Tech Notes: Hi-tech racing suspension

The Polaris racing support team get hands-on with ISOC
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Are all the high-tech shocks becoming so complicated that regular racers get lost with all the adjustment features? With both low- and high-speed compression dampening, as well as adjustable return dampening, will the regular weekend racer on the ISOC circuit be able to sort their sled out, or is that only a privilege for factory drivers with a professional backup team of technicians or engineers?

This may seem to be the case, when seen from the outside perspective of a spectator, but as we dug into the world of ISOC racing, we were actually very encouraged and surprised with the support that factories now provide for their “customer race sleds.”

■ Support before, during and after

The Polaris factory effort is a good example as we found out from Ben Hayes, who works out of the Polaris Race shop in Wausau, WI. Ben is the Polaris engineer in charge of making sure all the drivers get the best performance out of their Walker Evans Race Shocks. Here is how it works at the races: Before the season starts in Duluth on Thanksgiving weekend, the teams hit a full week of testing. The results and updates are then made available to the Polaris customer racers. Before practice even starts, Ben and fellow engineer Rick Bates - who comes down from Roseau, MN - go to every trailer with a registered Polaris racer and give them the latest factory recommended setup. Ben and Rick then watch the customer racers through all the practice sessions and make notes on the performances. Then they visit each trailer again with the feedback on their observations and to get the driver and the team’s opinion. This is when the work really begins with the sled tuning.

Not every driver has the same driving style nor the same skill level. In some cases, there may not be anything wrong with the basic factory calibration, but the racer may not be as experienced, so Ben and Rick become driving coaches instead, giving feedback on driving technique.

The other extreme is the hard-charging expert that thrashes through the base setting. The rookie may need a softer setup to use all the travel, while the expert driver may want to stiffen up the suspension action. That’s when each driver starts tuning to adjust the handling to work the very best for their own style of racing.
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Not your grandfather's IFS! Polaris sent us all the components that go into a Walker Evans Racing Shock. In front are all the parts that belong to the rod and piston assembly, and in the back, the main body with the piggyback high pressure reservoir.
Ben and Rick then spend a lot of time teaching the driver and crew how each circuit works and what circuit to adjust to get the desired result. A modern racing shock has a main body in which a piston mounted at the end of the rod travels through oil that is under as much as 300 PSI of pressure. The pressurized oil reservoir may be part of the body piggyback style, or connected via a hose and mounted away as a remote reservoir. On both sides of the main piston sit shim stacks, which are basically thin metal discs of different thickness and diameter. Each stack may consist of as many as 8-10 discs, which will give a certain preload over the passages that allows the oil to flow from one side of the piston to the other as the piston is pushed through the oil by the rod.

During the compression action, when the piston is pushed into the shock, the shim stack on the rod side controls the flow and dampening force. During rebound, when the outside spring forces the rod out again, the shims on the bottom of the piston control the rebound action. This main piston is calibrated to provide the major part of the dampening action. The shim stacks flex and open the quicker the piston travels and are therefore often called speed sensitive. On the early high-pressure Ohlin shocks, the only way you could change the calibration was to take the shock apart and rebuild the shim stacks. This works for a factory technician, but not for customer racers.

First of all, it requires a lot of equipment, like special oils, and the parts must be under pressure when assembled. While pressurizing the reservoir back to 300 PSI, if you are not an expert and did not get all the locks in correctly, the result might be parts flying through your trailer like bullets. This is both dangerous, messy and not recommended for any of the racers or team members. If the main piston needs to be recalibrated, teams hand the shocks to the factory technicians and they do it in their service trailer, which has all the parts, tools and high-pressure nitrogen charging equipment. Teams may not even be allowed into the trailer to watch the process for safety reasons. This is why the shocks are now adjustable from the outside, which makes it a lot easier for the racer to finalize their calibration.
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■ Science and style
There are two important areas Ben Hayes and Rick Bates concentrate on in their instruction to the drivers: knowing how the circuits work and how to use each of them to get the best result for the program. First comes knowing how and why the adjustments work. The easiest to understand is the rebound adjuster. This is a thin rod with a tapered tip that sits in the middle of the main rod. A hole coming into the rod from the compression side connects with a passage exiting on the return side of the main piston. On the return stroke, oil flows both through the return side shim stack and the center of the rod regulated by the tapered tuning rod. In order to prevent oil from flowing in the other direction and changing the compression dampening, there is a one-way ball check valve underneath the adjustment orifice, which blocks the flow through the passage on the compression stroke. This is a straightforward and easy to understand circuit which is adjusted by turning a screw at the outer shock eye, which then moves the tuning rod in or out of the tuning orifice.

