Wednesday, September 25, 2013

Lotus 'Ice Mode' Explanation

The Lotus have suffered from a phenomena under braking that feels like you've lost your brakes - this is something we mentioned in our recent blog entry with our Switchblade.  I found an explanation for this issue from a noted Lotus Engineer at the UK factory.
The symptoms being described are a result of the Electronic Brake Distribution (EBD) system operating. This system is also referred to as Dynamic Rear Proportioning (DRP) and is, as the name implies an electronic system which, through the ABS control valve block restricts the line pressure to the rear brakes automatically to a pre-programmed algorithm. You can consider it as an electronically controlled proportioning valve which measures parameters like the rate of deceleration and rate of pedal application and uses this data to anticipate a rear wheel lock-up and then reduces the braking effort at the rear wheels as necessary. If the ABS system is left to do this, it can only react to a wheel as it starts to lock and therefore the car can already start to spin before the ABS can start to work. In extreme circumstances, if the driver brakes very suddenly the EBD system can lock off the pressure to the rear wheels completely; what pressure was at the rear brakes as the EBD system engaged remains there and the rear brakes are still working as a result, but further increases in pedal effort will not increase the braking at the rear of the car because the pressure to the rear brakes cannot increase. When this happens the brake pedal goes hard, as it is now pushing against the front callipers and a closed valve only, instead of against the front and rear callipers. The rear callipers are single piston and therefore quite flexible, so they are a major factor in making the brake pedal feel 'soft'. When the valve closes, the brake pedal pressure no longer flexes the rear callipers, hence the increase in pedal hardness. The front brakes are still working just as well as before the valve closed and will give more braking if the pedal effort is increased, while with the rear brakes working as hard as they can the braking is NOT affected. The problem is the driver feels like braking is reduced (even though it is not) because of the change in pedal feel. If the driver continues to push hard on the pedal, the car will continue to slow as fast as it possibly can in the circumstances. If he increases the pedal effort the front braking effort will increase and the rear effort will remain where it was. If he was to back off the pedal for a fraction of a second, the valve will reopen and the rear brakes will operate as normal again, with the pedal feel going back to normal.
In the case of releasing and re-engaging the pedal the car should not be able to slow any faster than it was with the system engaged unless either 1: the driver triggered the system in the first place by stamping on the pedal too fast or 2: the system triggered because a rear wheel was unloaded when the brakes were applied and would have locked up but is now fully loaded once again and able to sustain a greater braking torque. If the rate of deceleration does improve when the pedal is reapplied then it is telling the driver that he is over braking either in terms of the ultimate ability of the brakes (cause 1 above) or the track condition (cause 2 above) and needs to adjust his driving style to suit. If the system were not fitted or disabled and he continued to drive that way he would be in danger of spinning when applying the brakes. 
The suggestion that the system is running out of vacuum is just plain wrong. The system carries an internal reservoir of vacuum sufficient for three full brake applications. As with every servo system ever fitted to a car there is a one way valve which prevents the vacuum being lost when the car is on boost. The only way this reserve can be depleted is if the driver is maintaining boost while applying full brakes, i.e.: left foot braking very badly. In this instance I would argue that depleting the vacuum is probably a good thing as it should provide him with a warning that he is doing something awful to the car and it may reduce the speed of impact when he finally hits something as the brakes fade to nothing!! In normal use the throttle is closed when the brakes are applied, there is therefore no boost and the vacuum is automatically replenished as it is used."
So there it is, 'Ice Mode' is an inherent issue that can be addressed with some old school brake pedal pumping.  I think the pedal feel, when it goes 'hard', tends to be the most disconcerting as it happens when you are approaching the corner at high speed.  Many racers have disconnected the ABS system in an effort to reduce this issue and frankly it is a good solution for advanced drivers.  

We think that the brakes on the Lotus are still awesome even though they lack the firm feel that you get from other sports cars like a 911.  The flexible single piston rear caliper appears to be a key reason for the softer pedal - but we'll take it as we go 2-3 cars deeper into every corner than those massively powered GT3, ZO6 and M3s...

Another solution is to upgrade the front brakes to our 308BBK  or 308vBBK and move your front calipers to the rear with our FCRbracket.  We disconnect the ABS with this set-up.  One nice feature is that you can keep the stock rear caliper so you can retain your handbrake.  For about $2k you can effectively have a complete BBK kit for your Lotus.  We ran this successfully on our Art Car with great results including two track records in Lotus Cup.

1 comment:

cm said...

I'm sorry but this "Lotus Engineer" has clearly NOT experienced the actual problem while driving the car. Anyone who has experienced the problem can tell you the break pressure is indeed pulled if you manage to hit the pedal at the wrong time. The ABS system in the Elise is simply awful -worst of any of the 10 cars I have owned and run at events. Having run it in haste for many years, it is very obvious that if you hit the pedal when a front wheel has little grip you will have VERY reduced breaking even if after the reduced grip situation is gone- with the big problem being that the system will not try to increase brake force again until you release the pedal and reapply. I've never had an ABS system like this that doesn't continuously try to add back pressure after it pulls the pressure at a given wheel. That is the major flaw with the Lotus system. I love how they try to blame it on driving style though.