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Brakes

Nick - 15/11/18 at 05:56 PM

Help chaps
Just started gathering bits on a new build. I have Sierra diff shafts a d hubs from a drum brakes car. What do I need to get to convert to discs. ie what size discs and what calipers. Also have read that machining needs to be done. The part that I understand needs to be reduced would take it down too close to the wheel studs.
Also what size discs would you suggest for front to suit rear discs.

Ahhhh I'm going out of my mind trying to suss this out but getting nowhere fast. Don't want to spend a lot of money a d it's wrong.
Cheers

big_wasa - 15/11/18 at 07:47 PM

There are loads of combinations around. Depends on wheel size and budget.

I am sure you can get brackets to fit Sierra discs and calipers to a drum set up. This may need work for 13” wheels.

To fit the Escort or Fiesta discs onto the Sierra hubs the od of the stub axle needs turning down to fit inside the disc. The hubs are fairly hard steel maybe cromolley ? I found they laughed at high speed steel but indexable carbide soon shaved them down.

For the Escort discs they need turning down to around 164~165mm and the discs still center on the spigot so no need to set up with in a 4 jaw with a dti.

The fiesta discs are I think a better size for a 7 but they don’t center on the spigot and need much more meat taken of the stub.

I’ve got some little mx5 calipers for a diy conversion.

SPYDER - 15/11/18 at 08:17 PM

What makes you think that you need rear discs Nick?

big_wasa - 15/11/18 at 08:49 PM

I would say they are needed for two reasons,

A) bling

B) resale value

[Edited on 15/11/18 by big_wasa]

Nick - 15/11/18 at 09:21 PM

I like the look more of the disc brakes through a wheel. Only reason.
I hope to run 16 to 17” wheels.
Trying not to have to do any machining.
Confused about size of discs font to back.

JAG - 16/11/18 at 08:19 AM

Front to Rear brake sizing is driven by the cars dimensions.

1) Wheelbase; distance between the front and rear axles.

2) Height of the Centre of Gravity above the road.

3) Static weight distribution.

These dimensions govern the weight transfer during deceleration and that limits the size of the rear brake. The rear brake limit is set by the Legal Requirement to lock the front axle before the back axle - this is to manage vehicle stability during braking.

Long, low cars with a 50:50 weight distribution have similar sized brakes front and rear while short and tall vehicles, with a 50:50 weight distribution have very different sized brakes front and rear.

Our Sevens are short and very low with nearly 50:50 weight distribution.

Like this...

Calc' for IDEAL brake force.

Slater - 16/11/18 at 09:20 AM

Drum brakes are lighter and probably cheaper than disks. More that adequate for seven type cars...... in my opinion.

Who needs bling?

For more speed just add lightness.

mcerd1 - 16/11/18 at 09:23 AM

quote:
Originally posted by Nick
I like the look more of the disc brakes through a wheel. Only reason.
I hope to run 16 to 17” wheels.
Trying not to have to do any machining.
Confused about size of discs font to back.

if you were to use a sierra donor that had rear discs (like most people do) its 260mm vented on the front and 253mm solid on the rear - depending on the master cylinder sizes you pick this seems to work out not too bad for most folk

the std sierra rear disc setup uses different hubs that have the caliper mounting lugs (these have different stub axles for bolt-on driveshafts and different bearings so you can't just mix and match)
so would also need either hybrid driveshafts (push-in one end and bolt-on the other) or a diff that also has bolt-on flanges

but note that this sierra rear disc setup is a fair bit heavier than the drum braked version

other points to note are, drum brakes are better handbrakes and make it much easier to fit 13" wheels (13" wheels are generally accepted as the way to make the car light and faster, and still have a decent range of tyres available for the track

)

v8kid - 16/11/18 at 09:36 AM

quote:
Originally posted by Slater
Drum brakes are lighter and probably cheaper than disks. More that adequate for seven type cars...... in my opinion.
.

I'm amazed! Do you have the source info?

Intuitively the discs should be lighter as they are smaller volume than drums, so the backplate, shoes and slave cylinder has to be lighter than the calliper, pads and bracket.

The more I think about it the less I believe it can you educate me?

Cheers!

David

jps - 16/11/18 at 10:04 AM

quote:
Originally posted by Nick
I like the look more of the disc brakes through a wheel. Only reason.
I hope to run 16 to 17” wheels.
Trying not to have to do any machining.
Confused about size of discs font to back.

The really obvious answer to doing it with minimal fuss seems to be 'get a full Sierra disc backend' (i.e. diff, shafts,carriers). Then there's no machining at all and the only thing you need to sort is brake balance - which is not a new path to tread.

Why do you want to use the drum shafts and hubs you've got?

JAG - 16/11/18 at 10:33 AM

quote:

Drum brakes are lighter and probably cheaper than disks

Honest truth - they're very similar in weight but Drumbrakes can be a bit cheaper. Especially if you don't mind them rusting.

If you compare a car which is fitted with both, like the Toyota Yaris then you find that...

Drum system; 12.5kg

Caliper system; 11.98kg

That's per wheel and includes the brake and the rotor (drum or disc).

I do this stuff for a living

mcerd1 - 16/11/18 at 10:49 AM

quote:
Originally posted by v8kid

quote:
Originally posted by Slater
Drum brakes are lighter and probably cheaper than disks. More that adequate for seven type cars...... in my opinion.
.

I'm amazed! Do you have the source info?

Intuitively the discs should be lighter as they are smaller volume than drums, so the backplate, shoes and slave cylinder has to be lighter than the calliper, pads and bracket.

The more I think about it the less I believe it can you educate me?

Typically I can't find it

(I weighed my disc brakes years ago, and had someone elses weights for the drums)

but as I remember it the big difference is in the weight of the bolt-on driveshafts vs the push-in ones (so the whole system rather than the brakes alone)

JAG - 16/11/18 at 11:36 AM

Big picture - I have Ford Escort 8" Drumbrakes on the back axle of my Seven.

They are very capable of stopping a Seven and the only area where they suffer in comparison to Discs is in cooling.

So unless you do a lot of track driving or you race your car then you won't find any problems when using Drum brakes on the back axle of a Seven

Nick - 16/11/18 at 12:45 PM

Cheers for help and advice dudes. Given a lot to think about. Appreciate it.

SJ - 16/11/18 at 04:21 PM

The other thing you get with drums is a great handbrake - disks are usually worse.

Mine has drums and they work great. I agree disks look better though.