October

 

Three wheels on my wagon

 

There is one thing all cars share, whether it’s the Ferrari that Michael straps himself into on a Sunday afternoon or the car Auntie Maude goes shopping in or any of the Bugatti cars that are our chosen subject. Strictly there four things, the little patches of rubber that all of our efforts at controlling the beast are fed through. When you think about the dynamics of a ton and a bit of Type57 downhill on a mountain pass with the only thing between success and disaster are four patches of rubber, less than the size of the magazine you’re reading, scary isn’t it? It follows that keeping those patches of rubber in good condition and attached is extremely important!

 

Starting inboard and working out, the kingpins should have an interference fit of 0.001” in the axle, this requires a press loading of about 3 tons to assemble, the kingpins should be made of EN36 case hardened with the core refined to 55 tons. On the race cars I have seen kingpins bend in use, and I have seen pictures of crashed cars where they actually broke; these are highly stressed items and should be checked regularly.

 

Stub axles seem reliable, we have never had a cracked one, but crack testing (Magnaflux not dye penetrant) when a car is in bits can’t hurt. Wheel bearings are obvious, make sure you use good quality products. We run the “sealed for life” type, never reuse the tab washer. If the stub axle has worn, we have successfully had the bores and even the OD of the bearing plated up using electroless nickel, the deposit can be applied very accurately and there will be no Hydrogen embrittlement concerns.

 

This leads us on to the hub assembly. There are two systems employed within the majority of Bugattis. (I’m not talking about oddballs like the Type 59, if you’re fooling with one of those you shouldn’t need telling). The GP system that is common to Type 43’s and all the alloy wheel GP cars, secondly the touring set up. Probably the most important 5mm screw in Bugatti’s resides in the front hub of the GP car. This screw locks the bearing retaining ring to the hub body; if the ring comes undone the wheel comes off. Proper assembly procedure for this ring is as follows: on the bench, fit the rear hub bearing and screw on the ring tight, not too tight as you have to get it as tight on the car and this is not so easy. Set the assembly in the drill press and drill a 4.2mm hole through the locking ring and hub in the centre of the groove provided. Remove the ring and the bearing, refit the ring, line the hole up and drill fully through with the 4.2mm drill. Tap the ring and the hub as an assembly (5x0.75). When you fit the hub to the car, line up the tapped holes and insert the correct length 5mm screw, head drilled for wire, to fully engage the threaded hole. Wire up the screw. Locking any other way isn’t good enough, no dimples or loose holes, two screws are good insurance but not strictly necessary. The bearing lock ring should clear the backplate bolt heads by a minimum of 0.060”, it’s the distortion of the assembly under load, touching the ring on the bolt heads that causes it to undo. We do the same on the rear hubs but this isn’t so critical as the function of the retaining collar is only to withdraw the bearing with the hub. The touring assembly and it’s delights were covered in the article on brakes however a slight wrinkle that I’ve seen since (on a Type 30) is the SOFT soldering of the retaining screws to the drum (we wouldn’t do it) which actually seems to work quite well.

 

Bugatti hub nuts and all other centre lock wheel hub nuts, to be fair, are self tightening. This is actually a complex process which involves the friction of the wheel on the inside of the nut as the (loose) wheel moves up and down in relation to the hub. If you put the hubs on the wrong side, this process loosens the wheels however tight you do the nut. I’ve seen it done, not just on Bugatti’s as the most notable experience was on an Austin Healy; luckily the only damage was to a shame-faced mechanics pride. Although the new hubs, wheels, nuts and time expended before the exasperated owner came through our door could well be included as collateral damage. The hub nut should undo in the direction the wheel normally travels.

 

Moving on out, we come to wheels; here we have two choices, wire wheels or alloys. Bugatti alloy wheels as they arrive from reputable suppliers need a little careful finishing before use. Firstly, the split rim variety; when removing the ring, be very careful to draw them off square as they break very easily. Invariably, the wheel screws protrude from the back of the casting, this area of thread then corrodes and won’t come through the casting or damages the soft alloy thread. Shorten the screws and use copperslip when putting them in, it will save broken screws and other damage later. If the rim is stuck on, don’t be tempted to inflate the tyre to push it off as when it does go, the shrapnel produced could well damage you. Patience and penetrating oil are the order of the day. Make sure the rim is drilled and tapped for weights. These threads are sometimes omitted from the manufacturing process, they should be carefully deburred because the tube is inflated against them. For the non-detachable rim Type 51 wheels, the balance weight holes are the major consideration but on these wheels you can use a rim tape to protect the tube. Something we do is to balance the wheel by metal removal before we fit the tyre as this reduces the amount of balance weights required. Turning the tyre on the rim to use the natural heavy spots to balance each other out also helps, watch the length of the weight bolts as you can wind them into the tubes. Balance is done statically as really the wheels we use are not wide enough to require dynamic balancing and the cones on the balance machines don’t hold the wheels well enough.

 

On the wire wheels: Careful attention needs to be paid to the ends of the spokes, as they can puncture the tubes, file them flush with the nipples and use good rim tapes. The pen test can find loose spokes, watch for corrosion in the rim and on the spokes as some cosmetic restorations paint over scrap wheels. The areas of the wheel that locate on the hub & nut cone should be locally stripped of paint as this wears out quickly in use, loosening the wheel. We also check run out and as a rule don’t accept more than 1/4”, on racecars 1/8”.

 

Tubes and tyres (like wheel bearings) are obvious; use good quality items with the right size tube for the tyre. We use race spec tubes mostly as punctures tend to occur at the worst moment possible and they are extra insurance. When mounting tyres, use lots of tyre soap and use the French chalk on the tube. Most of the punctures we see on the big cars are caused by the fretting of the tube where the chalk had been omitted. Inflate the tyre and make sure it seats evenly all the way round, even spin it up on the front hub and check the run out, here you might have to accept ½” as most tyres are not very round. The triple stud tyres from Julian’s Blockley Tyre Co are very good in this respect as were the Englebert and Michelins that unfortunately are no longer available.

 

Grease the splines and the hub nut and knock the wheels up; on alloy wheels, we have a spanner with a knocking pad to remove the possibility of damage. Pressure the tyre and you’re ready to go. Tyre pressures vary with weight of car and the use to which it is put; as a general rule the higher the pressure, the more grip the tyre will have up to the point where the profile is reducing contact patch. More pressure stabilises the tyre and stops it holding heat, hence higher pressures recommended for high speed cruising on the freeway. It does mean that on a T57 or T50/46, tyre pressures will be high by modern standards, high 30’s to mid 40’s psi. Low pressures and the tyre/tube will fret and punctures occur. Similarly, race tyres run high pressures although here uneven pressures in all four tyres is not uncommon, to balance handling and deal with the predominant high speed corners at a given track. In the racecars, it’s hot pressures we’re interested in to allow the tyre to hold it’s proper profile, so if it’s raining or cold, base tyre pressures should go up.

 

Murphy’s law often rears its ugly head in motoring; there are a surprising number of ways we can lend it a hand in this simple area of hubs, wheels and tyres.