Dubstep Shep
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Also, if you're gonna be at Hallett (my "local" track) in June, cool if I stop by and bug you with a million questions? hehehehe
Vorshlag 2011 Mustang 5.0 GT - track/autocross/street Project
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ddd4114
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That's actually a misconception. The reason that golf ball dimples are more efficient is that they create a turbulent boundary layer on the surface of the ball, and that deters separation behind it. When there is less separation, the wake is smaller, and there is less drag.
Within a duct, ASSuming separation is similar, more energy is lost to create the turbulence, and your airflow is reduced. Sometimes, if there is a lot more separation when the flow is laminar, then you'll effectively shrink the duct size, and that might reduce flow by itself (Google "vena contracta"). However, in most cases, more laminar flow = better flow.
Regarding pad knock-back, I noticed a lot of it when I switched to fixed front calipers. When I was still using the OEM two-piston floating calipers, I never had a problem with it. The first event I ran with fixed calipers, it was an immediate change. It was so unusual for me that I actually spun off the track because the braking came on so suddenly when I wanted to just brush them into a corner. Pumping the brakes regularly (as mentioned, near the end of straights) helps a lot, but it can still be pretty distracting. My wheel bearings seem fine, but I'm guessing that the two-piece rotors flex a lot under cornering. As Dave mentioned, the whole assembly probably flexes a bit too.
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SoundGuyDave
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OOOOOOHHH!!! Hallet June 21-22 with NASA... Everybody that CAN make it should, that is also the American Iron and Camaro-Mustang Challenge "Summer Shootout." An annual pilgrimage of racers from a LOT of different regions, and it's not uncommon to have 30+ car fields. AI takes a rolling start as wave one, and then CMC comes to a screeching halt right behind them to set up for a standing start, on a green track... 30+ V8 engines, all hammering away at full chat is something to see, and it should be a LOT of close racing. This should be a mini-preview for Nationals.
Oh, and yeah, Terry will be running in TT as well... ;-)
Great observations from Terry. Pad knock-back (long pedal), plus thin pads (no insulator), plus locked wheels/extreme ABS (rapid heat transfer to the calipers) equals bent car, injured driver, and baked brakes. Not a good formula.
Terry, any ideas on the cause of the knock-back? Aero-supported cornering forces? Curb hopping? Bearings?
Oh, and yeah, Terry will be running in TT as well... ;-)
Great observations from Terry. Pad knock-back (long pedal), plus thin pads (no insulator), plus locked wheels/extreme ABS (rapid heat transfer to the calipers) equals bent car, injured driver, and baked brakes. Not a good formula.
Terry, any ideas on the cause of the knock-back? Aero-supported cornering forces? Curb hopping? Bearings?
Hmm, unless I am misunderstanding you, No... for maximum total flow with air (or with any fluid flow) you want smooth, laminar flow through smooth bore tubing, if at all possible. The rule of thumb in race car plumbing for fluids is: you need to up-size at least 1-2 sizes for non-smooth bore tubing/hose to match the flow of a (smaller) smooth bore tubing/hose.
This is partly why the OEM corrugated intake hose on the 5.0L engine makes less power than an aftermarket smooth bore "cold air" intake hose kit. Its all about smoothing out those ridges, smoothing and increasing the flow of air to the engine. The OEM hose is more flexible (accounting for drivetrain slop/movement) but it flows less total air.
And ideally we want more air flow to the brake rotors, to extract more heat...
They might indeed make a 6-pison caliper that fits inside an 18" wheel. We could upgrade to a number of 6-pison calipers, which with more mass can hold more heat, and potentially have a larger brake pad. But normally, on the S197, you don't see 6 pot calipers on OEM or most aftermarket Big Brake Kits until you get to 15" rotors, which don't fit inside the 18" wheels we need to use (for many reasons).
And Wilwood is.... not really my favorite brand of calipers. You never see Wilwood calipers on endurance or pro level race cars - for a reason. They are really just a bit cheaper, and more street car oriented. There are much better (aka: more rigid) calipers available from Brembo, Alcon, PFC, StopTech, and others.
We have a brake upgrade we are planning for this car with a 6-pison Brembo caliper and 2-piece 14.5" rotor that just fits inside the 18" wheel barrels. We'll see if it pans out. That is about as large as you can go on 18" wheels, but most kits come in whole inch increments (even when measured in millimeters): 12", 13", 14" and 15" are common aftermarket rotor sizes.
No problemo - again, answering questions and helping folks is why we're there! Getting to race is just a bonus.
