McLaren: Adjustable Rear Brake Ducts

With the switch to Pirelli tyres, their rear tyre construction has needed a lot of care in managing degradation. This drop in tyre performance happens when the tyres drop out of their operating temperature window, and this can come from the tyres running too hot or too cold. As the preceding RenaultSport article explains managing rear temperature takes a lot of effort and understanding. McLaren have been active in understanding this problem and over the past year have developed an innovative method of controlling tyre temperature via its relationship with Brake temperatures. It’s come to light in the past two races McLaren have an adjustable brake duct set up and this can have an impact on tyre temperature.

Brake discs (red) visible inside the Brake Duct

F1 carbon brake disk temperatures can peak at over 1000-degrees centigrade. The discs being 278mm diameter inside a 305mm wheel means that there is little space between the two and heat inevitably passes from the disc into the magnesium alloy wheel. In previously years this was avoided to try to reduce heat conducting into the tyre, but McLaren have found a way to harness this.

By altering the flow of heated air coming from the periphery of the brake disc, the amount of heat passed into the wheel and tyre and can be altered. Already teams tune brake cooling with different inlets scoops, but these tend to stay fixed from qualifying onwards (wet races excepted). If the team want to alter brake and thus tyre temperatures during the race then normally there’s no path open to them. However McLaren have fitted an adjustable window in the rear brake ducts. A mechanic can adjust this in the pits to tune the brake and tyre temperature to suit conditions. Depending on the wording of the Parc Fermé rule, this could also be altered on the grid as one of the areas allowed to be changed is brake cooling blankings.

The heated flow from the brakes (arrowed) exits around the brake duct

To do this McLaren have altered their brake cooling design from most other teams. More typically the round brake drum cooling ducts exit the brake flow through the rounded outer face of the duct; this passes out through the wheel spokes. McLaren’s brake discs vent their through openings in the outside of the brake drum, with its outer face closed off from the disc. So all the heated brake flow passes between the duct and the wheel before exiting through the wheel. To accommodate this flow, McLaren’s wheel spoke arrangement has been altered. The Enkei wheel features 29 drillings around the face of the rim, with the more conventional spokes positioned inside these drillings. Brake flow is directed through these drillings and relatively little passes through ten holes between the main spokes. With this set up the heated brake flow has far more contact with the wheel, both as it passes to wards the spokes and even the spokes themselves have more surface area to absorb heat from the brake flow.

The heated brake flow passes through the outer drillings (red)

Normally teams tune the brake cooling via the inlet, taping it over or changing to a different sized inlet scoop. With the McLaren system the inlet scoop remains largely the same, but brake cooling is tuned via a threaded adjuster (Yellow in the following diagrams) moving a flap to either open or close the openings in the brake drum. This is analogous to the cars engine cooling, the inlet tends to remain the same and the outlet area is tuned for optimum cooling. A larger than required duct will create extra drag, but I suspect the operating window the adjustable duct is within quite a small range. Probably smaller range than switching to the next size brake duct inlet.

The flap (grey) is closed to cover up the cooling outlets

When the flap closes the opening, more heat is retained within the duct for hotter brakes, but cooler tyres.

The duct is open to expose more of the cooling outlet (red)

Conversely opening the flap to expose more duct exit area, brakes run cooler and more heat is passed into the tyre.

Also, see these images comparing the adjustable brake ducts of Hamiltons car (right) and the unadjustable ones on Buttons (left). via Russell Batchelor XPB.cc.  The silver coated section inside the brake duct on the right, is the adjustable part.  This semi cylindrical panel winds in to open up more cooling outlet area.

We have seen the adjusters fitted to the rear brakes in Bahrain, but they’ve reportedly been on the car since China and F1 insiders tell me they were used even last year. I’m also told the front brakes are adjustable too, but I’ve seen no evidence for this. One thing is clear, these are quoted as secret devices, but most rival F1 engineers know about them!

I understand the brake ducts can be adjusted from a single point near the fuel filler flap, so I presume cables (Grey in the preceding diagrams) run from the threaded adjuster back to the middle of the car. At a pitstop the mechanic can adjust the brakes with a tool accordingly.

See this picture from the Spanish GP (via F1talks.pl) shows the adjustment in operation.  Cables from each of the adjusters meet at the fuel flller flap and the mechanic, who is usually there to hold the car steady and clear out the airbox, can adjust the ducts with a pre-agreed number of turns on the hand tool in the adjuster.  As the adjustment is done via a mechanic it is a legal change to set up, allowed once the race starts, but not during qualifying or whilst the car is in Parc Fermé. When the car is stationery at a pitstop, the system is not considered moveable aero.

