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Disc brakes can sound almost too good to be true but then the cracks start to appear.
Maintenance is expensive but the market is growing.
If the driver of a disc-braked truck coming down a steep hill should miss a gear and find that the brakes are his only salvation, the discs might get red hot but they will still work.
That resistance to fade is the ultimate appeal of disc brakes but they have other inherent advantages. Expansion does not bring more pedal travel; they get rid of heat quite quickly; they do not grab but have rapid response with a smooth and progressive action—and they are not prone to steering reactions.
So convinced are vehicle designers of the disc brake's merits that they are increasingly committed to their fitment to rear as well as front axles. Disc brakes also have a hi-tech image: a chassis manufacturer might feel obliged to fit discs to demonstrate advanced engineering.
Many hauliers are less impressed. They complain of short lives of friction pads and discs, component failures and high parts prices. A maintenance man sums up this attitude: "Discs might be a salesman's dream but certainly not a fleet engineer's."
David Payne manages the fleet engineering at Silvertown, London for Tradeteam, the Exel-Bass drinks distribution enterprise. "With disc brakes you must get the maintenance right." he says.
His experience centres on hydraulically actuated discs—Lucas technology on Volvo FL6s, and Perrot on Leyland 45s.
In a year, Tradeteam trucks on urban distribution cover up to 40,000km, Following cases of calipers sticking, brake fires, cracked discs and poor parkbrake performance, Tradetearn follows a
strict inspect-andstrip routine: rear discs are checked every six months and front discs are checked annually.
During these inspections everything is cleaned and pads are changed when they are worn down to 4-6mm (pad changes are needed during most checks). The park-brake has to be set to within 1 mrn or it will bind; any split or hardened rubber boots are changed. Calipers are checked for movement and discs are examined for cracks. This strict routine makes financial as well as safety sense: a new caliper costs about 4:600 and a disc £130.
The rear discs have to be watched particularly closely. "They glow cherry red in service," says Payne. "Not only are the discs shrouded by 17.5in twin wheels but also the four-bag air suspension blocks air flow." Dot 4 brake fluid handles the heat without boiling, "but mechanics must wear gloves," he warns, "because it can harm skin".
He believes fitting discs all round is a mistake, citing maintenance costs which are three times those of Leylands with front discs and rear drums—and four times those of Volvos with air-drums all round (in a larger wheel, to be fair).
Superficial cracks in discs are tolerable, he believes, but if a crack extends right through the disc it must be changed. Changing pads early extends disc life, and he is suspicious of long-life pads, believing that they can be deficient on friction, co-efficient and too hard to conform to irregularities on the disc, leading to hot spots on discs and cracks. Payne buys Duron pads, which cost £36 per axle.
Pressure modulators
Disc problems have eased on Tradeteam's urban artics since dispensing with loadsense pressure modulators on the trailersemergency-stop grip being assured by the anti-skid system. A lengthy investigation revealed that in the low pressure check braking that predominates in service (83% of applications were found to be below 2 bar) the trailer's air-drum brakes were hardly contacting; the disc-braked tractor was doing all the work because the trailer's loadsensing valve choked the air flow at low pressure.
Once the trailer brakes were made to do their bit, the work load on the tractor's discs declined, bringing lower temperatures and much better reliability.
This experience is contrary to that on airbraked combinations grossing above 28 tonnes. Almost always, chassis engineers taking a cautious attitude to disc brakes on tractors say that they resist the move until trailers are also discbraked. They believe that at low air pressures the responsive disc brakes on the tractor will be overworked. They are mistaken.
What usually happens is that trailer brakes start working at between 0.5 and 0.6 bar while tractor brakes do not usually contact until 0.8 bar—indeed, some continentals need 12 bar. Faster reacting discs would actually give better balanced braking of tractor-trailer outfits.
The unsatisfactory matching is not helped by the loose type-approval requirements that allow one-bar threshold pressure. British efforts to cut this down in European
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Community sessions have failed because of opposition from the Germans, who see a technological answer in electronic braking control. Straight air-mechanical calipers are robust and relatively trouble free, if somewhat clumsy and heavy (they are no lighter, and often a bit heavier, than equivalent drums). Still there remains the expense of doubled frequency of relining and the doubts about the thermal stresses between the very hot and merely warm parts of discs.
Research done by TeeN (parent company of Duron and Ferodo) reveals a critical relationship between the composition of a rust-iron alloy and the design of the spacing between two halves of a ventilated disc—and then the interaction with the pad material.
The metallurgical task is to use an iron that is rich in carbon and conducts heat well. The carbon works best when in the form of long flakes of graphite, preferably with the flakes running transversely. This ensures efficient transfer of heat from hotter to cooler. Not much attention seems yet to have been paid by foundries to casting techniques that might stimulate this process.
In the meantime, metallurgical research indicates that alloys work best when they are rich in carbon (meaning 3.5%) and with more than 0.5% of copper to restore conductivity diminished by incorporating 0.3% of chromium to give harder wearing and good hotstrength. Then the resistance to cracking is typically raised by between 60 and 9000.
It was by such close attention to the metallurgy that Iveco cured disc cracking on its EuroTech trucks. The salutary point for operators is that if the alloy composition is so critical, much cure or knowledge is needed when buying replacement parts. Alternative suppliers of will-fit discs and pads need to provide some convincing technical information for
astute operators who want to avoid trouble.
Unfortunately no independent technical institution is appraising parts quality, and only big operators can probably afford to get metallurgical or performance comparisons done. Risk-wise engineers therefore buy from original-equipment sources. but the temptation to do otherwise persist; because the prices are so high. For example. P&_0 pays £186 for an original disc and £90 for a set of pads for heavy-duty brakes. Logically, original equipment suppliers ought to be able to undercut the will-fits.
Cost reductions Cheeringly, cost reductions are in prospect. Disc-brake production is about to accelerate smartly, bringing economy of scale. At Lucas, production engineers have been working to such good effect on the big airmechanical Mk3 disc brake (for which the prime customer has so far been Renault) that about 20% of t le cost will soon be knocked out of it.
The latest ■ ersion has a load-sensitive adjuster that cuts out when clamp-load is
applied by the caliper. This means fling clearances, with faster resf fuller use of actuator travel. Besic version with a transverse actuator developed. This is compact enoul rear installations even when air st close by. Come autumn's Hanover sl and Mercedes-Benz will have disc round.
The explosion in demand should the economics of disc brakes. The rate of commercial-vehicle disc Lucas's South Wales factory has b 20,000 a year, but it is expected to b that before long.
By the end of the century disc bra cost no more than drum brake advances in materials and finishes ly promising better reliability tf brakes with longer lives. Perhaps di yet be a fleet engineers dream after LI by John Dickson-Simpson