Yep, the AI/CMC races during Summer Shootout is always fun to watch! The standing starts are THE BEST!!!
Cool, come on - we need the class entries. TT3 should be huge, as several of the Colorado TT3 regulars are coming down to join the Texas contegent (we usually have 5-7 in class for most events)!
An AI car (9:1) is a perfect fit for TT3. I have run that track for exactly 4 sessions (I broke the car on Day 1 and had to rush back to Dallas to get repairs, then back to Hallett the next day), and we ran both directions that day, so the existing TT3 records are weak... and we had the wipmy front aero, old rear wing, and skinny tires back then, plus they just repaved Hallett a few weeks ago. I suspect my old records will be CRUSHED! "By whom" is the question.
Looks like my best lap beat the fastest AI car (Patterson's Camaro) by about 2 seconds last year, but I barely knew corner numbers and took terrible lines (according to everyone who watched my Hallett videos, hehe). I'm hoping my back is in good enough shape to race there in 4 weeks, and if we can get a few more mods done to the car by then, and get a lot more seat time that weekend, I hope I can improve on my old best times by a few seconds. Otherwise it'll be Amy racing the car all weekend. With all of these out-of-region folks coming in, and AI racers jumping over, TT3 will be anyone's ball game. Should be fun!
Again, I don't think our brake failure this time was pad knock-back. Maybe it contributed a small bit, I don't know. The front wheel bearings are tight (we check them before every event; this is our 4th set in 4 years and they were fresh) and I was staying off of the curbing, as it was upsetting the car when I tried using curbs at RA. So while maybe it contributed a little... the fluid was BOILED GOO when we were done.
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Dubstep Shep
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So I did some referencing in my old fluid dynamics book and I may have found the answer.
Apparently turbulent flow has less skin drag than laminar flow. Basically turbulent flows don't stick as well to objects. However, turbulent flows in a closed channel flow will have a higher drag than laminar flows. It has to do with turbulent vs. laminar boundary layers. Fluid Dynamics: a subject I'm somewhat glad I didn't have to take more of lol.
As far as brakes, like any system on a road track car, is heat soak an issue? You say bigger brakes will hold more heat, which I agree with, but after a certain point won't they be just as susceptible to heat soak? Like you can upgrade your intercooler tank capacity all you want, but after running it hard for a certain amount of time it's all going to be hot. You're just delaying how long it takes to get there. I suppose a larger caliper would have more surface area though, which means it would dissipate heat quicker though.
Also, why not use more advanced materials like carbon ceramic? Not cost effective?
Apparently turbulent flow has less skin drag than laminar flow. Basically turbulent flows don't stick as well to objects. However, turbulent flows in a closed channel flow will have a higher drag than laminar flows. It has to do with turbulent vs. laminar boundary layers. Fluid Dynamics: a subject I'm somewhat glad I didn't have to take more of lol.
As far as brakes, like any system on a road track car, is heat soak an issue? You say bigger brakes will hold more heat, which I agree with, but after a certain point won't they be just as susceptible to heat soak? Like you can upgrade your intercooler tank capacity all you want, but after running it hard for a certain amount of time it's all going to be hot. You're just delaying how long it takes to get there. I suppose a larger caliper would have more surface area though, which means it would dissipate heat quicker though.
Also, why not use more advanced materials like carbon ceramic? Not cost effective?
Yes and yes.
YIKES! Oh yes... carbon ceramic systems are coming online in high dollar OEM applications, and the TOP levels of pro racing, but they are far from cost effective enough for HPDE/Time Trials/Club Racing use.
Go price a set of factory replacement carbon ceramic rotors for a GTR... five figures. And they CAN and WILL wear out with track use. Our customer's car (shown above) uses OEM rotors/pads on his TT1 prepped ZR1 Corvette. They have to be weighed (to a fine resolution) to know when they need to be replaced. And while they can last longer, they aren't cheap.
My car's Centric Premium 14" front rotors cost ninety nine bucks. Remember: I'm just a small business owner and the "opposite of rich", heh. I also try to test and race what our customers use, which - right now at least - doesn't have much to do with carbon ceramic brakes, I'm afraid. When/if the costs get low enough I'll be one of the first to switch. That material is better for brake parts than iron in every possible performance metric. Except COST.
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Nope. Air flow and air resistance are directly related to each other.
Golf balls have dimples because their shape inevitably forces boundary layer separation. The dimples cause some turbulence at the boundary layer, but they also yield later separation of the boundary layer from the surface. The net effect is an increase in air flow, a reduction of air resistance.