Changing the brake ducting will alter the amount of brake cooling, opening the duct will allow more heat to escape and reduce brake disc temperatures and vice versa with closing the ducts. Adjusting the rear brake temperature may be the sole reason for this season. With changing tyre balance and KERS usage the rear brakes have been prone to overheating. But the more likely benefit is the effect of the brake heat altering tyre temperature. As the brake heat passes through the smaller set of drillings in the wheel, this has a greater surface area than the more usual 8-10 spoke wheel; this allows more heat to transfer into the wheel. Heating the wheel will transfer heat into the tyre; this will be useful when the driver is struggling for tyre temperature. The contrary is reducing the heat transfer into the wheel to reduce tyre temperature when the driver is struggling with heat related degradation.

Of other teams are able with their current wheel and duct arrangement to alter tyre temperatures via heat radiated from the brakes, then this will be an easy modification to make to the car. However many other F1 Engineers suggest that they find little effect of brake temperatures altering tyre temperature, making the solution unattractive to them. So it’s not clear if this is a must have solution or other teams are able to tune tyre temperatures with more conventional means. As yet I have not seen any other team run these types of brake ducts.

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22 thoughts on “McLaren: Adjustable Rear Brake Ducts

  1. So presumably, this system could also help to keep tyre pressures in check? Do you think at a pit stop they might put on tyres at a lower pressure in order to run the ducts open, heating wheel and tyre and raising pressures?

    • I was thinking the same thing: P1/T1=P2/T2 so P2=P1 x (T2/T1). What sort of temperature differential would they be aiming for?

  2. Intersting as always Scarbs.

    Thoughts…..perhaps they should use a bimetalic spring / device in there, and preload it with the adjuster.

    So, if the temperature gets too high, the spring collapes, thus opening the duct all the way. Alos referred to as a fuseable link.

    Cheers

    Greg

    • I think that would partially defeat the purpose? Isn’t it meant to be controlled, not self-adjusted? The whole point is to keep more heat in at certain times, so you don’t want it self adjusting away from whatever “heat retention” setting you’ve put it at.

  3. Pingback: McLaren majstruje przy hamulcach | F1TALKS.PL - BLOG F1, FILMY F1

  4. There was talk on the BBC about Jenson changing brake compound and setup before FP3 and what a large change it was – yet Jenson said when asked about the difference between friday and sat that he hadn’t changed much. Can you shed any light?

  5. I dont get why mclaren when DRS is engaged their speeds don’t cross 300+ barrier.. Have you guys noticed.. For the last race at the straight with vettel battling jenson button.. vettel topepd out at 313km/hr while jenson button was about 298km/hr. Same with Lotus and williams.. I think Mclaren really need to look into this because LH could have improved his position so much if he had more pace to the car…

      • Yes, I suspect they are gearing for just below 18,000 rpm without DRS, so when DRS is activated they are hitting the rev limiter. That would seem counter-productive to me.

      • I think it has to do with their anticipated position… when they are fairly confident that they will be on pole, it makes more sense for them to be gearing as they have. If it weren’t for the penalty Lewis received, they would have had him optimally geared to be at front, in clear air – not using DRS.

  6. The convective/conductive heat transfer from brake rotor to air flow to wheel to tire carcass is quite a complex mechanism, and it is also a process with highly dynamic conditions. While the idea makes for an interesting topic of discussion, I don’t know if I truly buy into the fact that McLaren is able to control their tire temps using this approach.

    It is reasonable to believe that the brake rotor air ducts and wheel baffles kept the wheel bearings from overheating and prevented excessive heat transfer from the cooling air to the wheel ID. But that’s not the same thing as using an adjustable vent flap on the wheel baffle to precisely control tire carcass/tread temps.

    Just my armchair engineer opinion though.

  7. This is very a good explanation but i think something is missing … Why to bother changing the cooling setting only when you pit, when you can design it to be changed from the cockpit (steering wheel) whenever the driver wants to change it during the race ?! Just like the differential settings, the engine mapping, brake bias and so on. This way you can have full control of the system at every time.

  8. Pingback: McLaren: Adjustable Front Brake Ducts | Scarbsf1's Blog

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