Rotor slots

[rjwoods77]

I was just looking at a bunch of slots of aftermarket rotors of different types. It seem that most, if not all, dont continue the slot past the OD of the disk. I thought the idea was for gas release. WOuldnt you want the slot to continue past the OD of the rotor in order to vent any gas buildup. Also, I see alot of formula sae teams doing a wave style rotor. I am assuming for stress relief of the rotor under thermal expansion. My question is how do you assume for any expansion? What drives your shaping of the rotors to utilize that affect?

[Denny Trimble]

I would reason that continuing the slot to the edge of the disc creates a stress concentration. But, not continuing the slot means you don't clean the entire width of the pad with the slot. So, "wave"-type rotors can ensure cleaning of the entire width of the pad.

You don't need to continue the slots the entire width of the pad to "vent" the gases, they will vent when the slot passes the pad.

But, I've done close to zero real work on this area, so have a look at the FEA yourself.

[Charlie]

Originally posted by Rob Woods:
Also, I see alot of formula sae teams doing a wave style rotor. I am assuming for stress relief of the rotor under thermal expansion.

I don't understand your thought process there Rob, can you explain it?

Floating rotor mounts take care of that issue...

[Kevin Hayward]

Rob,

Denny's reasoning is the same as what we use. Slots to the edge cause a stress concentration ... hence the waves.

Avoiding stress concentrations are a pretty big issue. We use cast iron rotors and have destructively tested a few different rotors. End up seeing some nasty cracks on some of them. These propogate from the mounting region of the rotor which is much harder to minimise stress concentrations.

Kev

[BryanH]

Rob, necessity is the mother of invention. The slots were cut through to to the edge of the discs to relieve internal stress in the s.s. discs which builds up after several heat cycles and bellvues the discs. This is why motorcycle discs are very large in dia. and small diff btwn I.D. and O.D.
BTW carbon pads don't actually need venting. There are many factors to consider when designing fsae discs. If you use high c.o.f. carbon pads the discs can be reduced in diameter,losing weight. And with carbon pads in fsae events you need the discs to heat up very quickly so thin discs are needed, which also reduces weight. Cast iron is not safe below 5mm, so steel is the go. You will need to test compatability btwn disc material & pad. For a free T-shirt I'll tell all.
p.s. relative to this, if jonesy thinks that cornell is the only team that had seemingly insurmountable problems 1 week before comp he needs to get out more often.

[rjwoods77]

So correct me if I am wrong but the reson for a "wave" rotor is to serve the purpose of a "gas slot" because the rotor is constantly exposed and unexposed during the rotation and also for pad cleaning with is what a gas slots secondary funtion is. If that is correct are there any rules of thumb of how much to unshroud the rotor with repsect to the pad? Should the pad have absolute coverage at certain points of the rotation? So if you do a wave then no slots needed?

[John Bucknell]

I would be very surprised if at this level of racing you can find any performance differential between slotted or wave rotors and totally plain drilled rotors. Granted, wave rotors look cool - but are you really going to put enough heat cycles in the steel to crack them in the course of a season? Just my opinion - feel free to show me data.

[rjwoods77]

I was just looking at all my options. If anything I could probably loose a tiny bit of weight with a wave rotor depending on how deep the waves are. Doesnt a wave rotor also reduce the pad area used to stop the car with? Another thing to balnce the whole choice.

[Akos]

Rob,

We used a wave rotor design on the car just to save weight.

With the wheels I was using, the rotor had to be a relatively large diameter so the caliper can clear properly. The problem is you can't make the rotors too thin as they will warp, so you need big holes in it to save weight.

The key to a good wave design to ensure that you have roughly the same brake disk area inbetween the pads as the disk rotates. If you don't follow this rule or have too few waves you will feel it in the brake pad.

For thermal you should always slot your mounts to allow radial expansion (along with proper fasteners). If the brake disk creaks as it cools down, it doesn't have enough float.

Cheers,

[BryanH]

Rob the wave disc was designed to self clean on motorcross bike in muddy conditions and they do this very well. but less disc contact area = less stopping torque.

[MikeWaggoner at UW]

Originally posted by Bryan Hester:
Rob the wave disc was designed to self clean on motorcross bike in muddy conditions and they do this very well. but less disc contact area = less stopping torque.

As contact area goes down, pressure goes up, so stopping torque should remain constant. Pad heat and wear will increase.

