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Wurzel

Uprated Front Brakes, Mastercylinders,

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jonah
if you increase the braking force at the front you are moving more of weight to the front when braking , this obviously means you have less weight on the back meaning you need less braking force at the rear to avoid locking the rear wheels .

Sorry, this is wrong. Don't forget that the amount of weight transfer is proportional to the TOTAL braking force from BOTH axles, not just the front!

 

Uprating the front brakes alone does not change the grip between tyres and road, so at the point that the front wheels are beginning to lock up, there will be no more weight transfer than before. So you still need just as much rear brake force as before to make the most of the rear grip that's available.

 

there is no such thing as a decrease in weight transfer with bigger brakes fitted to the front axle.

You're taking what I said out of context. What I'm talking about above is the MAXIMUM amount of braking force available, which is where the front wheels are just on the brink of locking up. Yes, for a given pedal pressure, bigger front brakes means more braking force and more weight transfer. BUT this is only true up to the point that the front wheels lock, which does not change when you uprate the brakes. So the peak amount of weight transfer for an emergency stop situation does NOT increase. With larger front brakes this point will occur at lower hydraulic pressure, so there will be less braking force at the rear (provided that the compensator valve is not limiting the pressure), therefore less total braking force!

 

So, indirectly, this decrease in TOTAL braking force means a slight reduction in weight transfer, this is simple physics!!

 

I'm putting together a spreadsheet that calculates some figures to show the effect, will post up later...

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Rob Thomson

F*ck me - someone (Jonah) actually understands how brakes work.

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DaveK

nice one jonah, will be good to see this

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jonah

Ok, the forum won't let me upload a .xls file as an attachment, I'll have to put it on my webspace when I get home. Meanwhile a couple of screenshots...

 

Friction coefficient = 1.14 (dry tarmac):

post-143-1141740780_thumb.jpg

 

Friction coefficent = 0.7 (wet tarmac?):

post-143-1141741519_thumb.jpg

 

What it shows is that in high grip conditions, the compensator valve is at its pressure limit with both types of front brakes, so the peak g force is the same (big brakes still do not give any improvement!). But when there's less grip (I've guessed at a value for the tyre friction coefficient for wet tarmac), the lack of rear brake pressure comes into effect and the peak g force is about 9% lower with the larger front brakes.

 

More later...

Edited by jonah

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jackherer

sorry if you've already said, but which compensator(s) is that data describing?

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Pugnut
Sorry, this is wrong. Don't forget that the amount of weight transfer is proportional to the TOTAL braking force from BOTH axles, not just the front!

 

Uprating the front brakes alone does not change the grip between tyres and road, so at the point that the front wheels are beginning to lock up, there will be no more weight transfer than before. So you still need just as much rear brake force as before to make the most of the rear grip that's available.

You're taking what I said out of context. What I'm talking about above is the MAXIMUM amount of braking force available, which is where the front wheels are just on the brink of locking up. Yes, for a given pedal pressure, bigger front brakes means more braking force and more weight transfer. BUT this is only true up to the point that the front wheels lock, which does not change when you uprate the brakes. So the peak amount of weight transfer for an emergency stop situation does NOT increase. With larger front brakes this point will occur at lower hydraulic pressure, so there will be less braking force at the rear (provided that the compensator valve is not limiting the pressure), therefore less total braking force!

 

So, indirectly, this decrease in TOTAL braking force means a slight reduction in weight transfer, this is simple physics!!

 

I'm putting together a spreadsheet that calculates some figures to show the effect, will post up later...

 

OK , so i get what you're saying and all that (i'll get my coat blah blah)but at the same time i'de be very wary of of having uprated rear brakes on such a light/tail happy car.

 

 

F*ck me - someone (Jonah) actually understands how brakes work.

 

a very constructive post. Thanks for that.

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Rob Thomson
OK , so i get what you're saying and all that (i'll get my coat blah blah)but at the same time i'de be very wary of of having uprated rear brakes on such a light/tail happy car.

But all you'd be doing is maintaining the standard brake bias - rather than moving it to the front which is what you do if you only uprate the fronts. Uprating the rears without doing anything to the fronts would be very daft, but that's not the scenario we're talking about here.

 

a very constructive post. Thanks for that.

No problem.

Edited by Rob Thomson

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Anthony

I'm sure this will become apparent when you upload the spreadsheet, but with larger front calipers/disks (eg GTi-6) would removing the rear compensator and thus allowing more pressure to the rear brakes help matters?

 

I'm assuming it will in the dry as you're exceeding the pressure rating of the compensator and (if I've understood what you're saying correctly) it's the lack of extra rear braking that's holding back the bigger front brakes? But in the wet, you're locking up before reaching the compensator limit and thus removal will have no effect.

