilton control line into the spreadsheet. Its a bit hidden but I've shown the diagram as well so you can play with the internal calc.

Regards

Gerry

[patrics]

Hello Gerry

This is a good post and you have obviously put a lot of effort in to this.
Is this for a standard road car or for the track?
Is this a theoretical rear lock problem or reality? ? a standard road +2 shouldn?t lock the rears until after .92g by which time the fronts will be or about to be locked anyway.
In my opinion fitting a valve into a plus 2 will leave you worse off and under braked on the rear especially in the fully laden condition.
The Cof G would probably be lower driver only at 500 mm but your ? figures are good. The deceleration is too high at 1.1 and the 50/50 weight distribution is probably optimistic - there will be more on the rear.
Isn?t the ideal booster ratio determined by what?s available?

Regards
Steve

[mikealdren]

Steve,
Pressure reducing valves are fairly simple devices to cut the braking to the rear at higher brake line pressures to avoid premature lock up at the rear as the weight transfers to the front. They usually have a cut in pressure and a fixed % pressure reduction above this point.

If you really want to optimise the braking, you can fit a load sensing valve across the rear suspension as they do on commercial vehicles where the load variation is much greater. For a car like the +2 with such a low CoG and a reasonably long wheelbase, it really isn't necessary.

Unless things have changed significantly since I worked on such things in the 1970s, the valves are fairly crude devices designed to get vehicles through Type Approval testing. I would treat the specifications as indicative - it isn't an exact science.

Mike