Project Miatabusa Part 5 - De-Powering the Steering Rack
By Dave Coleman
That's right; this installment of Project Miatabusa has nothing to do with putting the engine in the car. This may be the only Miatabusa strory where I don't mention the 11,000 rpm redline at least 3 times. Don't get used to it.
Motorcycles don't have accessory drive belts. The alternator is driven directly off the nose of the crank, the water pump is driven internally, and there's nothing else that usually needs power. No A/C, no power steering. We have some plans for making a drive belt system for Miatabusa 2.0, but this first one won't have, or need, any belts.
That means it's time to de-power our steering rack. De-powering racks is pretty common in the Miata world. The cars are light enough not to need power steering, and power steering was actually an option on early cars. Still, depowering is not a change a normal Miata driver takes lightly. Manual steering gives a lot more feedback, but the change in effort is drastic enough to really change your driving style. The effort causes you to slow down your inputs and become a more deliberate driver, and the feedback makes it easier to drive right at the grip limit. But the weight also makes it a lot harder to keep up with a wagging tail, especially with instincts honed on power steering. With a power-steering Miata, I'm sideways all the time. With manual steering, I drive clean.
Whether you'll like manual steering depends not only on how much time you want to spend sideways, but also what kind of manual rack you use. There are three basic options.
The factory manual rack seems like the obvious choice, but I'd argue it's actually the worst. To keep parking effort reasonable, Mazda used a slower steering ratio on manual racks. Yes, the effort is lower, but the slow steering really adds an unwelcome bus driver element to any twisty road.
The quick-and-dirty de-power involves simply removing the pump and looping the power steering lines on the rack. This is effective, but it's less than ideal for two reasons. First, there is a fair amount of friction in a power steering rack, thanks to all the seals and the hydraulic piston. The hydraulic system is still in place, and every time you turn the wheel, you're pushing air and residual fluid through the system. This is why you have to loop the hydraulic lines. If you just capped them, you'd basically hydro-lock the rack. The piston seal is weak enough that the steering wouldn't actually lock, but it would really, really suck.
The friction isn't that big a deal, and if it were the only problem, there would be no reason tear down the rack and fix it. The real problem with the quick-and-dirty de-power is slop. The valve that controls power assist relies on deflection of the pinion shaft to act as a torque sensor. That deflection is still there when you de-power, and you can feel it most right at turn-in, or as you transition through some esses.
This quick-and-dirty manual rack is actually what we run on the Frankenmiata. The conversion is so quick and easy, we did it in about a minute in the middle of a race after a power steering fitting cracked and covered the windshield with oil. Until then, we were afraid manual steering would cause too much fatigue in an endurance race. Turns out it was no problem at all.
The third, and best, option is the one we're taking. Tear down a power rack, remove all the power-steering nonsense, and weld the spool valve solid. Here's how you do it:
First of all, Mazda has used several different suppliers for steering racks, and all of them come apart just a little bit differently. The objectives are the same no matter which rack you're de-powering, but many of the steps to accomplish those goals might be different. This one is a JKC rack...
OK, so the first thing you want to do is pull off the driver's side tie rod. You can leave the passenger's side inner tie rod in place through the whole rack teardown. To get the driver's side off, you'll have to bend back the lock tab with a screwdriver.
Now, when I say that's the first thing you want to do, I mean the FIRST thing. DO it while the rack is still in the car. On the bench, it's a pain in the ass. On the bench after you've already removed the pinion shaft, it's an even bigger pain in the ass.
The inner tie rod end is threaded onto the rack, usually with lots of Loctite, and if there's a whole car attached to it, it's reasonably easy to get a big wrench on it and turn it. Without the car, the whole rack housing spins. Without the pinion shaft, the rack spins inside the housing. Learn from my mistake.
Next you'll want to remove all the hydraulic lines and throw them in a bucket. You won't need them again, but you might want to use the ends as plugs. I'll get to that in the end.
Now you wanna pull out the support yoke dongusmajiggy. Start by loosening this massive lock nut.
Then just unscrew this framus here.
The giant spring behind the support yoke lets you use the adjuster to change the load on the rack without accidentally clamping it so hard you jam it. Needless to say, this would be a good time to start organizing all the stuff you take apart so it goes back together in the right order.
The support yoke itself is this little shoe that presses against the rack. Its main purpose is to sit opposite of the pinion gear and hold the rack in place when the loads on the gear teeth try to push the rack away from the pinion. When you put the rack back together, the adjustment of this shoe is critical. Too loose, and the teeth will be damaged. Too tight, and the rack will feel sticky. Follow the adjustment instructions in the service manual as a starting point and then fine tune until you're happy. We'll re-visit this once the car is actually running.
Steering friction is a funny thing. Generally you don't want much of it, but just a little friction does make it easier to hold a smooth line through a corner.