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Author: Subject: brake upgrade questions
rodgling

posted on 12/2/13 at 10:15 AM Reply With Quote
brake upgrade questions

Thinking about upgrading to some Wilwood calipers on the front with two-piece discs (i.e. with separate hats). This should save me 4 kg each side plus it will look shiny :-) Usage is road & track.

Some thoughts:

- 1. hats don't seem to have a hub-centric bit, so how do I ensure the wheel mounts centrally?

- 2. on a 700 kg 7 with a 320 bhp engine, will solid discs (as big as I can get under a 15" wheel) be enough or do I need vented?

- 3. if I keep the Golf rear calipers will this upset brake balance? If so, will tweaking master cylinder sizes and bias adjuster sort it out again?

- 4. any other suggestions on what setup would work well?

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Dusty

posted on 12/2/13 at 10:43 AM Reply With Quote
If your car currently has well balanced braking and you double the braking power at one end you will no longer have well balanced braking. Yes, sure that's fairly obvious. If you then adjust the front end system by way of changing balance bar and cylinder sizes to be in balance with the original rears won't you have pretty much reduced your new extra braking back to what it was originally?
I believe that to retain balance and make any progress you would have to upgrade the rears by a similar percentage to the improvement on the fronts.
Solid discs OK for road and most track days. Reduction in unsprung weight is good. The donor brakes are good for a car with three times the weight of yours, four passengers and some luggage. Locating wheels on the hubs can be by centre spigot or studs/bolts depending on design of hubs and wheels.

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rodgling

posted on 12/2/13 at 11:19 AM Reply With Quote
Yes, that's my understanding on balance too. There will be a little improvement from having a slightly bigger diameter disc, and maybe going from a single-pot floating caliper to a 4-pot will give less play/flex in the system. But mainly I guess the unsprung weight reduction is the big win.

The stock wheels are hub-centric... so are hats without a hub-centric bit a no-no then?

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MarcV

posted on 12/2/13 at 11:26 AM Reply With Quote
1. Not too sure which ones you have seen, but the ones I'm after are hub centric. On the BMW flanges there are two ridges, the first for the disc, the second for the wheel. No change there. Even if the hats would not be hub centric, that would be an issue for the disc, not for the wheel.

2. Good question, let me know ;-). From calculation it would seem that they are okay for a stop from 250km/h to zero with decent pads. Repeated braking is a bit more difficult though....

3. There would be a bit of brake performance gain with slightly larger diameter discs (more brake torque for the same pressure). Balance should be sorted easily with correct MC and bias adjustment.

4. Very interested in this one. Will keep watching this thread to see what others have to say.

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sdh2903

posted on 12/2/13 at 12:00 PM Reply With Quote
I can understand the want for lightness and shinyness too. However brakes are for braking and would the new brakes give that much better over what you have?. 700 kg is still a light car and I'm assuming you have the 328 calipers? should be more than up to the job. I have the 318is front brakes and I think they are well overkill really I can lock all 4 wheels with a good stomp quite easily. For weight saving I'm going to swap for some solid discs with calipers from a 318 for this summer.






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rodgling

posted on 12/2/13 at 12:12 PM Reply With Quote
quote:
Originally posted by MarcV
1. Not too sure which ones you have seen, but the ones I'm after are hub centric. On the BMW flanges there are two ridges, the first for the disc, the second for the wheel. No change there. Even if the hats would not be hub centric, that would be an issue for the disc, not for the wheel.


Oh yeah... I'm being a cretin. The ridge on the hub pokes through the disc to centre the wheel so that's a non-issue.

Currently on the 328 calipers with vented discs (5 + 7 kg = 12 kg total). Going to BMW calipers with solid discs would save about 2 kg, so down to 10 kg for discs and callipers. Wilwood calipers and light discs would be about 1.5 + 3.5 kg = 5kg total.

Even if braking is not noticeably better, the weight saving is huge and I think worth it if I concentrate hard on not thinking about the cost. Besides as Marc says there will be a small improvement if I can increase disc size.

