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Land Speed STi Wheels, Tires & Brakes

 

Have you ever wondered what a stretched tire looks like on a 6.5” wheel?

 

#rsdlsr 03/29/2019

 

An object’s moment of Inertia is its resistance to changing angular velocity. Basically, it takes energy to spin something, and the more it weighs and the further that weight is from the center increases the energy it takes to make it spin. This pertains to every rotating part on the car, energy you put into spinning parts is energy not being put into moving the car forward. So the smaller and lighter you can make a part the faster it will spin using less energy. There are complex mathematical formulas needed to accurately calculate the energy used rotating parts. So to make things easier to explain we are going to use an old rule of thumb that says you double the energy needed to spin rotational mass, or simply you multiply the weight of rotational parts by two when doing rough power to weight calculations. If you want to learn more about all this, there are countless hours dedicated to it on YouTube.

So, let’s talk about the brakes, wheels, and tires. There are a lot of factors to consider when choosing these for a Land Speed car. Most of the parts available were originally made for very different applications. Each choice comes with unique benefits and drawbacks. But they are all affected by rotational weight.

 

 

Brakes

Brakes may not seem very important on a land speed car, I mean this is not time attack, we’re going for a top speed record right? Wouldn’t no brakes be best? They add significant rotational mass, more complexity via the hydraulic system, and may even cause friction and drag, ultimately slowing us down. And really, if everything goes well, we should not ever use them, but what happens when everything doesn’t go well?

In the last episode we talked about weight transfer control through suspension adjustments, which you can check out here, but did you know you can use the brakes to manage the weight transfer as well? Yep, In the event the vehicle gets out of control, the brakes can play a pivotal role in keeping the nose pointed forward and the roof off the salt. Well.. provided you have the right equipment to do so.

Now our setup is going to be pretty basic, but for our needs, it’s going to be overkill which is exactly what we want. So we reached out to FactionFab and they provided their full brake kit.

The rotors. These are their swept slot rotors, now these are more ideal for track and street, but the slots are designed to keep the face of the brake pad clean, and should we ever need to come to a stop in a hurry, they will ensure nothing prevents the pads from making maximum contact with the rotors.

Next up is the F-Spec Brake pads. We use these on many of our cars here and know they can easily handle an emergency braking situation at top speed, and with their excellent feedback through the pedal, we should be able to achieve threshold braking, even on slippery salt.

And to keep everything working, we installed their Stainless Steel brake lines, due to their high strength, coated lines, and corrosion resistant fittings, but these should also help with brake feel and feedback.

Now ideally we will never have to use the brakes, but should that time come, we want to make sure we have a competent and otherwise “overkill” brake setup for the car.

Safety reasons aside, we could have easily run the upgraded components on the stock Impreza calipers. And honestly, we would have been just fine, but we chose this setup for one reason. The Spirit of the class...

This is the production class, and we wanted to run as many factory components as we can, and so we decided to dip into the budget and buy the Subaru 4-pot front and 2-pot rear brake calipers as these were OEM on the Version 6 STI. But….As a bonus, many wheel manufacturers make specialized wheels designed specifically around this brake setup, and that gives us a huge advantage when it comes to tire options.

 

 

 

Land Speed Tires

Like I’ve said before, racing at Bonneville is completely different than racing anywhere else, and tires are no exception. As always, traction is a major concern but what works best on the street just doesn’t translate to salt. And then you need to factor the amount of resistance and drag a tire makes when you are going for maximum speed. Since racing on salt is a lot like driving on snow you can basically get traction the same way, by running narrow tires at higher pressure. Reducing the contact patch of the tire puts more weight on a smaller area, and inflating the tires to the maximum pressure recommended by the manufacturer focuses that contact patch even more. That narrow contact patch also reduces rolling resistance, while the smaller frontal area of the tire reduces drag.

Competitors wanting to get close to going over 200mph start looking at tires with the absolute lowest rolling resistance and drag. A common choice is “frontrunners”, these are the skinny tires you find on the front of top fuel Funny Cars that are only 4 to 5 inches wide. For racers wanting to go even faster, they make tires specificity for speeds over 300 mph. All these tires have something in common, they were designed to roll along the ground using the least amount of energy possible. Other than just being narrow, one of the other noticeable feature used to reduce rolling resistance and drag is a tread design that is somewhat round like a motorcycle tire. But with anything made for a very specific purpose, there is always a tradeoff. These tires don’t generate much traction. High powered land speed cars can easily spin these at over 150 mph, this can make precise throttle modulation even more important than maximum power.
 

