Brake Calipers – Treat them to the Best -They’re Worth It!
Hit the brake pedal and some 4000lbs per square inch of pressure are applied to the brake disc – enough to crush bone. It takes that much as the small four discs have to slow the momentum of over a tonne of vehicle as it hurtles forward. So much heat results that the gas layer between the brake pad and the disc can become ‘super heated’ and resists braking force. This is causes brake fade and is the reason discs are drilled grooved and brake pads have a slot down the middle – to allow the gas to escape. Brake pads flex significantly when braking pressure which reduces the pad-disc contact area. Twin calipers aim to reduce the amount of flexing by spreading the braking force over a wider surface area of the pad
Brake pads are in permanent contact with the disc so are made of low friction material. Low friction but not zero friction. If you’ve ever had to push your vehicle you know that the hardest part is that initial push to get it moving. This is because the resting resistance at the pad-brake is a lot more than zero. If you were to take out your brake pads, it would make it a lot easier to get the vehicle rolling. Once moving the friction at the brake-pad becomes negligible compared to the comparatively huge forward momentum of the vehicle which is why you’re able to tell the kind passer-by ‘thanks I’ll take it from here!’
Anything which increases the pad-disc friction will make it harder for a vehicle to move off from stationary and when driving you have to rev to overcoming that ‘rolling resistance’. The revving engines of vehicles moving off – at traffic lights, roundabouts, over speed bumps – makes a massive contribution to air pollution. Pad-disc friction affects acceleration from standstill.
It stands to reason that anything you can do to reduce rolling resistance will improve fuel economy, acceleration and emissions. Your choice of brake caliper grease and how you apply it affects rolling resistance of your vehicle – here’s why.
Lots of heat is generated when braking.- see https://www.youtube.com/watch?v=-1N5nBDLQMg. This heat will eventually cause petroleum based grease to dry out. Not necessarily a problem if the brake grease has a ‘dry lubricant’ – it still operates as a lubricant.
Copper grease became the choice for lubricating calipers and this still persists to-day, unfortunately. Copper particles do not slide easily over each other and so copper grease cannot act as a dry lubricant. It is a better anti-seize compound, the idea being that copper resists corrosion, which must be the reasoning behind why it is mistakenly used on the brake pin – not so much lubricating as stopping them seizing.
Heat reaching the caliper pin means any suitable caliper pin grease must be ‘high temperature’ and this inevitably means a synthetic grease. Silicone grease resists high temperatures, is compatible with rubber and is cheap. It is very poor metal to metal lubricant however which is why it is not used in CV joints or bearings. Most ‘high temperature’ brake greases are usually simply cheap silicone.
If your caliper pin grease does not contain an ingredient to protect the metal surfaces of the pin and the pin seat then fretting corrosion can occur which shows as pitting on the surface of the caliper pin. Fretting corrosion is a severe wear pattern which occurs when two heavily loaded metal surfaces undergo repeated micro movements – exactly what happens when you hit the brake pedal.
Your caliper pin grease needs to be not only ‘high temperature’ and hence synthetic but also have an additive package which protects the metal. These requirements already rule out 99% of brake greases on the market. Even adding dry lubricants to silicone such as PTFE will not prevent fretting corrosion.
It is unclear what the ingredients of those greases with ‘Ceramic’ in the title contain. Many state they are ‘metal free’ but the significance of this is unclear. They are cheap however which suggests they have not been specifically engineered for brake caliper pins at least – that takes an investment in time and money usually reflected in the retail price. The science of lubrication is a huge topic and specialist lubricants can be very expensive.
While there is 4000lbs of force pushing the pads against the disc there is no force pushing them off. The only force which allows the pads to sit back off the disc after braking is the weight of the pads and the caliper. However for this to happen relies on two things. Firstly the caliper pin grease must allow this recoil of the pad – if it has dried out then it will resist this passive movement. Secondly if the ends of the pads are corroded into the clips then the pads will move forward to grip the disc under the large braking force but not allow the pad to readily sit back off the disc – caliper relaxation we call it.
It goes without saying if the pads are corroded into the clips so you have to knock them out with a hammer then more force is need to actually push them onto the disc in the first place. Not good, you don’t want to be jumping on the brake pedal. At rest the caliper should be adding zero extra friction to that of the pads resting on the disc. For this two occur the pads must be able to move in the clips and the caliper pin must be able to allow the pads to passively sit back.
This provides a clue as to a key physical characteristic of your ideal caliper pin grease – it must facilitate the micro movements of the caliper in both directions, one under force the other passive. As we’ve implied many greases will allow the caliper to grip, but very few will aid the ‘relaxation’ of the caliper. ProSlip PIN was developed precisely with this in mind and was synthesised to have a very high lubricity – be extremely slippy. This is noticed at the pedal in improved ‘pedal feel’ – the pedal feels less hard. This is because with the caliper properly greased the ‘recoil’ of the caliper relaxing is communicated by the brake fluid to the pedal.Other branded brake greases are not engineered with such low friction, to make a grease with this particular characteristic isn’t easy. With ProSlip you’ve found the answer to your search. One last thing – a dry piston seal offers very significant resistance to the piston moving . Watch ‘The ProSlip Way’ video here to see our method for lubricating that seal. Hope we’ve answered some of your questions here!