Superbike Racing, Speed, and SOLIDWORKS
The world of Superbike racing is full of ridiculous numbers. The thought of machines weighing 365 pounds with engines that produce in excess of 230 horsepower with riders balancing all that power onto a tire contact patch the size of a credit card; is almost incomprehensible. To put that into perspective, a Toyota Camry would have to make over 2,000 horsepower, and would be riding on tires as narrow as wagon wheels.
As you might expect, machines like these require equally impressive braking systems to reign in the madness. It’s not uncommon for World Superbike teams to spend over $85,000 on the braking system for a single bike!
What if you’re not part of a professional racing team? What if you’re a club racer trying to go as fast as you can when money IS an object? Well, that’s where our story begins…
Jerry has been my teammate for several seasons of road racing. Last season he was suffering from braking issues, particularly toward the end of a Superbike race; we refer to this condition as brake fade.
The Problem: Brake Fade
Brake fade occurs when the braking system begins to overheat and the mechanical properties of the caliper, caliper pistons, and brake fluid change due to increased temperature. This temperature increase is caused by the kinetic energy (motorcycle traveling at speed) being transferred into thermal energy as the brake pads squeeze the brake rotors to slow the rider down. Basically the speed freak’s real world application of the 1st Law of Thermodynamics.
There’s an easy fix, but it’s not cheap
The easy solution to the problem was to buy a set of race-specific brake calipers designed for the rigors of road racing. Easy but not cheap. A quality set of race calipers can cost upwards of $5,000-8,000, which at a club racer level, is pretty far outside of the budget.
Clearly, some ingenuity was required.
After investigating the stock calipers on Jerry’s bike and comparing what we found to other calipers we concluded that the severe brake fade had to be caused by the brake caliper pistons. They’re made of an aluminum alloy which is not uncommon, but these particular pistons happen to have a very thin wall compared to other pistons that we measured. Considering aluminum’s high thermal conductivity properties our hypothesis was that the heat from the brake pad was transferring into the piston which then transferred the heat into the brake fluid. Based on this we came up with three proposed solutions.
- Design and machine aluminum pistons with thicker walls
- Machine pistons to stock dimensions using a material with better thermal properties
- Machine an insert for the stock piston from a material with better thermal properties
To better understand our options we turned to SOLIDWORKS and SOLIDWORKS Simulation.
Using SOLIDWORKS to Evaluate Options
First, I modeled the stock piston in SOLIDWORKS 2016 to get a better understanding of what we had and where we wanted to go.
Figure 1. Section view of stock aluminum caliper piston.
Begin By Creating a Benchmark
Using the evaluate function in SOLIDWORKS, I was able to gather the mass properties of the stock pistons. This is significant as the mass of the pistons is a consideration due to the effects that increased unsprung mass has on the overall performance of the motorcycle. Without knowing the exact alloy material we assumed it to be 6061-T6 aluminum alloy with a mass density of 2700 kg/m^3. This material gives us a mass of 20g per piston.
Now that we have our benchmark we can start creating new designs that we will compare in a thermal study to see which option best fits our goal of minimizing heat transfer to the brake fluid with the least amount of additional mass.
1st Design Iteration – No Replacement for Displacement: Increased thickness
Our first design iteration was an aluminum alloy piston with much thicker walls. The idea behind this piston is two-fold; first, the thicker wall will take longer to heat soak and transfer that heat into the fluid. Second, considering that aluminum’s strength (stiffness) is reduced as it’s temperature increases the thicker wall should reduce flex in the piston itself. Flex in the piston is a minor consideration but marginal gains can add up so it’s worth looking into.
Figure 2. Section view of thicker aluminum caliper piston design.
With the thicker aluminum piston design, we were confident we would eliminate almost all flex in the piston and that we would reduce how fast the heat would transfer into the brake fluid but what about the weight penalty of the new design? We again turned to the SOLIDWORKS Evaluate to get our mass properties and we show a mass of 36.52 grams for the new design. That’s an 82.6% increase in mass, which sounds significant but what does that really mean to us? There are two, 4-piston brake calipers for the front wheel which add up to an addition of 132.16 grams (0.2913 lbs). While a quarter of a pound might not seem like much we want to make sure we’re keeping the weight increase to a minimum so we continued with our design progression.
2nd Design Iteration – Material World: Stainless Steel
Next, we want to consider using the original design (Figure 1) but with a different material, specifically AISI 304 Stainless Steel. Comparing the mass of this design was extremely easy in SOLIDWORKS. All we had to do was edit the material for the original part, select AISI 304, and check the mass properties. With this material we see a mass per piston of 59.37 grams, a huge increase of 197% giving us a total mass increase of 314.96 grams (0.694 pounds). Adding 0.7 of a pound in unsprung mass to the front wheel was almost certainly too much but if the thermal properties worked out it might be worth the weight increase.
Figure 3. Section view of shortened stock piston with AISI 304 (SS) insert.
3rd Design Iteration – Best of Both Worlds?
Our final design involved facing down the stock pistons and machining a steel insert that pressfit into it. There are several perceived benefits with this design.
- First, we get some of the heat transfer properties of the steel without the mass of an entire steel piston.
- Second, the stock pistons are treated with a friction reducing coating and replicating this finish was going to require sending the pistons out to a chemical coating company. Not impossible but not ideal either.
- Third, the thermal resistance between the steel and aluminum components would aid even more in our goal of minimizing the heat transferred to the brake fluid.
With the design created, we checked the mass properties and were very pleased to learn that the new design was only 28.62 grams per piston, with a total increase of only 68.96 grams (0.15 lbs). This is from the stock piston being shortened which offset some of the mass increase from the steel insert.
Now that we had our designs we had to validate their effectiveness. This, of course, meant turning to SOLIDWORKS Simulation! This powerful FEA (finite element analysis) tool would allow us to simulate our real world application to sort out which design to manufacture and begin prototype testing and ultimately race with.
And, that is a story for our next blog post coming soon!