HAVE ENGINE, WILL PRACTICE
Finally, after weeks of learning, shopping, and preparation, we have a track-ready kart. If you've been following along with our An Introduction to Karting series
you're probably just as ready to go as we are. Last time
we got our chassis, installed the seat, and finally spooned a little two-stroke Rotax 125 engine on the back. The only thing left to add is a driver, and wouldn't you know it we have one of those.
In this installment we'll hit the track with the kart, get the motor broken in properly, start looking at some tuning parameters, and have a heck of a lot of fun drifting around the track on some ancient tires that appear to have been hewn from slate or something similarly hard. We'll also get some further advice on driving techniques, troubleshoot some brake issues, and learn just how easy it is to make a kart point in the wrong direction at high-speed – captured for you in high definition.
First practice: motor break-in
Most folks don't treat new cars
much differently than old ones. Sure, they'll probably be a little more vigilant about avoiding puddles and might park at the back of the lot for a month or two, but, when it comes to actually driving
few abide by manufacturer recommendations for breaking in a new engine. Generally that's because those suggestions are hidden at the back of the manual, but plenty of others blatantly ignore them, believing a motor should be broken in like you drive it to ensure maximum performance, economy, and longevity.
Our new, high-strung Rotax FR125 came fresh out of the crate with a very specific break-in procedure dictated in the manual and, while some racers ignore it and run the motor hard from the first time it turns over, most recommend against rushing things. The suggested procedure is as follows:
- Fill with fuel/oil mixture of 33:1 (normal is 50:1)
- Select a rich jet (we went with 172)
- For the first 15 minutes on-track, maximum RPM is 7,500
- For the second 15 minutes, maximum RPM of 9500
- For the third 15 minutes, maximum RPM of 11,500
The idea is to accelerate hard up to your current RPM ceiling, gently let off the accelerator and coast back down to 5,000 RPM (or use the brakes if you need to), then get back on the gas again. Fairly simple in practice but frustrating when you're itching to get racing; right when the motor starts to get into its happy zone you have to lift off the throttle and calm things down again.
You'll want to make sure your coolant temp gets up to 130 degrees F at a minimum (more on how to achieve that below), and you want to be smooth with your throttle inputs. More detail on the procedure is available here
and, while you'll note that recommendation ends after the second 15 min session, our installer recommended a third just to be safe.
So, that's at least 45 minutes of running without reaching max revs (a little north of 14,000) and, thanks to the delays we had in getting our engine installed, those 45 minutes were all the track time that we could get on our first practice day. Despite not running our engine at full song we were able to spend time lapping at a considerably quicker pace than we had in the rental karts, but having to keep an eye on the tachometer meant we couldn't really focus on our lines. We also had to keep our other eye on a fast driver in a Rotax and an even quicker one in a shifter. To avoid further distractions we simply left our lap timing pickup unplugged.
One thing that became immediately apparent during our first day was that we had a brake problem. The system on this kart is probably six or seven years old at this point, quite stately for a setup of this type. The previous owner had warned us that frequent bleeding would be needed but we were surprised that, despite purging the brake system
just before turning our first lap, the pedal was already running long before the end of our second 15 minute session. If we weren't on the brakes half-way down the straight we weren't getting slowed down in enough time to make the turn. Not cool, and certainly not safe.
Bleeding the brakes is a quick process, in our kart requiring an elevated bottle that feeds fluid into the master cylinder (mounted ahead and to the left of the seat) while two small screws are opened on the top of the caliper. Run a little fluid through, tighten everything down, and we were good for another session, but we started looking into other options as soon as we left the track. Failing brakes do not make for safe races.
Knowing that the master cylinder had recently been rebuilt we figured the problem lay in the single caliper mounted to the rear. We priced replacements and, though we found some affordable options, Tim Hannen at the track recommended we first start by simply cleaning out the caliper. So, we took the thing apart, splitting it in two (being careful to not damage any of the shims that manage brake pad spread), removed the rubber seals that sit behind the pistons (a little compressed air will pop them right out), and went at eliminating the crud that had accumulated on the cylinder walls. A soft polishing bit on a Dremel made quick work of it.
The whole process took a couple of hours and we weren't particularly optimistic about the results, but would need to hit the track again to see.
A little advice
Before heading back to the track and properly beginning our quest for faster lap times we spoke to Leading Edge
Team Driver Gregory Liefooghe to get some direction. Greg had earlier told us that breaking bad habits is harder than creating good ones, and though we didn't quite have the funding for a dedicated instructor to coach us through our first day of driving our kart in anger, we hoped that what we learned at the first driving school and the tips he gave us would be enough.
The most important thing? Smoothness.
