Peter's Blog

Is Regenerative Braking the End for Friction Bakes?

Electric cars are coming, at least it appears so at the moment. I'm still not convinced about energy storage though. My data suggests that Li-Poly batteries will give about 150W-h/kg or 300W-h/l whereas petrol gives 12300W-h/kg and 9300W-h/l respectively. Even biodiesel gives 11700W-h/kg and 8500W-h/l. It looks like we've got a hell of a way to go to get something comparable or else we need to change peoples attitudes or requirements.

Anyway back to the subject. Being able to save the energy you were going to turn to heat has to be great but that does assume you need to brake in the first place. I try to imagine the brake pedal as a hole in your trouser pocket, every time you put your hand in some more coins fall out of the bottom and if you drive like that the hole gets bigger too. Brakes are just energy dumps and using them is no different to squirting the fuel pump on the floor, except the risk of fire is slightly higher! Now if you didn't have to use the brake as much because you think ahead and coast down then you'll be better off than regenerating but it will slow your arrival time by a couple of seconds.

I'm not a battery expert but a typical car stopping at a normal rate (0.3g) from 100km/h would dump 500KJ of energy in less than 10 seconds. At 160km/h that increases to 1.5MJ and in an emergency stop that takes just 4.5 seconds. I don't know how quickly you can soak up that energy but it seems like a nice challenge. Just look at the temperature rise rates of a brake disc.

Now most customers like to be able to lock the wheels or at least get a good ABS stop on dry tarmac, that's a deceleration of about 1g however most good cars are taking 6 second to get to 100km/h so the acceleration is more like 0.5g. Assuming motor designers are looking for similar levels of acceleration, and who wants more on a bog standard Mondeo, then there's a 100% mismatch in torque requirements between accelerating and braking, unless there's some clever motor tricks I'm missing I still believe the difference will need to be done by friction braking, the new challenge will be how thin can we make the pads and disc?

Brake Fluid

OK So I haven't got a brake page on the fluids used but I keep getting asked for it so here's a bit of information....

There are 3 basic types of fluids in use today, unless you know differently :-)

well in road going vehicles anyway as I don't know too much about aviation fluids.
The main type was traditionally called vegetable based fluid but of course it's been synthetic for years. It's generally a glycol ester based liquid.
The second type is mineral hydraulic oil. It's widely used in tractors with oil immersed brakes but was also used in Citroens using the name LHM (liquide hydraulique minéral) where it is coloured green.
The third type is a silicone based oil and is used almost solely by the US military although Harley Davidson used it for several years.

Now you may well know brake fluid as DOT3 or DOT4 etc. These are codes used in the US standard FMVSS116 but they tend to be used worldwide.

Now if you are at all concerned go and download the standard from here

This standards defines 'brake fluid' as a liquid compatible with SBR, EPR, CR and NR rubbers and a hydraulic system mineral oil as a liquid NOT compatible with SBR, EPR or NR rubbers. So that leaves type 1 & 3 as 'brake fluids' and type 2 as a 'hydraulic system mineral oil'

The standard doesn't define the base chemical only it's compatibility with certain rubber types so you can't assume that DOT 4 and DOT 5 can be mixed only that they will work with the same rubber types.

The main difference in the standard grades is the wet and dry boiling points and the viscosity.

What's wet boiling point I hear you shout!

Well, glycol ester based fluids absorb and chemically convert water over time. This is absorbed through the cap, hoses etc and after a year or so it will boil at a lower temperature, the wet boiling point.
Of course mineral oils and silicone fluids are much better behaved but if water gets into the system, run off from the windscreen or washer bottle are favourites, you'll find it sinks to the lowest point. If there’s a bend in a pipe it'll rust out and if it's the brake it'll boil at 100C, because it's still water! Leaving you with no brakes!

Now, of coarse the higher the number the better the spec, obvious eh? Well yes and no. Certainly the measured parameters are better but as an engineer you know there's no such this as design without compromise.

You may find the higher spec fluids have inferior lubricating properties and in the case of silicone inferior compressibility.
So what's this odd ball DOT 5 silicone fluid, SBBF as it's called?

Well if you want to deploy your vehicles to the Arabian Gulf at 45C and Northern Europe at -50C without stripping and rebuilding the brake system than SBBF could be for you.
If you want to change your brake fluid and drip it all over your custom paint job without damaging it, then SBBF could be for you. (Of course silicone on metal work prevents paint adhesion and is difficult to clean off)

If you don't fall into the above categories then it's probably better to avoid it.

If you can live with the spongy pedal feel at high temperatures and don't bottom the master cylinder out, are happy with it's poor lubrication properties and are prepared to strip and clean all the components, give it a try but be very careful pouring it in as it is prone to collecting tiny air bubbles.

Now this is where it gets complicated. Originally glycol based fluids couldn’t meet the low temperature viscosity requirement so DOT 5 was only SBBF but now it can so to avoid confusion it’s been called DOT 5.1. What a great numbering convention this is!!!

So you know all about brake fluid now and are going to fill your racing bike or car with DOT 5.1 because it’s the highest spec and is much less compressible than SBBF ah! there’s one more thing to consider. What if you could create a glycol brake fluid that had an excellent dry boiling point, much better than 5.1 but it didn’t have a good wet boiling point. To certify it for road use it may only be a DOT 4 fluid but providing you change the race fluid often the wet boiling point may be of no concern.

Racing fluid may only be DOT 4 but read the small print. What’s really important for you?

Brake Recalls

According to Auto Industry News BMW motorcycles are recalling 6600 bikes because of hose failures, (again) though it doesn't appear on the VOSA website

http://www.autoindustry.co.uk/news/21-05-10_3

http://www.dft.gov.uk/vosa/apps/recalls/default.asp?tx=VOSA

It's easy to speculate on the cause but I've experienced the different ways in which hoses from different manufacturers behave when bent.

Clearly the task is, get it right at the prototype and pre-production phase and don't change it to save money!

During my time at Honda we spent a disproportionate time studying Maru W (wiring and piping). Maybe there's another Japanese lesson to be learnt?