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Info guides and mods

Rear Disc Brake Upgrade

Modification Details

Rear Discs Brakes

They look good, no doubt. However, they can reduce braking and be potentially dangerous.

This Is Going To Be Unpopular

Much like my page explaining why motorcycle engined smarts are shit, this page is going to create a lot of hate.
However, as before, I will back up what I'm saying with science. You know what they say...

Brake Drums Are Shit Though, Right?

Agreed, however, smarts aren't performance cars. They don't need anything better than drums.
They may be inefficient when they get hot but for the amount of weight they brake, they won't get that hot.

Discs Are Better At Braking Though!

Correct but there is such thing as "over-braking" where the brakes are too good for the car.

Centre Of Gravity & Weight Transfer

On the smart fortwo and Roadster, the weight distribution is approximately 45% front, 55% rear.

This sets the centre of gravity a bit back from centre and approximately the top of the alloy wheels.

360Kg                                                                     800Kg Total                                             440Kg

The car, let's say 800Kg, has a 45/55 weight distribution giving 360Kg at the front and 440Kg at the back.

However, this weight is dynamic as it moves as you brake and accelerate. Let's see what happens when you brake.

The centre of gravity shifts forwards. The front of the car dives down and the rear rises up.
The weight transfer shifts forwards violently on emergency braking.

640Kg                 800Kg Total                                                                                                 160Kg

This gives an 80/20 weight distribution during heavy emergency braking.

Remember, we are dealing in 2 dimensions, so under heavy braking, the rear drums are only braking 80Kg each.

The Problem With Over-Braking The Rear

The biggest issue is fish-tailing.
That's where the rear wheels lose traction and the back of the car tries to overtake the front of the car by attempting to go around it.

That is why the rear brake drums are biased to only do about 15% of the braking.
It's enough to help but not too much that'd it's cause problems.

But ESP Would Kick In During Fish-Tailing

Correct but remember that ESP braking isn't as efficient as normal braking. It releases and applies the brakes to
stop the wheels slipping that means that ESP braking is only about 50% as effective as controlled braking.

Controlled Braking - 100% braking
ESP Braking - 50% braking
Tyres skidding - 5% braking

Also, as the front brakes will be doing controlled braking, the weight transfer will stay forwards.
The rear wheels will still be skidding for 50% of the time.

ESP can't break the laws of physics. It's there to reduce the chance of you crashing.
It certainly can't reduce that chance down to zero.

The Rear Wheels Are Mainly Designed To Track, Not Brake

What I mean is that the rear wheels just follow the front wheels.
In fact, if you didn't have rear brakes, you probably wouldn't even notice.


If you fit rear disc brakes, you are much more likely to activate the ESP on the rear.
This stops the rear wheels braking properly and reduces rear wheel tracking.
So overall you can have less braking than drum brakes and more chance of spinning the car.

Drums on the rear are perfectly fine for all smarts

What If I Upgrade The Front Brakes Too?

The weight distribution will move even further forwards and quicker than before.
That'll make the rear even lighter. So no, that wouldn't help.

What Are The Options?

You could fit worse brakes on the front. Obviously not recommended.

Fit the S-Mann rear disc conversion.

Are You An S-Mann Shill?

Nope, I pay the same for S-Mann parts the same as everyone else. My integrity is not for sale.

So, Why Are S-Mann Rear Disc Brakes OK?

We have to look at the size of the discs used.

Every other rear disc brake manufacturer just fits standard front discs onto the rear.
They do this because it's cheap and easy as they are available to buy from lots of places.
The size of the disc also makes it easier to fit different makes of calipers in place.

Standard drums are 203mm

Standard front discs are 280mm.

So if you put them on the back of the car, even with the 15% brake bias to the rear,
280mm brake discs will give over 60% more braking than the drums. That's nearly a quarter of the braking.
15% / 100 * 160 = 24% minimum. Anything more than 20% and you will upset the braking balance.

The S-Mann rear brake discs are machined down to 240mm. Removing 40mm from the diameter.

As I'm sure you know, reducing the size of the disc will reduce the braking power.
It's the same as torque on a ratchet. The longer the handle, the more power you can put through it.

So, you reduce the brake torque by fitting smaller discs and pads with a smaller surface area.
The S-Mann rear disc kit increases rear braking marginally but keeps the braking percentage under the important 20% of the over all figure.

What If I Think You're Talking Out Your Arsehole?

The Dunning Kruger effect suggests that you probably don't believe me, no matter how much evidence I throw at you.
This is because you probably hold preconceived ideas that can't be changed because you are too closed minded to actually learn
and you are too stubborn to actually admit you could be wrong. Admitting you are wrong won't shrink your penis.

But I Really Want A Cheap Rear Disc Kit

If you like the look of rear discs, can't afford the S-Mann kit and don't mind the increased chance of crashing.

Take a look at the Smart-Mania Kit.

Peter is a nice guy. He offered me a kit for free so I could do a fitting video for him and this site.
However, since I knew I was going to write this page, I turned down his offer.

I met Peter at an Ace Cafe meet and I explained my thoughts on over-braking and reduced disc size.

Am I Going To Immediately Die In A Fire Ball?

No. Under normal braking you'd be fine with over sized rear disc brakes.
Only in heavy braking (when you really REALLY need them to work), they will be far less good than standard
and you are far more likely to lose the back end into a skid. 

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