If turbulent air flow were better in general, you'd see dimples on aircraft wings and other aerodynamic surfaces. But instead, you see exactly the opposite, precisely because laminar flow is the most efficient. The design of a wing is such that it attempts to reintegrate the separate flows over the top and bottom in the smoothest way possible, to extend the laminar flow as far as possible and to minimize turbulence.
Even on aircraft, though, there are mechanisms that induce turbulence in controlled ways to improve the flight characteristics of the aircraft in certain flight regimes. When the boundary layer separates from the wing surface, the end result is a substantial loss of lift. This happens at high angles of attack, which is generally when the aircraft is flying at it slowest. That loss of lift is called a "stall", and aircraft have crashed as a result of it happening unexpectedly (e.g., when the pilot inadvertently slows the aircraft too much, or attempts maneuvers that increase the angle of attack beyond the stall point). To combat this, vortex generators are often used on the leading edge of the airfoil. While these add a little turbulence, their main effect is to direct additional air flow into the boundary layer at the point where separation would normally occur at high angles of attack, and thus delay separation of the boundary layer from the wing.
ETA: I see my reply was just in time, well before anyone else was able to reply.
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Dubstep Shep
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Hahaha no kidding KC. Like you mentioned, turbulent flow results in a loss of lift, which makes sense. The air isn't holding onto the wing as well and you lose all the lift generated by it.
From my understanding, turbulent flow produces less skin drag but flows slower, if that makes sense.
Check out the mythbusters episode where they put golf ball like dimples on a car. They actually improved their gas mileage.
From my understanding, turbulent flow produces less skin drag but flows slower, if that makes sense.
Check out the mythbusters episode where they put golf ball like dimples on a car. They actually improved their gas mileage.
I suspect it has more to do with boundary layer maintenance than anything else, but fluid dynamics makes my head hurt, and is as crazy complicated as thermodynamics (no surprise, really, since the latter is strongly dependent on the former when you start talking real-world applications)...
Know the season and episode off the top of your head? Sounds interesting.
But if that were the case in general, then you'd expect aircraft manufacturers (for subsonic aircraft -- supersonic flight introduces even more complication) to use dimples on those surfaces that are not lifting surfaces, so as to reduce friction...
ETA: to expound a bit, I suspect that carefully inducing turbulence can yield some benefits if you're talking about a shape that isn't optimized very well for flow through the air. But for shapes that are (e.g., aircraft bodies), doing that would be detrimental.
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SoundGuyDave
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Just to expand on Terry's uncharacteristically brief answer... Yes, the "bigger brakes," meaning larger diameter and thus higher mass rotors as well as potentially more massive calipers will indeed be able to withstand more heat before the fluid inside the caliper is compromised.
At the risk of over-simplifying, all a braking system is, really, is a device to convert kinetic energy (rotation) into thermal energy (heat). It does this via friction between the brake pad and the rotor, with the waste byproduct being heat. A more massive rotor will be able to assimilate more heat before the metallic structure crystalizes (creating those characteristic "hot spots" in the rotor face). This is combated in a race setting by using vented rotors, and blowing cool air into the center of the rotor, and exhausting it out of the perimeter, pulling some of the heat out.
On the hydraulic side, the heat generated by the pad/rotor friction is relatively well insulated from the fluid by the friction material of the pad itself, then the small heat-sink of the backing plate, then the piston material. Different piston materials have differing rates of heat absorption, but remember that EVERYTHING will eventually heat up, and the more insulating materials then hold that heat in. Titanium is a great example. It takes forever for it to actually get hot, but then it STAYS hot forever... In any event, eventually this heat makes it to the fluid in the caliper, but the mass of the caliper helps to dissipate what heat makes it to the fluid.
The way the "game" is played, in terms of thermal management, is to try to design a braking package where there is sufficient cooling to rapidly pull heat out of the rotor and caliper, while still retaining a deep enough "sump" for the inevitable spike in temps during and immediately after a braking event. Like the supercharger example, yes it will eventually heat-soak, but if you can work in a large enough heat exchanger you can either prolong that point past where it matters (think 55gal reservoir), or manage the heat to the point where eventual stasis temp reached is within manageable levels. In the intercooler example, if your system was efficient enough to stabilize at, say 170*F, that's a WHOLE lot better than 230*F!
Getting back to brakes: If you have sufficient airflow (4" instead of 3" ducts, smooth tubing as opposed to corrugated, spindle plate seals the duct to the hub of the rotor, etc.) that goes a LONG way towards pulling heat out of the rotor. A cooler rotor means less thermal energy makes it past the pad material into the fluid. If you also have a more massive caliper, that provides more radiating surface area for the fluid as well. If you were to duct to both the rotor AND the caliper, you would gain that benefit also.