The compression dampening mechanism on the other hand is a very complicated and sophisticated piece of engineering made to obtain a linear and wide range adjustment of the high-speed compression dampening. The basic principle is to control the flow of the oil into the reservoir. Oil flows into the reservoir because it is displaced by the main rod as it travels into the shock.

The volume that is displaced may just be around 20-30cc, but if it is not moved into the reservoir during the compression stroke, the shock would lock hydraulically or blow itself apart.

The compression adjuster controls how fast the oil travels to the reservoir. It sits in the passage between the main body and the reservoir itself. In the reservoir there is a floating piston that moves out when the oil flows in, but it is resisted by 300 PSI of nitrogen pressure on the other side. The pressure pushes the floating piston back down when the rod moves out of the body again and the oil then flows back into the main body.

The mechanism consists of a calibration plate with a shim stack for compression flow and one on the other side for return flow. Through the center is a calibration orifice which is tuned by a tapered needle. This is the low-speed adjustment. To adjust the high-speed flow, the pressure must first build up enough to open the shim stack on the calibration plate. The shim stack is not adjustable, it just “blows off” at a certain pressure. Next comes the clever part!

Flow is actually controlled by a tapered sleeve which is moved in and out by the high-speed adjuster and opens or closes a circular slot to control the oil flow. This passage cannot be completely closed so that the shock would hydraulic lock if both the high and low adjusters were turned all the way in. On the way back on the return stroke, the oil bypasses these adjustments through a shim stack that is set at minimum pressure for easy flow.
■ The hard part
To use the adjustments there are a few things to remember: First, the front end (IFS) requires less changes as the race goes on. Setting up the front takes about 20 percent of the time, while getting the track suspension right as the racing surface deteriorates is the main challenge and usually takes about 80 percent of the effort, according to Hayes. Return dampening can be a problem. With too much dampening you get a condition known as packing or ratcheting. This means that the suspension doesn’t extend all the way out before it hits the next bump. With successive bumps it may actually ratchet up and lock in a very stiff position which will make it ride very hard over bumps. Hayes therefore advises to adjust return dampening slightly on the loose side. The low-speed compression dampening works well on long shallow bumps, but in the front it is also used mainly to control roll in corners. Stiffer low-speed dampening on the front will let the sled handle flatter in the corners. Roll is also affected by pretension and rate of the springs, but Hayes feels that a stiffer front end that rolls less in the corners may be fine on longer bumps where the driver can compensate by lifting the front end with the throttle.

Getting the track suspension right is the most important part, as it is located directly under the driver and this is his main communication with the track.

Hayes uses a thick paint to check out if the rear suspension works well. He recommends painting the top of the bump rubber on the rail, which the cross-shaft hits. If the paint is all gone at the end of your session, the suspension is set too soft, but if it is still there it means you are too stiff and not using the full travel. It would be very difficult to cover all tuning options in a short story, but Ben Hayes has some good advice: Know how all the adjusters work and spend most of your time making the track suspension work once the front end works well.

There was a time when setups were a big secret that factory racers kept to themselves. Times are changing and Polaris feels that customers are entitled to have good information in order to get the best performance out of their expensive racing equipment. Having a team like Ben Hayes and Rick Bates watching everyone at practice, heat races, qualifiers and the final makes it possible for customer racers to feel they have a good chance to compete and that their success as a Polaris racer is important not only to them and their team, but the people who design and build their racing sleds.
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