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sdh2903

posted on 12/2/13 at 12:47 PM Reply With Quote
I think the increase in braking would be difficult to even notice. It's one of those cost vs gain conundrums. I had a look at this a while ago the cost was too great for the little improvement. I've figured the best way for my car to lose 10kg is for me to go on a diet

You could always speak to Pete/fibreform about the possibility of alloy uprights as you could use the GKD ones as boat anchors they're that heavy.






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rodgling

posted on 12/2/13 at 12:55 PM Reply With Quote
quote:
Originally posted by sdh2903
I think the increase in braking would be difficult to even notice. It's one of those cost vs gain conundrums. I had a look at this a while ago the cost was too great for the little improvement. I've figured the best way for my car to lose 10kg is for me to go on a diet

You could always speak to Pete/fibreform about the possibility of alloy uprights as you could use the GKD ones as boat anchors they're that heavy.


All very true but it's rotating unsprung mass which counts for a lot.

I've spoken to Pete more than once but he can't justify the investment in uprights - which isn't unreasonable given that he'd probably only sell a few. That said I think he's just bought a CNC machine so it would surely make a good project for him...

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sdh2903

posted on 12/2/13 at 12:59 PM Reply With Quote
Well I would be up for a set, my uprights are heavier than my engine

out of interest which wilwoods you looking at?






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rodgling

posted on 12/2/13 at 01:05 PM Reply With Quote
Not sure yet. Marc favours the Dynapro, which is one of the heavier options (about 3.5 lbs) - but even that is very light compared to stock. Caliper choice is probably the biggest question we have, there are a lot of options.
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hughpinder

posted on 12/2/13 at 01:23 PM Reply With Quote
The amount of braking force you getfor a given pedal force depends on:
Disk diameter (measure to the centre of the pad)
Caliper area - so measure the diameter of your piston(s) and calculate the total area, sliding types count twice as the pull the pads with the same force on the other side of the disk.
The coefficient of friction of your pads
The amount of hydraulic pressure you generate to push the pistons.

A bigger diameter on the disk obviously gives more braking force if everything else is the same, and no change to pedal travel.
A bigger piston area on the caliper piston - same effect, but you will need more pedal travel to fill the cylinders if they are bigger. The information I have for a 328 is that it has a 54mm piston, so a willwood 4 pot with 31.8mm piston has 70% of the piston area(so braking is worse), or the 38.1mm piston ones have identical area (as the pads are a bit narrower the centres will be a bit further out, so you effectively gain slightly (15mm approx) on disk diameter), and the 44.5mm 4 pots will give about 35% more area/braking force (and 35% more pedal travel for the fronts).
A higher coefficient of friction on the pads - more braking force, no change to pedal travel. As was pointed out to me some years ago, try to have the same pads all round so they heat up the same. Mintex 1144s seem to be the ones recommended by most people.
A smaller master cylinder means you will generate more pressure in the hydraulic system for a given pedal pressure, and so more braking force, but you will get proportionally more pedal travel.

As others have said - be careful about brake balance.

Hope that helps
Hugh

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rodgling

posted on 12/2/13 at 01:41 PM Reply With Quote
Thanks Hugh, this is all in line with what I've researched.

I don't think brake balance or caliper piston area is a big issue. If I adjust the master cylinder in the same ratio as the change to the caliper piston area, then clamping force remains the same (for a given strength of push from my leg). So as I understand it I can adjust the MC to suit the caliper - this is simply a way of adjusting mechanical advantage.

I'm not sure about pedal travel. Given that the amount of fluid moved is tiny, is this more down to flex and play in different parts of the system than actual caliper piston travel? I don't know.

Getting back on topic though, my big questions are, for a lardy but fast 7 doing trackdays:

- vented or solid fronts?

- any specific recommendations on caliper choice?

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coyoteboy

posted on 12/2/13 at 01:56 PM Reply With Quote
Ive been mulling this over myself (though with slightly different bounds) and I'm in the middle of writing a bit of code to run through options of disc sizes, types and caliper sizes. There's a few out there already if you hunt around but I didn't like the lack of transparency and valued the process as much as the result so I did it again myself. It started off in excel to be as transferable as possible but that became too cumbersome so it's now going to be in Scilab scripts.