 

On a heavier car, does the increased weight mean increased weight transfer and thus the front brakes doing a higher proportion of the work? Just wondering, as obviously stuff like 306 GTi's have the same size rear brakes with the same calipers as 205 GTi's, and one assumes Peugeot wouldn't have made a complete mis-match with the front and rear brakes...

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Rob Thomson
I'm sure this will become apparent when you upload the spreadsheet, but with larger front calipers/disks (eg GTi-6) would removing the rear compensator and thus allowing more pressure to the rear brakes help matters?

Yeah - absolutely. That's the easist way to upgrade the rear brakes. I'd like some means of regulating the pressure to the rears, though - an adjustable bias valve is preferable to no pressure reduction at all.

 

I'm assuming it will in the dry as you're exceeding the pressure rating of the compensator and (if I've understood what you're saying correctly) it's the lack of extra rear braking that's holding back the bigger front brakes? But in the wet, you're locking up before reaching the compensator limit and thus removal will have no effect.

This really depends how the standard compensators work - whether they're pressure limiting or proportioning valves. I don't know.

 

On a heavier car, does the increased weight mean increased weight transfer and thus the front brakes doing a higher proportion of the work? Just wondering, as obviously stuff like 306 GTi's have the same size rear brakes with the same calipers as 205 GTi's, and one assumes Peugeot wouldn't have made a complete mis-match with the front and rear brakes...

Although the calipers may be the same, I'll bet anything that the compensators are different. Making bespoke compensators is much less expensive than making calipers with different sized pistons for each model.

Edited by Rob Thomson

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Anthony
Yeah - absolutely. That's the easist way to upgrade the rear brakes. I'd like some means of regulating the pressure to the rears, though - an adjustable bias valve is preferable to no pressure reduction at all.

Thought as much - I run GTi-6 fronts, standard rears, and no compensators, and I've always found that setup to be very good. Was just starting to wonder if that was some kind of halo effect in light of the above...

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jonah

Here's the spreadsheet...

 

http://pages.zoom.co.uk/jonahn/205brakeupgrades.xls

 

The yellow boxes are the variables, you can adjust the strength of the front and rear brakes in cells B7, B8, C7 and C8. The units of pressure are arbitrary so the numbers themselves don't mean much, but what's important is the ratios between them. The values for standard brakes have a front to rear ratio of 2.56:1, this is the ratio of the caliper piston areas on the 1.9GTI. For 307HDi front brakes, the figure for the front is scaled up by 45% (as calculated above).

 

Change the value in B6 to see the effect of different surface conditions, 1.1 - 1.2 is about right for road tyres on dry tarmac, wet tarmac will be lower (0.5 - 0.8???)

 

The numbers in green are the calculated values, higher is better! (they can never be higher than the friction coefficient)

 

Where the other numbers come from... I have a copy of an Autocar road test of the 205GTI (1.6 115bhp) which gives various dimensions and weights:

Mass (weight) 1023kg as tested by Autocar (including, I assume, 2 people on board as the kerb weight was 883kg)

Wheelbase: 2420mm

Centre of Gravity horizontal position: calculated from the weight distribution (61.6:38.4 gives 930mm, adjusted rearwards slightly to account for driver and passenger in seated position)

CoG vertical position: Estimated (450mm is roughly level with the bumpers).

Friction coefficient: 1.14 gives a maximum deceleration of 1.1g (as measured by Autocar) for dry tarmac.

Compensator pressure limit: found by trial and error, 46 (using these arbitrary pressure units) is just low enough to prevent the rears locking before the front over a wide range of friction coefficients. Adjust to see the effect of using an adjustable bias valve (assuming it is a pressure-limiting type), or set to a very high value to simulate removing the compensator.

Change of friction when wheels locked: this is to model the reduction in grip with the brakes locked up, 0.9 is a guess (means that only 90% of the full grip is available once the tyre's grip limit has been exceeded).

 

The columns of the large tables should be self-explanatory - the figures in blue show the Front:Rear weight distribution as the brake force increases.

 

Hope this is useful - might try and expand it to make the hydraulic pressures real units, experiment with different types of bias compensators, etc...

 

Have fun! :unsure:

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Rob Thomson

That all makes sense.

 

Not that it really matters, but CoF won't be 1.14 on dry tarmac - remember that braking from high speed is assisted in no small way by aerodynamics. If the tyres/brakes contribute 0.9G you're doing well.

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jonah

Good point, hadn't thought about aerodynamic drag... although the 1.1g measured by Autocar was from only 30mph so it can't have been a huge factor.