In endurance racing, brakes are obviously a pretty big deal. If the system isn't properly engineered (sufficient cooling), then you heat-soak the hardware pretty quickly, and that is tough to recover from. Sitting in pit lane, bleeding the brakes while your competition is whizzing by on track sucks. So far, the longest I've run the car is five hours straight (less fuel stops) with no full course cautions, and the brakes held up. Admittedly, they were starting to get thin, and I was starting to feel the heat in the pedal (starting to get a tick spongey), so if we had to push to six hours, we would have done a pad change and a quick bleed up front. "Pro" endurance teams are set up for this type of change in a pretty unique way. They have a dry-break fitting in the middle of the brake hose, so they pull the wheel, pull two bolts (knuckle to caliper mounting bolts), pop the dry-break fitting, and yank the rotor, caliper, and half the line off in one piece with asbestos-lined gloves. The replacement is already set to go, bedded in, with fresh fluid and pressurized to hold the caliper in place. Pop the replacement assembly on, run in the two spindle-mount bolts, and click in the dry-break fitting, and add a new tire/wheel assembly. Less than ONE MINUTE per corner. At that point they have fresh fluid in the caliper, and room-temp brakes.
That is a case of putting off the heat-soak until it just doesn't matter, because the race will either be over or they will replace the hardware before it becomes a factor.
ford20
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Wouldn't that be the advantage though with going to a 2 piece rotor? Less heat soak, less chance of warping and with full or semi floating rotors less or eliminated chance of pad knock.
Dubstep Shep
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I would guess it's for a few reasons:
It looks retarded on a plane or car
It REALLY complicates the manufacturing process
It introduces a ton of complexity and added weight to the structure of the body
It's a lot louder because turbulent flow is louder than laminar flow
Dubstep Shep
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Also, those are some really good points/analysis Dave. Thanks!
I'll be racing HPDE 3 at Hallet in June. Looking very forward to it. Unfortunately I can't make ecr tomorrow. Because my dang car is still in the body shop getting hail damage repair. 4 f-ing weeks I've been mustang less.
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It probably depends on the size of the dimples. I definitely agree with you as regards passenger cars, but on race cars? Those guys are looking for every advantage they can lay their hands on.
Not really. You just have to have molds of the proper shape.
Weight is the primary factor, and could explain why you don't see it on passenger aircraft. I doubt it would make enough of a difference for a car, though...
Right, but in a way, that's somewhat contradictory. More sound means more energy being expended, which suggests reduced efficiency...
For race cars, I somehow suspect they don't care about loudness.
So if we stick our cars into a hailstorm, will they be faster afterwards?
Dubstep Shep
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True.
Yes and no. Depending on what part you're looking at and how its manufactured it could be much harder. If it's a stamped part, then yea the complexity isn't that much more.
I would gamble that for the same material and "strength" a dimpled object would add at leas 5-10% to the mass.
This has to do with the boundary layers.
In laminar flow, the layer that touches the object actually doesn't move. It never moves. The "noise" you hear is from the sliding and separation between the layers.
In turbulent flow, the air swirls and eddys. It causes buffeting and other issues because it isn't consistent flow.
For race cars, I somehow suspect they don't care about loudness.
I agree, noise isn't the issue there. But as any racer will tell you, raw performance ultimately takes a backseat to consistency of the performance. Laminar flow is consistent. The way the air flows and the forces it applies are predictable, and thus useful. Turbulent flow isn't really predictable, so it's less useful for things like wings or spoilers.
Think about when planes hit "turbulence." They rock back and forth and up and down. It's terribly inconsistent.
So if we stick our cars into a hailstorm, will they be faster afterwards?
Faster? I don't know. Maybe hahaha, though I somewhat doubt it. From everything I've seen they may get better highway mileage. That mythbusters experiment was VERY well setup.
SoundGuyDave
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No, yes, no, in order.
1) Less heat soak. Technically by going to a 2-piece rotor, you're actually reducing the ability of that rotor to dissipate heat. The mechanical coupling of the rotor hat to the ring is MUCH less efficient at transferring heat away from the ring, into the hat, and from there to the wheel, hub, spindle, etc. If thermal transfer was the only criteria, 2-piece rotors would be the worst possible choice, assuming the mass of the ring was held consistent. Generally speaking, though, the advantages of the 2-piece rotor (lighter unsprung weight, sometimes a more massive rotor ring) outweigh the extremely minor drawback in thermal transfer. You could also make the point that the LAST place you want heat going is into the wheel bearings.