I've also read that wave rotors were designed to decrease heat at the leading edge of the pad. In a standard rotor app, there's supposedly a temp differential between the leading edge and the rest of the pad. By waving the rotor, the leading edge contact is not constant, decreasing the temp. This strikes me as BS, because it seems like you'd get the same non-constant effect from holes in the rotors, and I don't think the phenomenon holds true as described.

[Marshall Grice]

If wave rotors actually gave a performance increase of any sort I would think they would show up on some sort of race vehicles, besides off road bikes and the buzz word crazy kawi's. I'm not too sold on the idea.

[Z]

Trailing edge of the pad gets hotter than the leading edge. Think about how the temperature of the disc rises - it gets hotter the further it travels against the pad (absorbing the "frictional energy" on the way).

The pad, apparently, wears more when it is hotter, hence smaller diameter piston at the trailing edge. That's how I understand it???

As for "wave rotors", well they look good, as do slots, neatly drilled holes, etc etc. All of that stuff works, but so do flat discs in moderately stressed applications... like FSAE.

Z

[Chris Boyden]

As for "wave rotors", well they look good, as do slots, neatly drilled holes, etc etc. All of that stuff works, but so do flat discs in moderately stressed applications... like FSAE.

I agree. IMHO, a simple disc with no slots or holes, is the way to go for FSAE. The pad wear is excessive with slots. If the benefits of slots can be proven to outway the cost of pads, then great. A decrease in rotating mass is beneficial, but practically speaking, who wants to change their pads that often in a weekend autocrosser?

[mtg]

Originally posted by Z:
As for "wave rotors", well they look good, as do slots, neatly drilled holes, etc etc. All of that stuff works, but so do flat discs in moderately stressed applications... like FSAE.

Z

If a system is only moderately stressed, doesn't that mean it's too heavy?

Just because FSAE cars are small doesn't mean the parts should see less stress, it means the parts should be smaller.

[Z]

mtg,

Brakes are sized for two different conditions:

1. A single maximum force panic stop. For this the brake components mainly have to be strong enough not to break, and also there has to be enough disc mass to absorb the kinetic energy of the car without too great a temperature rise.

2. Many rapidly repeated brake applications . For this the brake has to have enough cooling to get rid of the heat that is repeatedly being put into it, otherwise the discs get red, yellow, white hot... The maximum cooling power dissipation required is equal to the engine power - eg. brakes on while driving at full power in first gear. In practice probable worst case power flow is ~1/3 engine power. (Worst case for trucks is a heavy load and a long, steep downhill road...)

FSAE brakes should be sized for the first condition, eg. end of the acceleration run, but the track layouts are such that the second condition isn't too bad (which is what I meant by "moderately stressed"). For the second condition the worst tracks have straights that are just long enough for the car to get up to maximum speed, then a hairpin, then another straight/hairpin, etc...


Z

[Charlie]

Originally posted by Z:
1. A single maximum force panic stop.

2. Many rapidly repeated brake applications .

FSAE brakes should be sized for the first condition, eg. end of the acceleration run,

Z, we've logged our brake temps with contact and IR thermocouples. I don't agree with this synopsis, in FSAE brakes are used hard and repeatedly for the whole endurance run. They reach a saturated temp that is much higher than one acceleration run braking and if sized properly the #2 condition as you put it is much more applicable. Red hot is pushing it, but just below that is about right and mans you have as little rotor mass as possible...

[MikeWaggoner at UW]

The other complication for temp in brake pads is that the pads that have a good coefficient of friction at low temps (eg when starting an autocross), tend to have lower service operating temps than normal pads.

For example, EBC greenstuff is good at low temps, but doesn't like getting hot.
EBC Temp Chart (http://www.online-racer.com/ebc_chart.JPG)

[MikeWaggoner at UW]

Originally posted by Z:
Trailing edge of the pad gets hotter than the leading edge. Think about how the temperature of the disc rises - it gets hotter the further it travels against the pad (absorbing the "frictional energy" on the way).

God, I LOVE theory...

" 2. Longitudinal taper: Longitudinal taper is apparent when viewed from either the inner or outer surface of the pad. The trailing section of the pad is partially floated in the boundary layer of outgassed volatiles and incandescent particles of friction material torn from the leading section. The leading edge of the pad will therefore always run hotter and wear faster than the trailing edge . This phenomenon is more pronounced in long pads and is one of the major reasons why racing calipers are designed with a multitude of small pads. (See "Multi piston calipers") "-Stoptech.com

OR

"We were familiar with the pad taper phenomenon from our experience with the stock calipers, where the leading edge of the pad wore quicker than the trailing edge due to the forces involved, causing increased pedal travel (often confused with brake fade) and decreased confidence in the brakes."

