So, we made a huge number of changes to the car this year, some of which I haven’t mentioned on the blog until now. Our essential goal in 2013 was to try and reduce the massive drag on the car. At the 2012 Runoffs, I had a top speed of approximately 137mph, down in Canada Corner at Road America, compared to the 147+ that the best cars had. We were approximately 3 seconds off the pace. The same delta was true at the season opener in Texas, where we had the same ~136-7mph speed limit, compared to other cars in the mid-140 range. So, finding and eliminating drag was one of the biggest goals for the year.
At this year’s Runoffs, The car’s top speed was in that needed high 140 range, with the car touching 147-148 as we got the car dialed in. Had the week gone better, we would have continued to dial out rear wing due to the extensive rear grip, and found even more rear grip. Broken endplates and everything else made that a lower priority.
In short, we fixed the drag problem, and the car is now at least somewhat competitive in terms of straight line speed. Next will be to try and gain some corner speed to try and keep up with the pace-setting Citation and JDR cars.
So, some of the changes have been obvious, some of them not-so-obvious.
BRD Rear Diffuser
I want to mention what I think was the biggest change first. Jesse Brittsan made me a copy of his rear diffuser, which we installed on the car for Runoffs. The rear of the car was SO PLANTED that we continually had to keep reducing rear wing throughout the whole week, as well as raising the rear of the car. The amount of extra rear downforce is something we haven’t had all year, and even better, the car’s top speed was excellent – high 140’s, and within shouting distance of the smaller cars. In short, finally something that you could fight a little bit with, rather than being tens of miles-per-hour down.This seems to have been the largest single contributor to the car’s increase in top-speed, as even at the race before, at Miller Motorsports Park, Jose and I in our Stohrs were still stuck around the 137mph speed limit that the factory diffuser apparently had on our cars. Flow-Viz on the factory diffuser showed huge amounts of air rolling around the top of the diffuser and infiltrating in the holes for the lower wishbone, resulting in huge separation on the inside of the diffuser. Whatever the interaction, it seemed to create tremendous drag, and it’s nice to have off the car.I’m really looking forward at continuing forward with the BRD diffuser, and the level of grip it appears to give the car. Gathering some more data at High Plains Raceway, where we have lots of comparative data, will be really interesting as the 2014 season starts.As with Jesse’s excellent Dry Sump systems, contact Brittsan Racing Development for more info.
Front Anti-Roll Bar
Halfway through the season, Dave from FRM developed a front anti-roll bar for the car. This not only allowed us to lower the front spring rates, but lessened the roll of the front of the car. By making the bar adjustable, it provided some in-race adjustment, which was very useful at the very hot Miller Motorsports Park race, where the front tires really suffered from the big heat and long, high-speed turns.These ARB kits are available from Dave at Front Range Motorsports, if you would like one for your Stohr.
Zebulon Motorsports Front and Rear Wing
I’ve been lucky enough to spend most of 2013 working with a pair of bright young engineers who make up Zebulon Motorsports. One of our first studies was to examine the wings on the Stohr, as our first attempt at finding and reducing the drag on the car. The result was a slight reshuffling of the rear wing configuration, which is now the Stohr factory setup – a large single beam wing, with a dual-element upper. The Stohr elements for the rear tested quite favorably in CFD. Zebulon drew me a nice swan-neck mount for the Stohr factory beam wing, which maximizes rear wing performance over the traditional bottom-mount. style.
These are available for purchase if you’d like one for your car, and Stohr has the shape for the top-side Swan-neck wing brackets – you can see the final version of the swan-neck mount in the diffuser shots above.At the front, CFD showed significant problems with the factory front “flat bottom” wing, so Zebulon designed an outstanding front wing, with an innovative endplate treatment that makes outstanding downforce. The wing was CFD optimized for a low drag coefficient over a wide range of downforce settings, while minimizing downstream flow disruption.The rear wing changes, combined with a new front wing, netted a 20% increase in downforce on the car as measured by the shock pots, and a few MPH of top speed at High Plains Raceway. At high downforce tracks like Sonoma, this enabled me to outpace the other Stohrs with room to spare.I highly suggest Zebulon’s replacement front wing package, which is also available for sale.
This has become commonplace on all of the Stohrs now, but one of the biggest bodywork problems exposed by our CFD study were the large “flip ups”
just inboard of the rear tires. in CFD, not only did these make substantial drag, but these contributed lift as well. We ran a 3-part test with stock bodywork, modified bodywork where the flip-ups had been extended out to the tires, and then a third test with the flip-ups removed.Predictably, the factory configuration was worst. Moving the kickups out to actually
shroud the tires did pick up 1-2mph, and removing them entirely also picked up the same 1-2mph. As such, you’ve now seen that most Stohrs have removed those flip-ups. Owing to this
change, Stohr has now developed a new sidepod shape that tucks in tightly to the rear spar, that streamlines the rear of the car substantially. This should be even better
than the raw cut edge that the car has now, once they make it available to more than just the factory car.
Rear Tire Fairings
Copying a bit from the Citation guys, we made some rear tire kickups out of some foam from Home Depot, and a bit of gaffer’s tape. These seemed to net
about a 1-2 mph gain on the data at High Plains Raceway, when coupled with the front wheel spats. As with many of the other modifications we did this year, Stohr has taken
our idea and will be making production rear tire fairings you can get for your Stohr. Or, you can get the originals from Mike Devins at Hurley Racing Products, since he’s about the nicest guy in the business.
Taking a page from the Formula Atlantics, we developed some simple front wheel spats that cover off the front wheel space. With brake cooling requirements so low on our F1000’s, covering the wheel reduces drag and lift. Fitting the fronts along with the rear tire fairings made a measureable increase in top speed, as well as a definite change in seat-of-the-pants feel in the car.Copies of these are available from Dave at Front Range Motorsports.