What has happened so far…
We left the last installment with me replacing my old clutch basket and hub in an effort to stop the Busa bull ride. Read Part 1
The old components had definite wear on the hub due to the steels digging into it from hard launches. (Hard by my standards, someone like Keith Dennis may beg to differ.) Also on the clutch basket there was wear from the tabs on the fibers slamming into the basket.
Grooved hubs, baskets and warped steels
The bucking I described in the first part of this article could be caused by the aforementioned findings. The grooves could be keeping the fibers and steels from merging smoothly together as you release the clutch. If any of the steels hang up in a groove the bike may lurch as soon as they snap free of the groove and slam into the fiber. At the very least it would seriously hinder your consistency in releasing the clutch smoothly.
Having warped steels could cause this also; as you are slipping the clutch and if any of the steels are warped they would start to catch the fibers on the peaks of the warp causing surges in power until the clutch was completely released. This is why it is important to check your steels periodically on a flat surface. If they rock, they are no good.
Both of these situations probably would not cause the problem to the degree of severity I was experiencing; although, they were a contributing factor.
Performance Clutch Baskets
The stock clutch basket is good up to 210 hp or so. This seems to be the general consensus if you talk with engine builders and performance parts suppliers. Anything over that and you are running into the likelihood of the basket literally coming apart. When one does come apart it is not a pretty picture. It leaves metal in your oil pan and can get between the gears of the basket and the crankshaft.
For applications above this range you will want to invest in a billet clutch basket. While there are many companies who manufacture billet clutch baskets, the one in this article was supplied by Hays Machineworks for my use.
A billet anything is made from one solid block of material. When machined correctly there are theoretically no weak areas or stress points. Therefore the billet item is less likely to come apart.
In the case of the Hays basket there is a feature built in that allows you to upgrade to a multistage clutch in the future; also built by Hays which is driven by engine speed as opposed to rear wheel speed. It also is designed with “Surface Area Reduction Technology” to help the clutch fibers move with less friction.
On the back side of the basket you will notice there are a series of springs. Some heavy and some lighter. These springs are a form of buffer to help soften the shock in the input shaft and crankshaft of a hard launch. Under normal riding circumstances they probably will last the life of the motorcycle. Yet in a drag racing application they will get abused like a temporary laborer.
Over time, the springs become compressed and start to loosen up in the spring pocket. This allows the basket free play to snap into the springs instead of being cushioned and therein lies the heart of my bronco ride problem. I had anywhere from 1/8 to 3/16 of an inch play in the spring pockets of my clutch basket.
As I started to ease the clutch out, the clutch basket snapped forward that distance slamming into the springs. Next, as I let the clutch out more, the springs were more than likely already bottomed out from the momentum and the bike bucked forward from having no cushion left. When I eased the cutch back to try and soften the launch the springs released starting the whole sequence over again.
It took three years of drag racing for this to happen with my clutch basket.
Talking with Tim Hays about this issue he had to say, “We have tightened up the spring pocket for this problem.” Tim mentioned it is important from a performance standpoint to not have play in this area as it will adversely affect the consistency of your launch.
The hub takes a beating from the steels slamming into it. Overtime they become gouged. I attempted to stone them smooth in a last ditch effort to make the bike launch smoothly. It was a 3 hour waste of time and effort to file each cog.
Hays Machineworks sent me a billet hub to evaluate for this article.
A billet hub has two major advantages:
- The Back Torque Eliminator is no longer needed as the hub fits directly onto the input shaft.
- They are stronger then the factory hub.
The Hays Machineworks hub incorporates their “Surface Area Reduction Technology” to the design. This allows the steels to move more freely on the hub.
Pressure plate and single-stages
This is an area that makes or breaks a winning run down the track. Whether you are ET, heads up or grudge racing, if you do not slip the clutch just right you can blow a race or worse yet end up on your back.
If you do not use a single stage on a 230+ horsepower motorcycle with stock clutch springs you will notice pretty quickly that you are bouncing off the rev limiter and your back tire seems to be spinning all the time. In fact it is you driving through the clutch. The way to fix this is by increasing the static pressure of the springs by going to thicker ones. The higher in static pressure you go the more difficult it is to pull in the clutch and you run into the possibility of cracking your hydraulic clutch housing.
The first level of help in this area short of a Single-stage set up is the Brocks Performance Clutch Cushion. By its “simplicity carried to the extreme” design it helps the release of the clutch, lessening your margin of human error. It does this by adding a small amount of slippage as the pressure plate comes down on the clutch pack.
The next level is the Single-stage lock up. It allows you to use lighter springs for your static spring pressure on high horsepower applications.
A single-stage uses centrifugal force to apply weight to the pressure plate. As your RPMs go up your clutch pack gets compressed more. This allows you to throw the clutch off the line and not worry about trying to slip it as the single-stage does that for you once set up correctly.
It is called a single-stage because the arms are controlled solely by their weight which is typically adjusted by the addition or removal of washers. Adding weight causes the device to lock-up earlier and removing weight causes it to lock up later.
With a single stage the goal is to get the first part of your launch smooth by allowing the clutch to slip some. I started with 6 springs that were slightly heavier and longer than stock. This turned out to be too much static spring pressure, when I threw the clutch the bike bogged. Next, I removed three of the springs and added 3 stock Hayabusa springs. On this launch the clutch noticeably slipped as the engine rpm’s went way up. Once the weighted arms of the single stage came on I took off. This resulted in a 1.8 60’ and 9.50 pass.
Next, I took one of the Hayabusa Springs out and added back one of the heavier springs. This resulted in another noticeable slip of the clutch although not as bad as the first one. On this run I had a 1.7 60’.
Brock Davidson had sent me some spacers to use when adjusting spring pressure a while back. I used 2 of them to put on the two Hayabusa springs. This was the trick. I threw the clutch and ran a 9.21 with a 1.585 60’. The bike left smoothly with a slight power wheelie. I did screw up and short shifted 2nd and 3rd on this pass.
After letting the bike cool, I went back and made another pass. This was a 9.03 with a 1.61 60’. My fastest pass ever. On this run my 60 foot was off because I did not go to wide open throttle as quickly as I should have.
I have to admit, the single stage is nice and it is easy to use. Yet I can already see there is more room for tuning. As I have it set up now it is on the edge of being set right, I think I can lighten up a hair more on static spring pressure as the front wheel wants to come up slightly on the hit. This is the type of thing the Multi-stage set up allows you to tune for.
Coming up next…
The Hays Machineworks convertible Multi-stage clutch and how it works.
See ya at the track…
- Paul Cavanaugh