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Mk6 fiesta 280mm front discs and 280mm rear discs conversion?


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I have seen plenty of brake conversion posts here but this might be unique. I was looking at 300mm front st170 brakes on my car but sadly the 15" wheels will more than likely not allow the conversion due to clearances. I have therefor decided to buy the st150 280mm discs for the front brakes. I am aware that the st150 rear setup is a smaller disc than the front measuring 253mm. This setup would mean the brake bias is towards the front.

My question is what would happen if I installed 280mm st150 front discs and 280mm st170 rear discs? They are the same size as each other, but would this create an unbalanced brake bias. 

If anyone can give me their opinion or if they have the technical knowhow and are able to give expert advice on if this would be an issue, please do not hesitate to give reply.🙂

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don't know for sure and can't remember off the top of my head so don't quote me but under braking, something like 60 odd% of the braking effort is loaded onto the front wheels, as the weight transfers forward. of course at this point there is less weight on the rear wheels so if the same braking effort is applied to all four wheels the rears will lock up.

designer's use a combination of smaller brakes on the back (less friction area) and a control valve to decrease the braking effort at the rear in the brake lines themselves. to end up with say a 60/40% front/rear bias

So logically if you use the same size discs and calipers at the back you will even out that bias, to say 55/45 meaning not only will your front brakes become less efficient (60% of your weight is still there with only 55% of total brake effort, they'll probably stop quicker than the old ones but not their full potential) but your rear becomes over braked potentially leading to lock up.

This is assuming of course that that same size discs/calipers and friction material is used.

But these mathematics are complicated and you will find yourself very uninsured very quickly if you start mixing and matching.

Don't know if this is specifically checked front to rear but I'd imagine the braking would be shown to be unbalanced at MOT.

So all in all  it seems logical to me that if you fit the 280mm front you should also fit the 235mm rear.

And at least using the same models front and rear even though not your specific st at least an st, so similar weight and driving characteristics

Any change of course must be made clear to your insurance company who will then see it as their duty to relieve you of another load of your hard earned cash. but believe me it is galling when they refuse to pay out because you made your car "safer"

If you have not significantly added to the power of your car. The brakes fitted at the factory are more than capable of doing the job. Indeed taking the power to weight ratio into account (and tyres) they are probably nigh on perfect.

Imagine having discs the size of cart wheels' doing 85 down the motorway, touch your brakes, instant lock up. put the same discs on a train, stand on the pedal, nothing. It's all just a game of balance.

Hope a little food for thought. enjoy.

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20 minutes ago, gornalwolf said:

don't know for sure and can't remember off the top of my head so don't quote me but under braking, something like 60 odd% of the braking effort is loaded onto the front wheels, as the weight transfers forward. of course at this point there is less weight on the rear wheels so if the same braking effort is applied to all four wheels the rears will lock up.

designer's use a combination of smaller brakes on the back (less friction area) and a control valve to decrease the braking effort at the rear in the brake lines themselves. to end up with say a 60/40% front/rear bias

So logically if you use the same size discs and calipers at the back you will even out that bias, to say 55/45 meaning not only will your front brakes become less efficient (60% of your weight is still there with only 55% of total brake effort, they'll probably stop quicker than the old ones but not their full potential) but your rear becomes over braked potentially leading to lock up.

This is assuming of course that that same size discs/calipers and friction material is used.

But these mathematics are complicated and you will find yourself very uninsured very quickly if you start mixing and matching.

Don't know if this is specifically checked front to rear but I'd imagine the braking would be shown to be unbalanced at MOT.

So all in all  it seems logical to me that if you fit the 280mm front you should also fit the 235mm rear.

And at least using the same models front and rear even though not your specific st at least an st, so similar weight and driving characteristics

Any change of course must be made clear to your insurance company who will then see it as their duty to relieve you of another load of your hard earned cash. but believe me it is galling when they refuse to pay out because you made your car "safer"

If you have not significantly added to the power of your car. The brakes fitted at the factory are more than capable of doing the job. Indeed taking the power to weight ratio into account (and tyres) they are probably nigh on perfect.

Imagine having discs the size of cart wheels' doing 85 down the motorway, touch your brakes, instant lock up. put the same discs on a train, stand on the pedal, nothing. It's all just a game of balance.

Hope a little food for thought. enjoy.

Thanks for that explanation, I had an idea it might not be such a good idea to do. I am very happy that you gave me your opinion and for the information. I will stick with the st150 front and rear standard sizes. It makes much more sense now than before.👍

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14 minutes ago, FiestaForever said:

Thanks for that explanation, I had an idea it might not be such a good idea to do. I am very happy that you gave me your opinion and for the information. I will stick with the st150 front and rear standard sizes. It makes much more sense now than before.👍

Another factor to consider is the "unsprung mass" this is a major factor to consider when changing to bigger rims or bigger brakes aswell as brake conversion as the greater unsprung mass contributes to the level of power required to get everything rolling and reduces the 0-60 speed which has an impact when pulling out of junctions and on to roundabouts; 

Unsprung mass is the term given to everything which is carried outside of the main chassis, meaning everything that isint on the springs, 

It is the rotating mass, you could balance things out by fitting some light weight alloys such as something supplied by Rota, but there lighter and more susceptible to buckling, 

Then fitting a hybrid turbo that is assuming that your engine is a diesel; well doing so would restore the power to the wheels which was once there before the bigger rims and disc brakes upgrade, 

I recommend fitting EBC green stuff pads and some cross drilled & grooved discs to the front and paint the rear drums with hammerite smooth finish silver paint, that is ofcourse after you wire brush then and wipe clean using an old sock and some petrol to provide a clean surface for best application. 

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2 hours ago, BigLen said:

Another factor to consider is the "unsprung mass" this is a major factor to consider when changing to bigger rims or bigger brakes aswell as brake conversion as the greater unsprung mass contributes to the level of power required to get everything rolling and reduces the 0-60 speed which has an impact when pulling out of junctions and on to roundabouts; 

Unsprung mass is the term given to everything which is carried outside of the main chassis, meaning everything that isint on the springs, 

It is the rotating mass, you could balance things out by fitting some light weight alloys such as something supplied by Rota, but there lighter and more susceptible to buckling, 

Then fitting a hybrid turbo that is assuming that your engine is a diesel; well doing so would restore the power to the wheels which was once there before the bigger rims and disc brakes upgrade, 

I recommend fitting EBC green stuff pads and some cross drilled & grooved discs to the front and paint the rear drums with hammerite smooth finish silver paint, that is ofcourse after you wire brush then and wipe clean using an old sock and some petrol to provide a clean surface for best application. 

Sound advice thanks biglen. Was going to throw a supercharger on at some point so i guess that may help.

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