Re: Bike Handles Like Ass
author: Max McAllister
Honda did something very strange with the RC-51 that has spilled over into this bike. The fork springs have no internal preload on them. Ordinarily, a fork spring will have about 15mm of internal preload, even when the preload adjuster is backed all of the way out. When we measured the sag with a 165-pound rider, we were shocked to find 60mm of sag! This bike sagged more under its own weight than it should have with a rider on it! I typically like to see around 20mm of free sag, and 35mm with the rider on the bike. I can’t comment on the spring rate in this bike, because we simply couldn’t even come close to setting the sag.
This Showa fork has a new style of preload adjuster on it as well. It turns internally, and you have to count turns, much like an Ohlins superbike fork. This is not good in my opinion. It gives you no visual reference for your spring preload adjustment.
There was a more serious problem with fork than the lack of preload on the springs. This fork had horrific stiction. You can measure the stiction in a chassis while measuring sag. Find a fully extended measurement on the fork or shock. Then lift the chassis and let it settle gently (stuck up). Record this measurement. Then push down on it and let it rise gently (stuck down). Record this measurement. The distance between these two measurements is stiction.
5mm is good.
10mm is a sign something is wrong.
15mm is a sign something is REALLY wrong.
2mm is good
5mm is a sign something is wrong.
7mm is a sign something is REALLY wrong.
This set of forks had 15mm of stiction. Frequently, the lower fork legs will be in a slight 'V' from changing the tires and not aligned properly. I tried to correct this on the CBR and couldn’t make the stiction any better than 15mm.
I do theorize that since the springs were nearly 'dead' in the fork with no preload on them, that this may have contributed for a few millimeters of the stiction. But that doesn’t explain 15mm worth. This was the worst stiction I have ever measured in a production fork. Ordinarily, I recommend that a racer with this much stiction not use the forks until the problem can be found and repaired!
A spring without preload on it will also 'feel' dead. I pinned the preload adjuster and it didn’t really do much. I moved the rebound adjuster (top of the fork) to 2-½ turns out from full hard, and set the compression to 1 turn out.
The CBR954RR has the Honda 'HMAS' damping in it. This fork had no feel in it at all. It didn’t feel like it had springs, and it didn’t feel like it damped. It is the worst feeling fork I have ever pushed on. It is by far, the weakest link in any of the suspension systems of any of these bikes.
The shock spring seemed to be of a decent rate, just slightly soft, and we got decent sag on the #4 ramp position. It had way too much low-speed rebound damping, and we had the screw (bottom of the shock) set to 3-½ turns out. The compression adjuster didn’t really do anything, which is typical of Showa street shocks. We set it to ½-turn out.
This bike was wobbly when going fast. This is easy to explain. While the geometry numbers are very conservative for racing use, the problem lies again, in the fork spring preload. This bike has so much sag that when the bike is riding down the road, it is more than halfway through its travel. At this point, the bike has very little rake or trail. Without trail, you have no stability. So basically, this bike is going to take a beating in the magazines for not having about 10 cents worth of additional spacer on the fork spring! Unbelievable.
This condition will be severely aggravated under braking, as the fork will only be working in the last 15% of the travel, which is taken up by a stiff hydraulic bottoming mechanism. For sport riding, this is not a good situation, since there is no functional travel left to absorb bumps in the pavement.
To upgrade this fork, you will need to change the compression and rebound pistons (to remove the HMAS) and have appropriate springs installed for your weight. (With the right preload, of course!
The frame does have a ride height adjuster, which will be great for some street riders. It is not at all convenient for racing use. For track use, regardless of how the shock is built, the compression adjuster will just be useless, so I suggest you change to an aftermarket shock.
With the right fork springs installed, you could raise the back of the bike up to gain about 1.5 degrees of swingarm angle (to get to about 12.5 degrees), and leave the forks at the stock height. This would bring the rake and trail down tighter, and make the bike just about right for racing use.
Note: You definitely would not want to raise the back of the bike before fixing the fork springs!