Question I’ve been wondering recently is which would be better, a lightweight brake package with a big disk, or a more powerful brake with a smaller disk – the brake systems would work out at similar weights so it would be interesting to find out if there are issues with either option.

EG

Hope X2 with 203 vs Hope M4 with 160

Or

Avid Elixir with 203 vs Avid Code (2011) with 160.

Interesting experiment for a mag to do I think.

you might need to define “better” more clearly

i’d take the big rotors every time.

By better I was thinking; outright stopping power, modulation and feel for techy manouvers, heat resistance on big descents and repeated braking efforts.

I know big rotors can help – but where I ride they are frequently smacked by large rocks (when riding and/or when the rider is bucked from the bike and it’s gets thrown down rather carelessly). Hence the question… would a smaller rotor and a more powerfull brake work as well?

It’s probably going to be a personnal preference thing as it’s going to be about feel and perception.

Interesting to hear of anyones thoughts and experiences.

I think the modulation thing is moot, I prefer less movement and find I get better control over a shorter range, others don’t.

The braking force at the tyre can be estimated from f=(P*x*A2*Ub*Rb)/(Rw*Y*A1) where
f= braking force at tyre
P= lever force
x=leverlength from pivot to hand
A2 = total caliper piston area
Ub=effecitve disc to pad coeff of friction
Rb=effective disc radius
Rw=tyre rolling radius
Y= leverlength from pivot to cylinder
A1 = master cyl piston area

Assuming all other things are equal like friction etc then to increase braking force at the tyre (F) then increase :

f= braking force at tyre
P= lever force
x=leverlength from pivot to hand
A2 = total caliper piston area
Rb=effective disc radius

or decrease:
Rw=tyre rolling radius
Y= leverlength from pivot to cylinder
A1 = master cyl piston area

so to answer your question if you fix everything except effective disc radius and piston area then you can compare the relative effects of increasing or decreasing the disc size or piston size.

Whilst we all know disc size, I don’t accurately know piston diameters, anyone measured any?

The other issue is that by fixing everything other than disc and piston area(s) we have no knowledge of the relative differences of master cylinder piston and lever dims between manufacturers or different models..

Get yer verniers out..

Whilst waiting for an adapter, I rode over the Malverns with a Saint M810 brake & 160mm rotor on the front. It was terrifying.

Right I was bored on coffee brake and did some fag packet calcs here is a graph.
I’ve assumed single piston, friction coeff of 1 and appropriate dims for levers etc, given that its all linear it will scale easily. So actual no’s could be miles out but the relationships give you an idea of whats going on..
This shows that for a 140mm disc you need a 30mm dia piston in your caliper to equal a 203 mm disc with a 24mm dia piston.. Hmmmmmmmm.

Does anyone know any of the real no’s for master cyl, and lever levarage?

Nice graph.

However, these portray linear mechanics.

There’s no consideration of momentum/inertia/impulse. Ft = mv-mu, F=ma to get started. Then factor in temperature, friction coefficients, moments around the axle (torque) and acceleration (+ or -).

The net result will be that bigger rotors fair better.

Brilliant answer Toys19 – I guess for M4 and Code you could work out the equivalent piston diameter for the two pistons? WOuld the size of the pad have much effect since 4 pot callipers tend to run larger pads???

Pedalhead – care to ellaborate on “terrifying”?

I dunno why my pic didn’t come up, I’ll try again

I guess for M4 and Code you could work out the equivalent piston diameter for the two pistons? WOuld the size of the pad have much effect since 4 pot callipers tend to run larger pads???

Yeah you would have to measure the length of the lever arms involved from hand to pivot and pivot to master cyl piston contact, then dia of master cyl, dia of pistons, more pistons=more area so more force at the caliper. The pad area won’t make a difference to the brake force applied to the disc, but it may well make a difference to the performance as larger pads will cool faster and might have different wear characteristics.

Rocket science for well established principles – See “Motor Racing”.

Bigger discs = better brake perfomance. Magic words mentioned above:-

Leverage, Area, Heat dissipation.

I find Saints with 200mm rotors work pretty well. Unless your bothered about a bit of extra weight why not go big brake & big rotor.

but…..

most uk riders will never have issues with a lack of heat dissipation causign brake fluid to heat up or pads become inefficent.

They are likely to have problems with a larger disk being more prone to striking obstacles on the ground.

It’s a balance. For most people a 180/160 setup is more than adequate for their braking needs and protects, as far aspossible, the disks from damage during normal riding or falls.

wwaswas has it spot on with all 3 points

All very good points… but I’m not most people… and where I ride I need good brakes… hence the question.

I suppose I should whip the 180/160 Elixir 5’s off my Whyte hardtail and bung them on my Nicolai Helius AM to replace the 205 Hope V2’s and see if they can take some of my favourite descents… that would answer the question… but since those descents are under snow right now I’ll ask on STW 🙄

Good fun replies so far 😆 Keep’m coming 8)

As it happens I swapped a set of Hayes this week from 8″ to 6″ and they are definitely more powerful. Quite surprising as I expected the opposite. The 6″ discs are a slightly thicker and ‘wavy’ (as opposed to the 8″ sawtooth) so I assume there is more disc on the pad at any given time and 1/2 worn pads might have something to do with it.

well I’ve got a set of elixir 5’s with 160 discs which blow my mind how good they are, not as good as my saint 810’s with 203’s, but ruddy good anyway.

My analysis above was stolen from a couple early 90’s PHD from Bath uni on friction and wear in train brakes. One guy works for Dunlop Aviation developing carbon carbon brakes for aircraft, the other worked in the rail industry and now works primarily in research.

I think this …

most uk riders will never have issues with a lack of heat dissipation causign brake fluid to heat up or pads become inefficent.

They are likely to have problems with a larger disk being more prone to striking obstacles on the ground.

It’s a balance. For most people a 180/160 setup is more than adequate for their braking needs and protects, as far aspossible, the disks from damage during normal riding or falls.

…makes sense.

Holey thread resurrection!

I’ve put “some” my money where my mouth was and done some experimenting.

I went from a Hope V2 200/200 combo on my Helius AM to a Formula RX 200/180 – I prefer the feel of the Formula RX but I missed some of the outright stopping grunt of the V2 (although sometimes I found the V2 grabby which is one reason I wanted to change… the other was I could never get the lever comfortable or get it to feel quite right). I liked the RX and it was good but not good enough, but since I loved the feel of the RX lever I bought some Formula TheONE brakes which have a bigger piston… and to test my theory I bought 180/160 disks figuring that if they are not up to the task I can go back to the 200/180 I had with the RX’s. My seat-of-the-chammy testing is ongoing but so far so good; TheONE is an increasable feeling super powerful brake, with the 180/160 combo they are definitely better in the feel and stopping department than either the RX or the V2. Only downside I have found so far is clearance between calliper and disc which is an issue when you bend them a little in transit. No issues with overheating despite some ridiculous brake heavy descending in the high temperatures of Arizona.

My AM looks a little funny with the dinky discs 8)

Don’t think I’ve ever had problems with rotors hitting obstacles, at least while riding. Is this really an issue for people?