 

There's no reason that the CoF can't be higher than 1 though - I've been looking around on the web for figures on this, I've seen it quoted as high as 2.0 for racing tyres! (the type used on top fuel dragsters.) All a bit academic anyway since the CoF for tyres changes with vertical load - doubling the weight does not quite double the available grip. This effect is not modelled in the spreadsheet...

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Rob Thomson

Yeah, I was talking about road tyres really, but it's amazing how much grip you can get from a proper sticky slick.

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DaveK

so if you guys r using semi slicks or whatever, what sort of friction coefficient would they produce? its all becoming oh so clear that tyres change the braking efficiency massively

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TEKNOPUG

So if fitting larger front brakes (307 283mm) in this instance, would it be more beneficial to fit: A.) no compensators, B.) standard 205 1.9 compensators or C.) compensators from, say a 306 GTi-6 or 307 (if they have them)?

 

Just ordered some new 205 compensators and will be a bit disappointed if I've just wated £70 :)

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jackherer
So if fitting larger front brakes (307 283mm) in this instance, would it be more beneficial to fit: A.) no compensators, B.) standard 205 1.9 compensators or C.) compensators from, say a 306 GTi-6 or 307 (if they have them)?

 

Just ordered some new 205 compensators and will be a bit disappointed if I've just wated £70 :D

 

D.) Bias valve :unsure:

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TEKNOPUG
D.) Bias valve :D

 

Bastard!!!

 

:unsure:

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max0

intresting read! just woundering how much weight the bigger brakes are over the factory items.

 

Reason for asking is if you increase the un spung weight (think thats the correct term) wont that reduce the tyer contact with the road on standard suspension and slighly reduce braking?

 

edit just noticed i got post 69 lol

Edited by max0

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ricdat

Warning - possible waste of time reading this post!

 

Change in front braking force:

 

283/247 = 1.146 times due to disc size increase

(54/48)^2 = 1.266 times dues to piston size increase

1.146*1.266 = 1.45 times total (45% increase)

Well actually.......

 

I have just read through all the posts and you have done great work there Jonah.

 

Just to make it perfectly correct I'll point out one minor error:

The change in front braking force is not proportional to disc diameter. It is proportional to the diameter at which the pad pressure is centred. The centre of pressure of the pad should be pretty close to the centre of the piston. The centre of the piston is about 21mm in from the edge (std 1.9 caliper on a std disc) - as I have just measured as well as I could on an old pad. The change in braking force is then

 

(283-42)/(247-42) = 1.176

 

...hmmm, not that much different to 1.146. I think I'm wasting my time here. Note that this is for if the std caliper is retained but moved (283-247)/2 = 18mm further out. A different caliper such as the 307HDi (which I know nothing about) with its larger piston, may have a larger pad and so the centre of the piston has to move back in a bit. If it moved in (54-48)/2 = 3mm (which it does not have to do) then the change in braking force becomes

 

(283-42-6)/(247-42) = 1.146

 

:wub: Surely that's just a coincidence. Now I know I'm wasting my time here. I'd better put a warning up the top of the post.

 

Anyway, I'm fitting RX7 4 pot calipers to 306 GTI6 discs on my 205GTI S3 and expect to have to do something about front/rear balance but I'll sort that out later. The RX7 pistons are 36mm, which is (36+36)^2 / 48^2 = 1.125 times the piston area. This is not as bad as the 1.266 gain when using 307HDi calipers. The discs are 283mm, as you know.

 

Rick

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jonah

Well spotted :wacko:

 

While we're at it, for your RX7 calipers it should be 36^2 + 36^2, not (36+36)^2. But you got the answer right anyway :angry:

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ricdat
Well spotted :wacko:

 

While we're at it, for your RX7 calipers it should be 36^2 + 36^2, not (36+36)^2. But you got the answer right anyway :angry:

Bugger! I wrote it down wrong.

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Emmy Seize

While we are at it: The front braking force is not proportional to the piston size either.

 

This is due to the fact that the pad size of the uprated brakes is roughly 20 % larger than the size of the standard pads.

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jonah
While we are at it: The front braking force is not proportional to the piston size either.

Eh? :( Yes it is. (piston area that is, not diameter)

 

This is due to the fact that the pad size of the uprated brakes is roughly 20 % larger than the size of the standard pads.

Assuming that the friction coefficient is the constant and the same for both pads (i.e. both made of the same compound), then the pad size doesn't matter.

 

 

Here you go, from Wikipedia:

http://en.wikipedia.org/wiki/Friction

For sliding friction, the force of friction does not vary with the area of contact between the two objects. This means that sliding friction does not depend on the size of the contact area.

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Emmy Seize
(piston area that is, not diameter)

 

That´s what I meant.

 

You are right about the friction area, of course.

 

What i had in mind was the "Hysteresis"-thing between tyre and road surface, where pressure becomes an

 

issue.

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