posted on April 03, 2011 19:00
Project Sipster Part 8: All the Drag-Reducing Details
by Dave Coleman
Photography mostly by Jared Holstein
When we set out to save the world, we promised three 7's; so far we've only delivered two. With 0-60 in 7 seconds and 70 mpg in the bag, we still owe you an explanation of how to reproduce this fine driving machine for $7,000.
First the how-to part. A few weeks ago we explained, in excruciatingly greasy detail (maybe by TopGear standards, but it was pretty shallow by MotoIQ standards. Sorry 'bout that...)
, how to put a modern TDI engine in a crusty old Rabbit
. Since then we figured out how to reduce the amount of work that engine has to do to motivate that sexy little box.
If we make the car lighter, it will take less power to accelerate, and less power to climb hills, two of the most fuel-intensive things a car does. Over the course of this project, our readers kept suggesting we strip the interior to save weight. Clearly they've never been in a Rabbit before. The accommodations are so bare bones, there's nothing left to remove. No, our weight savings was accomplished up front, when we removed the heavy 2003 Jetta from around our engine and replaced it with a bantamweight Rabbit. We've done all we can do here.
Friction is our next enemy. All of a car's moving bits take power to move, so we need to make them move more easily. The faster you spin the engine, for example, the more power you waste on friction between all the slidey bits inside. The tall gearing we installed lets the engine turn only 1800 rpm at 60 mph.
Disc brakes can drag, too, if the rotors are warped or the calipers are in anything but top shape, so the Sipster got nice, straight new rotors from a GTI, and we made sure the caliper pins were nice and greasy so the caliper could release without undue resistance.
Our rear drum brakes are adjustable, so we adjusted them slightly loose.
We had plans to get fancy with lubrication as well. RedLine oil sent us a few cases of gear oil and LSD friction modifier. Start with MTL, they suggested, and add a little LSD additive at a time. LSD additive is super-slippery, which is great for reducing drag, but sadly, synchros require friction to do their thing. By adding just a bit of friction modifier at a time, we could work our way up to the point where the synchros start grinding, then we could add a bit more straight MTL to dilute the oil back to the point where the synchros work again. This seemed like a great idea, but we never actually had time to do it.
Tire Rolling Resistance
Tires are a huge source of rolling friction, but good luck figuring out what to do about it. Tire companies are surprisingly reluctant to divulge any rolling resistance data, even if you beg. The only data we've been able to find is vague, old, and buried in a dry government study. Consumer Reports claims to be the only independent body testing rolling resistance, and they're so proud of that fact that they never actually publish the results. They just factor that data into their overall recommendation and keep the good stuff secret.
There is a movement afoot to standardize and publish rolling resistance data, but until then, the frugal-minded will have to rely on speculation and marketing, which is exactly what we did. Observe: when we were building this car, Yokohama was about to release their new dB Super E-Spec tire that they say has 20% lower rolling resistance than the original equipment Prius tire.
Marketing: "Yokohama says."
Speculation: "Prius tires must have low rolling resistance."
The study did give us something to get excited about, though. A 10% reduction in rolling resistance, it said, was good for 1-2% improvement in fuel economy. 20%, then should give us as much as 3 mpg by itself. And that's compared to Prius tires, which are surely better than the ancient BFG Comp T/A's we found in the snow behind CWS Tuning. Excited, we called Yokohama and asked for a set.
"Sure!" they said, "just hop on a boat and go get them." The first set due in the country was halfway across the Pacific and wouldn't land until we were done. Instead, they offered a set of Avid Touring tires. They're not specifically low rolling resistance tires, but they are a brand-new design which should make them better than what we had. Or so we thought. Our testing actually showed them to be exactly the same.
That testing, by the way, consisted of cruise-controlling across a marked stretch of flat, mostly-windless road and using an aftermarket fuel economy gauge to calculate the average fuel economy over that run. A few back and forth runs with each configuration made the numbers seem believable.
Back in that dry, old government study, though, there was a bit about tires breaking in and how their rolling resistance could drop significantly over the first 4,000 miles. So maybe we just have to drive on the Yokos longer.
Our tests did show that pumping the tires up from 32 psi to the max pressure on the sidewall (44 psi on our old tires, 51 on the new ones) was good for just over 1 mpg on either tire. So at least we had that...