The leap from solid to vented, at least superficially (simple model), makes a couple of fairly large differences, apart from the vast increase in unsprung mass. The step from solid-vented is usually in the order of 2-3kg per rotor, this effectively doubles the rotor mass and so the warm up time, but also the time to overheat, even staying at the same overall dimensions (bar thickness). Personally I'm falling around the opinion of about ~280mm vented (but only 22ishmm thickness) as the ideal region, going up to 300mm seems to be massive overkill.






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rodgling

posted on 12/2/13 at 02:14 PM Reply With Quote
I got fed up with the crappy Wilwood website so I made a spreadsheet of their caliper options. Hope this is useful. It's all scraped from their webpage, except the Midilite data which I entered manually off the Rally Design website.

https://docs.google.com/spreadsheet/ccc?key=0AuhBzuVF2_3YdHNnQ1RDdjEyUEd3MHRvbG9BclhBU2c#gid=0

Going up to 300 mm, regardless of heat/weight, does have the advantage that you get more braking torque for a given clamping force, so I think it's best to go as big diameter as possible.

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MarcV

posted on 12/2/13 at 02:17 PM Reply With Quote
quote:
Originally posted by sdh2903
I've figured the best way for my car to lose 10kg is for me to go on a diet


You will notice it a lot if you can shave 10kg of the front brakes (mainly rotors of course), you will barely notice 10kg saved in body weight (well from car / driver performance point of view that is..)

Reducing the rotating mass up front will give a noticeable quicker steering response. You will be able to tune the spring / dampers a bit better (these light cars suffer from relatively high unsprung mass) giving better performance over road irregularities. Also it will require far less power to get it all to the required rotational speed (either faster or slower) resulting in quicker acceleration and deceleration....

I think light wheels, brakes etc. are all very well worth the effort...but hey... I'm still building.....

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BangedupTiger

posted on 12/2/13 at 02:25 PM Reply With Quote
I'm ditching my 280mm vented discs with single piston caliper for 4 pot wilwood 257mm solid discs.

I can only see a benefit.

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russbost

posted on 12/2/13 at 06:31 PM Reply With Quote
quote:
Originally posted by sdh2903
I think the increase in braking would be difficult to even notice. It's one of those cost vs gain conundrums. I had a look at this a while ago the cost was too great for the little improvement. I've figured the best way for my car to lose 10kg is for me to go on a diet

You could always speak to Pete/fibreform about the possibility of alloy uprights as you could use the GKD ones as boat anchors they're that heavy.


Lose 10kg? If you're running a car type battery then switch to one of our Lithium ion ones, that'll save about 12kg, even if you're running a bike battery or an Odyssey it would save 5-7kg, ok it's not unsprung, & it's not very shiny but it's all weight & batteries are usually high in the CoG so must make it go round corners better to say nothing of acceleration & braking - it's also much cheaper & doesn't require rebalancing the braking system (probably!)





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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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rodgling

posted on 12/2/13 at 09:21 PM Reply With Quote
It's on my list Russ. The specs of those batteries are amazing but unsprung weight comes first.
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coyoteboy

posted on 12/2/13 at 09:31 PM Reply With Quote
quote:

Going up to 300 mm, regardless of heat/weight, does have the advantage that you get more braking torque for a given clamping force, so I think it's best to go as big diameter as possible.



Massive rotating mass, massive sprung mass, if you can lock your wheels with 240mm discs you won't fundamentally gain anything by going up to 300's other than a bit of heat capacity?






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rodgling

posted on 12/2/13 at 09:47 PM Reply With Quote
Yes, the extra mass is bad but the braking torque increases (because the pads are further from the centre of the wheel). This should improve feel and make the brakes feel less "on/off". Think about turning the steering wheel - if it was 10 cm across you'd have no fine control, if it's 30 cm across you have much more "feel" and control over it. That's the theory as I understand it, anyway.
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coyoteboy

posted on 16/2/13 at 10:26 PM Reply With Quote
But if you can lock the wheels now all it really is doing is changing your braking feel, and FWIW I think it'll make them more on-off. Assuming the same calipers but larger discs, the same pedal pressure will achieve higher braking force. With the same application rate of the pedal, the brake force will increase proportionately faster.






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