By John Hale, Driver of Big Jim Dunn’s “Oberto’s” F/C and John Hale’s “One Bad Texan” NFC
Throughout my years of involvement in Fuel Racing I’ve heard it often said that the Clutch is 90% percent of the way the car goes down the track.
Let’s take a look inside the clutch with a focus on the discs and floaters that make up a clutch pack.
Clutch packs are made up of a number of discs and floaters; a five-disc clutch would have four floaters and a six would have five.
Clutch discs used in fuel cars today originated from heavy equipment wet clutches and are made of sintered iron. Through its involvement in racing the ingredients and the way the batches of discs are made and who makes them have come to play a big part in the tune-up of a car as well as how the clutch works with the motor. The discs are dated and have a Rockwell number on them indicating the hardness of the disc. Lower numbers are softer and less aggressive where higher numbers are more aggressive. Larger teams will usually check them and put their own numbers on them. Depending on the batch, sometimes the same number can be more aggressive than the same number of another batch. Oftentimes a team will order a year’s supply in advance so they don’t have to adjust to a new batch of discs’ midyear and can stay consistent with their clutch program. A disc may be used up to 3 runs before it is too thin for use.
Floaters are flat steel plates that are made usually of a boiler plate type of steel and are a parallel ground to make sure both sides are flat and consistent. Floaters have grooves cut in them called cutters to facilitate and control the wear of the discs they are mating with. These cutters not only cut material from the disc they also channel away the dust. A lot of times this dark dust can be seen trailing behind a car when it leaves. Anywhere from 1 to 6 grooves can be cut into each side of the floater with between 3-6 being the most common in the pro ranks today. The floaters see so much heat that they’ll usually warp and are not worth the time to cut them so they are discarded after each run.
Crew chiefs will have the clutch guy make up packs using a combination of discs and floaters. Clutch packs will contain one or two new discs and then several used ones; maybe a one run and two thinner discs. The CC will insert one new aggressive disc if the track is good, say on a night run or he might take a chance and put two aggressive new discs in to get the car moving early. If the track is not as good he’ll go with a softer disc that he knows will get down the track and not smoke the tires just to be safe.
Clutch pack wear is measured after a run to see how much the discs wore on the run. Crew chiefs will have a number anywhere between .070 and .150 that they are looking for and want to stay within say .020 of that number. Some nostalgia fuel cars only wear .020 total. If the clutch is not wearing enough for the levers to come in properly the CC will put in a floater or floaters that have more cutters in it to wear more. Just the opposite for too much wear.
The pack can be responsible for making the clutch too aggressive which pulls down the motor causing engine damage. Also if the clutch wears too much and takes the load off the motor it can cause the motor to drop a cylinder, or cylinders and hydraulic the motor. This usually results in a big fire ball as the process tries to lift the head off the motor.
These packs, whatever the disc and floater combination, are responsible for the car going down the track or up in smoke and are one of the most important components of the cars we see going 330mph today.