Things we needed to consider when choosing a tire:

You already know we like doing things a little different than everyone else, so we decided to not run skinny frontrunners off dragsters. We noticed something when we looked up top speeds for the other production classes, almost every lower displacement record was set by an AWD car. If traction is that important, we can probably use a little more grip than we can get out of those frontrunners. In a car shaped like a brick and with only one liter of displacement we just aren’t going to have the power to accelerate very fast once we get going over 100 mph. We will want to get going as fast as we can as quickly as possible than crawl our way to the new land speed record. And a Subaru Rally Car gave us an idea…

If you have ever seen a rally snow tire, you might know exactly what we were thinking. These tires are very narrow, with a section width between 155 and 165, that’s just a little wider than the frontrunners most land speed competitors use but with aggressive tread for maximum grip on snow and ice. Unfortunately, there were a few obstacles and drawbacks that meant we weren’t going to be able to run actual rally tires.

For Starters they are specifically made for racing and aren’t DOT legal they don’t have an official speed rating, and without a speed rating the race organizers might not let us out on the salt. We didn’t want to risk getting to the event and not being allowed to run. Another reason is that rally tires, like most tires, are relatively expensive. Remember we are on a pretty tight budget. Rally tires and dragster frontrunners can both run $240 or more per tire. Spending almost $1000 on tires meant we would need to cut corners somewhere else.

One last thing, those rally tires are 25.5” tall. That might not sound like much and running a taller tire reduces friction losses on the salt. The downside is a taller tire actually raises the car, for every inch of tire diameter the car goes up .5”. We will get more into aerodynamics in one of our upcoming videos. But needless to say, we want the car to be as low as possible to improve airflow around the car.

 

 

 

The Tires

After spending way too much time digging around on the internet we narrowed it down to one tire that was the best fit for our needs. The 145/65R15 Dunlop Enasave 01 A/S. These are the OE for Mitsubishi’s little electric i MiEV. The tire is specially made out of a low rolling resistance compound, and the tread was specifically designed to promote traction with minimal energy loss. All that sounds pretty good, but wait there’s more. These tires are as narrow as a rally snow tire with around 4.5 to 5 inches of tread. They are as small and light as a dragster frontrunner, weighing in at just 12.2 pounds and 22.4” tall. And very importantly they are DOT legal with an H speed rating, as per the SCTA regulations H rated tires legal up to 150mph.
When we compare the 145/65R15 Dunlops with the 20 pound 205/55R16 tires that came stock on the Version 6 STi you get a tire with 1.5” narrower tread. The Dunlop is about 2.5” shorter, that drops the whole car almost 1.5” before touching the suspension. And last but not least each eco-friendly Enasave weighs 7.8 pounds less than the STI’s stock tire… Using our rule of thumb of 2 to 1 for reciprocating weight, we are saving 62.4 pounds just from tires!

 

 

 

The Wheels

The Version 6 STi came with 16X7 wheels that weighed about 16.5 pounds. That’s pretty light, but our friends at Rays Engineering know a thing or two about making lightweight race wheels. This is the Volk Racing TE37, one of our favorite aftermarket wheels of all time. This wheels specifically is the TE37 Gravel, this forged racing wheel was originally designed to survive the abuse of rally competition. Forging is one of the best ways to make a wheel. During the forging process, a massive press smashes things together with enough force that the aluminum‘s internal granular structure aligns to shape of the wheel, this all makes the wheel very strong. Our wheels are 15x6.5 inch and come in at just under 10.8 pounds, that makes them one of the strongest and lightest wheels Rays makes for our Subaru. Forging doesn’t actually make the wheel any lighter, the extra strength that comes from the forging process allows the wheel to be made with less material while still being stronger than a heavy cast wheel.


When we compare the 145/65R15 Dunlops with the 20 pound 205/55R16 tires that came stock on the Version 6 STi you get a tire with 1.5” narrower tread. The Dunlop is about 2.5” shorter, that drops the whole car almost 1.5” before touching the suspension. And last but not least each eco-friendly Enasave weighs 7.8 pounds less than the STI’s stock tire… Using our rule of thumb of 2 to 1 for reciprocating weight, we are saving 62.4 pounds just from tires!

As we already talked about, the lighter something is and the closer its mass is to the center of rotation the easier it is to get spinning. With most of the mass that affects the moment of inertia sitting way out in the barrel of the wheel, dropping down to a smaller size overcome even more rotational resistance. It might not seem like much but when you add everything up and multiply everything by 4 wheels you get a very noticeable 22 pound reduction in rotation mass from the outside of the wheels. Again if we use 2 to 1 rule of thumb that’s another 44 pounds of weight off the car.

62.4 pounds from the tires and 44 pounds from the wheels…That’s a saving of 106.4 pounds in equivalent reciprocating weight!

 

Now this setup may not scratch the itch of our stance friends, and certainly won’t provide enough grip for track use, but if you’re trying to break the world record, on salt, in a 1 liter Subaru... Well, it just about perfect.

And that’s it guys, I hope you enjoyed this episode and if you want to see more about our build and attempt to break a land speed record in this Subaru, make sure to subscribe, hit the bell icon, and follow us on our Instagram, facebook, and twitter. Thanks so much for watching, we’ll see you next time.