OR,Brembo Gran Turismo Big Brake System,
"The calipers are equipped with differential piston bores in order to eliminate pad taper. Without this feature, the leading edge of the pad will wear more quickly."


or, for an image of a properly mounted caliper http://www.wilwood.com/Products/001-Calipers/002-P6R/mtg2.gif

Has someone been installing their calipers backwards?


Originally posted by Z:
Tsk, tsk, mutter, mutter....


/EDIT
If you do have one pot, or multi-pot same size calipers, you can taper the pad in order to get similar effects as the above system.

[John Bucknell]

Charlie has it right - even one stop from Vmax (~90mph in FSAE, how do I know that?) is nowhere near the same peak temp as a roadcourse/fsae endurance. The reason has something to do with the difference between radiative and convective cooling modes as I hinted earlier. He and I are sayin the same thing.

Mike - the reason for taper wear that isn't mentioned directly (but alluded to) is the moment generated by the friction forces between the pad and rotor, which is much more apparent in road car applications where the aspect ratio of the friction material thickness over the length is a much larger number. You can size the differential bores based upon the contact pressure differential induced by the moment.

[Z]

Mike,

Re: pad taper. Yep, I'm wrong! (I did add the comment "That's the way I understand it???".) Thanks for correcting me/ clarifying this. Now where's that differential piston caliper so I can hit myself on the head with it? Oh, that's right I've never used them, too modern for me... (Seriously, I don't mind being corrected - that's how I learn. http://fsae.com/groupee_common/emoticons/icon_smile.gif )


Re: 1) Panic stop vs 2) repeated applications.

What I was getting at was that condition 1 is mainly a structural problem with a bit of thermal capacity also needed, whereas condition 2 is mainly a thermal problem about dissipating heat energy, a bit like sizing a radiator.

To satisfy condition 1 a brake can have a small diameter and thickish disc (although still relatively light), all buried deep inside the wheel. As long as the mounting bolts, etc. are strong enough, then OK.

However, to satisfy condition 2 the brake needs a lot of cooling. That means a large surface area (which for a given mass means large diameter and thin disc), and/or internal vanes (so that it acts like an air pump), and definitely some ducts taking freestream air to the disc and caliper. Also the disc and pads should run red hot because that gives the biggest delta T and so most cooling (radiative and convective).

I've never seen vaned (hollow) discs, air ducts, or brakes glowing yellow in FSAE, hence my assumption that condition 2 isn't too bad.

But!!! Just say the organisers changed the Acceleration event as follows:

Call it Longitudinal Acceleration, and test it both ways as with Lateral Acceleration event (Skid-pad). Place two cones, say, 120 metres apart on straight stretch of track. Car starts from line next to one cone and accelerates towards other cone. After about 80 metres car starts maximum braking, then hairpin corner around the second cone. Then accelerate/brake/hairpin in figure-8 (to discourage weight biasing) for 1 complete lap. Repeat for ten laps. Fastest ET is the winner.

This would be a good test of the brakes under both conditions 1 and 2. Also a good test of the whole drivetrain. No offense intended to anyone, but I doubt many current FSAE cars would finish this event with their brakes unaffected, ie. no fade, smoke, etc...

Who thinks their car could do this? Has anyone actually done something like this?

To the organisers - how about it? http://fsae.com/groupee_common/emoticons/icon_smile.gif


Z

[jack]

to add fuel to the fire everybody dig out "engineer to win" and read "the stagger piston caliper" on page 188.

(i'm suprised nobody has mentioned this yet)

[TG]

I'd like to add that any edge, this being a slot, drilled hole, or disc edge, on the rotor not concentric with it will add to the bite of the brake pad when brakes are applied. So you therefore have the wave design to create this edge at the OD and ID of the disk without creating stress concentrations. Larger car brakes might already have a large enough surface area with slots or holes that they would probably not need the waves I guess.

I've heard of brake glazing on the brake pad during braking which the slots, holes, and waves serve to take it off. I don't know if this is similar or the same to the gases released on metallic pads that you guys have been talking about.

Does anyone know how the carbon brake rotors Yeungnam ran faired at FSAE this year? In a previous post they said that the pads would be sticky enough to be up to the task at hand.