Carrying speed around the corners is what makes a good kart driver. Braking late is overrated...Observe what other drivers are doing and try to reproduce it. Try to push your limits in steps and be smooth: a kart will go a lot faster with slow inputs, even though it's less comfortable to have slow hands.
To encourage smooth steering inputs we saw some younger drivers at the track affixing zip-ties to the top of the wheel, one on either side of the flat spot at the top such that they point straight in the air. The idea was to then drive as quickly as possible without causing the ties to flex and the tips to sway.
We also asked Greg about setup, wondering whether an oversteering or understeering kart is preferable. Naturally the answer is a perfectly balanced one.
Jetting, taping, and tuning for a cold day
A kart is not a car. If it understeers, you can't carry enough speed, if it oversteers, you can't accelerate off the corners. You are not playing with the transfer of mass like you are with car: any slide will create drag and will slow you down. You need to roll around the corners.
Our second day of practice at Oakland Valley Race Park and our first day with a broken-in motor was a cold one. Rain was in the forecast and, with temperatures hovering around 30 degrees, there was even a threat of snow. The track was dry when we arrived but covered in leaves and properly frigid. As we unloaded the kart a Rotax Jr. driver was out there trying to find some grip and finding himself pointed in the wrong direction quite a few times. Soon we'd be out there doing the same.
This would be the first session when we'd have a portable weather station, needed to get the jetting right for the motor. The jet screws into the bottom of the carburettor and dictates how much fuel is mixed with the air going in the intake. Colder, denser air needs more fuel, though too much (running rich) saps power and generally make a mess. But, too little fuel (running lean) raises engine temperatures and, since the oil is mixed in, can result in insufficient lubrication -- the sort of thing that causes motors to go bang. It's a delicate balance that must be mastered to get a Rotax engine running at its peak, but as newbies we intended to run more rich than needed to play things safe.
By the time we were ready to roll our weather station indicated the temperature had risen to 50 degrees. Pressure was 1000 mb and humidity was 43 percent. We plugged that into BBR Karting's online jet calculator
and got a recommended jet of 172 -- exactly what was already in there. That site indicates that results will generally be rich and our testing proves that point, but again we wanted to stay safe... even though it meant a lot of black goopy oil deposits to clean up later.
Then there was the matter of engine temperature. The Rotax doesn't really get going until over 120 degrees and should be running somewhere around or north of 130 for optimal power. On a cold day the only way to get there is to put some tape over the radiator. Run multiple strips vertically and fold over the bottoms so that you can easily reach and grab one. Keep an eye on temperature and, when you start exceeding your safety zone, rip one off and try to stuff it in your seat or, if you can, into a pocket. Tossing the tape on the track is common but not always smiled upon.
Second practice day
On this day due to the miserable forecast we basically had the track to ourselves in the morning and would get four sessions in, just about an hour of time on-track. We started out with freshly bled brakes and much to our pleasure at the end of the day they were just as strong as when we started. This wasn't the longest and most thorough test of our caliper cleaning job, but with no signs of a long pedal we were confident we at least had a safe system.
Finding enough grip to use those brakes would prove to be more challenging. The years-old MOJO D1 tires that came with the kart had plenty of rubber left on them, as evidenced by healthy looking wear markers, but had lost their sticky essences many moons ago. As we motored around the cold track any application of power before straightening the wheel sent the rear in a lurid slide and braking too strongly brought the tail around as well. Most of the time we could catch it, but sometimes we didn't as you can see at the end of the video below. Karts have a heavy rearward weight bias and, with the braking done at the rear axle, surprise 180-degree spins are easily executed in the braking zone.
When we stayed on
track we had a lot of fun, the aged tires offering outrageous slip angles that we took advantage of, drifting around every corner we could. But, in the back of our mind we could hear Greg telling us that we were losing seconds, and our timing system showed it. Our fastest lap in the first session was a 0:38.7 and, at the end of the day, had migrated down to steady 0:38-flat times. We'd need a mid-to-low 0:35 lap time to have any chance of actually passing someone in a race.
However, our immediate concern was more about being safe and, after talking to the track staff, we were told we wouldn't just be a moving chicane at that speed. We were assured we'd be at the back of the pack, but not unsafely so... assuming we kept our head on straight. That we figured we could do.
Next is the culmination of all this work: going racing. We've had experience in different types of racing before but, as we'd soon find out, a kart event is a little bit different. In the next installment we'll take you through the race day, dazzle you with lots of helmet cam footage, and describe as best we can one of the most amazing things we've ever experienced: the grip of a racing kart on brand new racing slicks.
Tim Stevens writes for Autoblog sister site Engadget.com where, among other things, he covers all things tech-related in the automotive world. He recently took up the hobby of karting and will help you do the same through this series, An Introduction to Karting.