Smaller brakes?

[AdamR28]

So here's a weird one... I'd like to fit smaller brakes to my track car.

Does anyone know if there are other Toyota brake parts that swap straight over to allow smaller rotors? Mainly thinking carriers, I'll be able to figure discs out from a catalogue.

Thanks in advance!

[JB21]

Yaris maybe. I know the toyota Will uses the same pads 😆

[1979scotte]

Curious as to Why?

Weight saving i suppose.

[Joesson]

Curious as to Why?

Weight saving i suppose.





With all the weight lost what's left would be over braked I believe.
My daily has front discs and rear drums for a similar reason.

[1979scotte]

Curious as to Why?

Weight saving i suppose.





With all the weight lost what's left would be over braked I believe.
My daily has front discs and rear drums for a similar reason.

Is over braked a thing?
Can you have to much braking force on track?
You can have too much power that's for sure.

[Joesson]

Overbrake:
verb (used with object), o·ver·braked, o·ver·brak·ing.
to brake (a wheel, vehicle, etc.) excessively.
verb (used without object), o·ver·braked, o·ver·brak·ing.
to apply an excessive amount of braking power.
Dictionary.com

Progressive braking is also a thing, the rate of progression I believe needs to be limited, not too little and not too much. Too much would be over braking.

[Petrus]

Only when a car looses traction too easily on braking can it be called overbraked. The standard discs are most definitely not thát powerful even with 25% weight reduction.

If there is a problem with braking then it is most likely an issue with modulation of the brakes. On our Spyder this can be caused by the ABS oddly enough.

[AdamR28]

Why... mainly so I could use smaller wheels. Drops the centre of gravity without affecting roll centres, lowers the effective gear ratio, lighter, because I can.

Personally find the standard brake setup on a standard car (when in good condition with decent pads) too 'aggressive' (servo way too big). So as a secondary reason, smaller rotors would help with that.

[1979scotte]

Why... mainly so I could use smaller wheels. Drops the centre of gravity without affecting roll centres, lowers the effective gear ratio, lighter, because I can.

Personally find the standard brake setup on a standard car (when in good condition with decent pads) too 'aggressive' (servo way too big). So as a secondary reason, smaller rotors would help with that.

Not something I've ever come across someone wanting smaller brake discs. I can't say I've ever thought that MR2 brakes are over assisted. In fact most people who are new find the brakes to be under assisted. Most modern hatchbacks seem to put 50% of brake force on with the first 5mm of travel or thats how it feels to me anyway.

[AdamR28]

Indeed they do - awful isn't it!

For 'performance' driving, you're better off having the locking point at a much higher leg force, somewhere around 40kg is my preference. At a guess I would say the MR2 is more like 15, so it feels weird on track.

[1979scotte]

Indeed they do - awful isn't it!

For 'performance' driving, you're better off having the locking point at a much higher leg force, somewhere around 40kg is my preference. At a guess I would say the MR2 is more like 15, so it feels weird on track.

I must remember that you are coming at it from a different point than myself.
I'm looking for a fast road car not a track  car.
And I value torque above all things.
Dont think a 6l V8 and a whipple will fit in a 2 though.

[tets]

I think the problem with going down to 14" rims is your profile and width of tyre - you'll be lucky to get bigger than a 185 width and 60 profile which sort of defeats the object.

If it's a weight thing, wilwood do lightweight callipers that are about 1kg a side and around £120.00

If it's a feel thing, can you not just get rid of the servo and abs? I used to get rid of the servo's in my rally cars for that very reason!

As always I may be proven wrong on all counts   

[AdamR28]

Told you I was weird... 


I would like to go down to 13s ideally. I use 185/70R13 standard road tyres on my 550bhp/ton kit car and it is a hell of a lot of fun, yet still rapid. I don't actually want to go that fast on track in this car, just 'optimise' as much as I can without buying new stuff, and have a set of tyres that will do any track day, any season, any condition without having to faff about swapping them.

The Mk1 mx-engineinwrongplace 's front brakes are 235mm and will clear 13s quite easily (the rears are solid and even smaller). I did this on a previous track car and it made a huge different.

The MR2 rears are bigger than the fronts at about 260, and I am absolutely certain the car will stop just fine at the weight I'm planning it to be even with mx-engineinwrongplace  sizes discs (indeed, the mx-engineinwrongplace  at a ton including driver does fine on stock brakes with standard engine).

Weight - not really that bothered (in a £ : kg way) and like to use OE brake components where possible.

I think I will get rid of the ABS - but not sure about what to do with the servo. I could mod the pedal ratio I think to get some power back if I ripped it out (they feel impossibly 'firm' with the servo just disconnected!). Could go twin masters and bias bar but that is a lot of fabbing...


So, bodging Aygo or Yaris brakes onto the MR2 - any ideas? Or am I just gonna have to buy some scrapyard bits and get all Project Binky on it?

[tets]

at least if you go to 13's you can get some Avon CR28's. Best tyre i've ever rallied on in cold wet conditions - I used to take my Michelin TB's off at times and put these on! Super soft and grippy

I think the MR2 rears do more with having the extra weight that end although if you went down in proportion i'm sure you'd be fine - using a mid range track pad like yellowstuff you'd still have as much stopping power as a road car on Halfords specials

I used bias pedal boxes without the servo but thats a tarmac / gravel thing as I used to wind the bias to the rear on gravel to lock the back up before the good old Scandanavian flick!

I'd probably start with Yaris or even see if you can graft Mazda ones on if they are cheap enough - I used something like Austin Princess fronts on the back of my sunbeams and escorts and it worked well

[Petrus]

For 'performance' driving, you're better off having the locking point at a much higher leg force, somewhere around 40kg is my preference. At a guess I would say the MR2 is more like 15, so it feels weird on track.