2) Less chance of warping. Yes, with a full-floating design, the hat isn't actually bolted to the ring, it's retained by side-loaded drive pins, which easily and conveniently allow for the differential in expansion rates between the iron ring and the aluminum hat.
3) Reduced knock-back. I'm not sure you're really grasping the mechanical cause of pad knock-back. Imagine a front view of the suspension, with a cutaway vertically through the centerline of the spindle, perpendicular to your axis of view. In other words, you can see all the guts exposed looking towards the rear of the car. The spindle itself is pressed into the back-side of the steering knuckle, and has a hub cartridge slipped over it, retained by a nut. That hub cartridge has a pair of bearings inside that ride on the spindle, as well as the flange with wheel studs on the outside. The rotor hat slips over the studs and sits flush against the hub flange. The (fixed Brembo GT500) caliper is slipped over the rotor and bolted in place onto the knuckle. If the rotor were to rock left-to-right (aka in and out) that would apply leverage against the pads and push them (and the pistons) into the caliper. The rotor, however, is unable to do that; once the wheel is slipped over the studs and tighened down with lugnuts, it very effectively sandwiches the rotor between the hub face and the backside of the wheel, locking it firmly in place. Now imagine that you had bad wheel bearings, which allowed the hub to wander in an arc, instead of being held completely true to the centerline of the spindle. That would cause knock-back, as the rotor (and wheel) are free to rock back and forth a bit, and since the caliper is no longer fixed relative to the rotor face, you have leverage against the pads. In the case of load-induced knock-back, the mechanism is quite different. So much load is being applied to the tire and wheel (whether from cornering forces or a berm/pothole impact) that the spindle (pressed through the knuckle) and/or the area of the knuckle surrounding the spindle flexes slightly. This has the same effect as bad wheel bearings, in that the rotor shifts position relative to the caliper, with the same results. The only thing different is the mechanism behind the shifted position. Even though the spindle is machined steel, and the knuckle is cast iron, they still can and will flex slightly given sufficient load.
There's just no way for a non-failed 2-piece rotor to have the ring go out of plane with the hat. There are a lot of advantages to 2-piece rotors (cost NOT being one of them!), but eliminating pad knock-back just isn't one of them. FWIW, I have a pair of Girodisc 2-piece rotors sitting in my living room. They're slightly larger than the pizza I had delivered for dinner...
Most body panels are stamped, especially on cars. Dunno about aircraft, but I wouldn't expect it to be much different.
Well, except for the really new, exotic stuff, where they're doing carbon fiber shells and such. Jet aircraft manufacturers pull out all the stops for their upper-class models. At a price of tens of millions per unit, that's not much of a surprise...
Strangely enough, the same is probably also generally true of the larger airliners.
Well, actually, I would expect an increase in the strength (stiffness) with a proper design of the dimples. But yeah, it'll definitely add to the mass otherwise.
Right. But mixing, which happens with turbulent flow, induces friction (collision of molecules at sharp angles), which thus increases parasitic drag. Hence, I have a hard time believing that turbulent flow is inherently more efficient (i.e., disspates less energy) than laminar flow is. Indeed, all of the (basic) reading I've done thus far suggests quite the opposite, and says that the golf ball is a special case, a situation in which the effects of delaying the boundary layer separation are greater than the increased friction due to the use of turbulent airflow.
So I would expect that whether or not dimpling the surface of a car would yield a more efficient car depends entirely on the shape of the car itself and, in particular, where boundary layer separation occurs.
True, but in this case, I wouldn't expect the turbulent flow to be on a scale large enough to introduce that kind of inconsistency.
That's because the scale of the changes in flow is large relative to to the size of the aircraft. For turbulent flow induced by variations in the skin, however, the changes should be small compared with the size of the aircraft.
Were that not the case, the flight path of that golf ball would be highly unpredictable. While that is certainly going to be the case with any golf balls I might hit, I suspect that has more to do with me than with the golf ball. But it does sound like a really cool excuse to use! "My shot would have gone into the hole if it weren't for all that turbulent airflow over the surface of the ball!"
Let's just say that I won't be volunteering my car for that experiment ...
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Dubstep Shep
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All good points KC.
I'm going to back out of this particular tangent at this point though. I don't wanna muck up this thread.
I'm going to back out of this particular tangent at this point though. I don't wanna muck up this thread.
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