Possible 300mm conversion problems

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tossaxe

New member
Joined
Aug 15, 2013
Messages
71
Location
Lithuania
Doing my 300mm disc conversion for R15 windmill wheels I've ran in to some problems not reported elsewhere or downplayed. Thought some might find it useful and save some quid.

I've used :
3mm spacer
ST170 Ford Focus Mk1 discs (the base plate is ~1.3mm thicker than that of a Puma)
Ford Mondeo Mk3 calipers

Carriers

Had to grind the carriers to a thickness of 6.5mm. That is almost half of the thickness and left a gap of ~2.5mm between the carrier and the rim.

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Also the corners of the carrier had to be rounded off. Beware, the iron wall of the slider pin tunnel gets really thin, quite easy to overdo if you rush it.

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Calipers

If the brake pads are worn to only few millimeters, then everything fits. Yet, if you install new or only slightly worn pads, the caliper itself sits so far outside you'll also have to grind ~3-6mm of the caliper itself. Truth be told, I haven't grinded the caliper. When I tried fitting the wheel, to my eye, there seemed such a big gap between the wheel and disc, that I lost hope.

ST170 disc and Mondeo caliper mating

I apologize the photo quality is not great. So much of the disc is left unused, I wonder if a 280mm conversion doesn't offer as much heat dissipation with less excess weight.

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Its not as bad on the side closer to the center, however, again, ~3mm of the disc is unused:

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Weight

Unsprung weight is an important characteristic of a car.

Calliper: Mondeo Mk3 – 5.3kg vs Puma 3.7kg
Disc: ST 170 – 7.1kg vs Puma 3.8kg
Total difference: +4.9kg per corner

Brake balance

A wonderful, detailed article is written at turnfast.com
http://turnfast.com/tech_brakes/brakes_balancing" onclick="window.open(this.href);return false;

In short, bigger brakes are only for heat dissipation. You don't get better stopping by increasing the brake size. The front has as much braking power as the front tires have. If you put bigger brakes at front and do nothing to the rear, you gain nothing at the front and loose stopping power at the rear. Because now, when you press the brake pedal, say, halfway - the front already starts to lock-up, but the rear is no-where-near it maximum braking point.

These are purely theoretical and VERY rough at that, I used the formulas provided by turnfast.com.
Say 2% (according to Nissan engineers) of the car's weight moves to the front when braking and the standard puma brakes are spot-on for that brake balance - 65-35. So the front and rear would lock up at exactly the same moment.

300mm disc are way more powerful than the standard 236mm. So I guess that 300mm disc at front and standard drums at the rear would provide a brake power balance of maybe 90 – 10. Front still has the same braking force/stopping power, so I calculated roughly how much braking force I would lose at the rear with these numbers.

Standard front 270 rear 158
90/10 front 270 rear 30

True the rear tires do less work at braking, but now they almost do nothing at all. Again this is only to illustrate a point not to provide exact numbers/data. I recommend reading the turnfast.com article and drawing conclusions for yourself.

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what wheels were you using? the mondeo calipers can be tough to fitt behind some wheels, because of what you just described :)
 
The standard brakes on the Puma are lacking in comparison to the 300mm setup in my experience. The wheels do not lock on as the tyres can more than cope with a full on stomp on the pedal - again in my experience. Add the fact that you have ABS they're not going to lock anyway and the only time your not going to get full stopping power is in damp/slippery conditions.

A big ratio of the braking on a car/bike etc is done with the front brakes anyway (under heavy braking) due to the transfer of weight when braking (way more than the 2% you stated) which in turn puts even more force onto the tyres meaning more pressure can be applied to the brakes without the wheels locking up. Rear brakes only do a decent job when braking at a steady rate and that ratio is dependant and close to the weight distribution of the car. They do very much less in comparison under heavy braking and are there basically to give stability during the braking phase. The higher the centre of gravity of the car and the wheelbase also effects the amount of weight transfer to the front under heavy braking.

I understand where your coming from as you can only brake as hard as the tyres will adhere to the road. With a bigger set of brakes less pedal pressure is needed to get to that braking point whereas with a poorer set you may not even be able to get to that point therefore braking force and deceleration in most cases is better - or indeed feels better as your getting more of it for less energy on the pedal.

That's my take on it and reading it back makes sense to me so I must be right and I'm sticking to it :lol:

The theorists may say this that and the other but if they were right in their conclusions why do higher performance models have bigger brake setups, simple, to stop the car more efficiently. I say efficiently because that covers a lot of factors that could be debated :lol:
 
Stated in the first paragraph "R15 windmill wheels". Yes, I'm sure not implying that this setup will be hard to fit with ALL R15 wheels.

Well nissan engineers on DriftTengoku or VideoOption said its 2%. Okay they were talking about cars like Silvia or FairladyZ. I agree a puma will transfer more to the front. Regardless, 2% or 10% the rear tires do help with braking. And upgrading just the front won't help. Higher performance models, and race cars for that matter, have bigger brakes for heat dissipation. It's easy to research this.

There is also an option to upgrade the brake pads. It increases the friction coefficient. Way easier to do, weighs less. However in the long run this might be more expansive.

<Agreed, ABS will help sorting out the brake bias. Yet I'm forced to delete mine>
 
oh... didn't catch that. no the 300 mm set up wil not fit behind the original rims. I have not found one 15" rim that wil take the 300mm setup. I belive it may be posible whit some motorsport rims or so, but the most sensible thing to do is fit 16"s

and to add the rear discs from a focus :)
 
Yeah, too bad when I was making my parts list I found many post saying " just grind the casting marks and its a bolt-on". In the future I'll either A) fit 25mm hubcentric spacers(sticking with R15), the rear of a focus, as you say, and proportioning valve, B) or just get some good pads.
 
Yes I fond many saing the same, but as that may be true whit other 15" rims, it is not whit the propellars :)

A tip. Don't do spacers, they wil knacker your bearings in half the time. And don't fit a proportion valve. The abs wil handle the distrobution, and the 300mm front 260 (or whatever) at the rear is a super combo. It's what the frp has (almost) and it's what I have. It's exelent stopping power. Everyone who drives my car alwas complements me on how awsome my brakes are :)
 
I've deleted puma's ABS, hence the proportioning valve ;)

Thanks for the tip. But maybe its the cheap not-hubcentric-spacers that cause this?
 
No, it's spacers in general, by extending the axel byond the center point, your efectivley increesing the strain on the bearing. The bigger the spacer, the bigger the load.

Imagien a boom crane. The longer the boom, the les weight it can lift. Same prinsipal.. :)
 

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