If my memory serves me correctly, from when I was looking at replacing the abs pump by a bias valve, the UK spec has 40 kg. for the pedal pressure.

[Alex Knight]

This thread is mental.

I'd only, ever, ever want more braking ability. Never less.

Over braking is not relevant to fast driving IMHO.
Pedal progression, however, absolutely is.

The driver controls braking force with the progression of the pedal.

Why you would ever want to hamstring a car with less capable brakes for any reason other than cost is either so novel/leftfield as to be beyond my comprehension, or just plain stupid.

FWIW.

[Petrus]

I prefer to use ´modulation´ over progression of the pedal as it is about increase in hydraulic pressure and not pedal travel. The less pedal travel, the easier to control, modulate the force thus hudraulic pressure.

´Overbraked´ means too little pedal force needed for max brake effort.
The problem lies in the difference between 10%, 50% and 100% brake pedal force being smaller in kilos with the reduction of the max force; smaller differences in force are more difficult to control.

The ONLY critisism I have on the bake system is the ABS introducing (an albeit small) extra initial pedal travel.
This is btw why harder brake lines give only a small improvement. Well, that and the OEM lines being quíte good already.

[Petrus]

@AdamR28 ; I suggest you have a look at removing the ABS and installing an adjustable bias valve.

The OEM system has two lines coming out of the brake master cylinder which go to the ABS module. 
Within the ABS module they split between front left and rear right, front right and rear left.
The front brake lines are at the firewall side, the rear brake lines at the top.

To remove the ABS module use T-Pieces to pair up both front pipes and both rear pipes. Run one port from the master cylinder to the front T-Piece and run the other port from the master cylinder  to the rear T-Piece.
Depending on the type of valve you fit the bias valve either at the master cilinder between front and rear line (usual) or in the line to the rear.

Adjusting the bias valve is a matter of closing it more and more from full open till the rear wheel júst begin loose traction on max braking.

I am not aware of anyone on the fora having done this. The only example I have found on the web was the Spirit car.



This what you will chuck (photo Impreza):

[AdamR28]

This thread is mental.

It is a bit, isn't it 

I don't specially want 'worse' brakes - they are just in the way of my goal.


Goal: Make car faster by running smaller diameter wheels, which in turn drops the gearing (effectively adding power - 17% if I can fit the tyre size I want, and who wouldn't want 17% more power for the sake of swapping a few brake parts? (I already have the wheels)) and centre of gravity (nearly 2" without any change in roll centres, which is very significant - I can do the maths on that if you like). Has the side effect of looking pretty poopoo, which is another bonus for this car, and meaning a better selection of cheaper tyres.

Problem: Standard brakes are too big to fit under 13" wheels.

Solution: Make brakes smaller.

Consideration: Will smaller brakes be adequate to stop the car?

Answer: Yes.

Evidence: A heavier car will do 90 minutes flat out (racing) on road pads with smaller brakes, with no issues.

Other consideration: The car will be around 20% lighter than standard, so need 20% less thermal transfer (kinetic to heat) capability.

Conclusion: I can fit smaller brakes, if I can find them.



Thanks for the info Petrus, I'm familiar with this stuff from previous projects    If you like to play with this stuff. http://www.brakepower.com is a good site.

For circuit driving, increasing the pedal pressure required to lock the brakes is a big benefit. On the road you'll rarely make 0.5g without making your passenger/ dog feel sick, on the track you'll hit 1g every braking zone (or should!).

The weight of your leg alone is then adding ~20kg to the brake pedal. That makes it really hard to modulate when the brake locking pedal pressure is barely above that value. This is why F1 cars have brake pedal pressure requirements of around 140kg - sounds mental, but at 5g the driver isn't actually doing that much.



Back to the original question... does anyone know if other smaller (Toyota?) car brake parts will fit on the MR2?

[thetyrant]

Smaller discs shouldnt be a problem but the getting a suitable oem caliper that mounts up and gives you a enough decrease in outer diameter of brakes (to allow for the smaller wheels) could be tricky,  do you know the wheel barrel size on those you want to fit ?  it might be possible to redrill/mod the oe mounting bracket to move it inboard more but i cant remember how much meat is on it.

[thetyrant]

Looking at discs specs Mk1 Yaris fronts at 235mm should go on MR2 front and rear, PCD and C/bore same and slightly less height but easy to sort that with shims if needed. Calipers though im not sure...

[AdamR28]

That's awesome, thanks Ian! Looks like the Yaris discs will go straight on front and rear, height is close enough I reckon (sliders should take out that couple of mm).

Not sure of wheel barrel size, but they will fit if I can get the calipers in the right place for the 235mm discs - that's exactly the same as the MX-5 brakes which I have had these wheels over.

Time to go and see if there's enough meat to re-drill the carriers... cheers again!

[AdamR28]

So, a small update:

Bought some 14" mx-engineinwrongplace  wheels that JUST fit! https://www.mr2roc.org/forum/index.php?topic=69843.msg836378#msg836378

I have ordered some 195/45/15 tyres, which drop gearing by 10% and the car by an inch: https://www.mr2roc.org/forum/index.php?topic=69843.msg836438#msg836438

The front calipers / carriers / discs are actually really close to fitting on the rear, the carrier bolt spacing is just a few mil different - my backup plan was to slot the carrier holes in the uprights and bolt front brake carriers and calipers (with 255mm rotor) onto the rear (normally 262mm) if these 14" wheels didn't go on.

With the car being lots lighter, and the smaller wheels, I am now going to do away with the servo and adjust the pedal ratio by moving the clevis pin up nearer the pivot (details in the Mr Poo thread in my signature).

My preferred pedal force is 40-45kg, so the 7/8" (22.22mm) standard master cylinder works perfectly.

You cannot view this attachment.