Written By Carlo Forni
Last month Carlo reviewed several solutions for changing gear ratios more eﬃciently, and this month he tried out the Predator system at Viterbo.
First session: 83 teeth
We proceeded with our test using the Predator system as we needed something that could be mounted and dismounted quickly. On our main test track in Viterbo, we started the ﬁrst session with a base set-up, 140cm rear carriage width and 139cm front carriage width, neutral caster and camber setting. We decided to use our LeCont LH08 tyres, putting together performance with durability. Pressures were set at 10psi. The air temperature was 23°C, a nice sunny day. We set the transmission ratio with the front sprocket having 12 teeth and the rear sprocket at 83, which is an average ratio at the Viterbo track.
Impressions during this session were of a good speed at medium and high revs, but diﬃculty exiting slow corners where revs went down below 9000 revs per minute. Maximum revs obtained along main straight reached 16,129 compared to a maximum limited revs value of the Super Rok set at 16,700. A few hundred revs were missing, but this was not the main problem. Acceleration in slow corners, in particular a corner with a slightly uphill exit, was deﬁnitely a problem and a lot of time was being lost.
Second session: 85 teeth
We decided to add two teeth on the rear sprocket and try to have better push at low revs. Actually the eﬀects of increasing the number of teeth on the rear sprocket are both higher revs at the same speed and greater torque, but for a smaller range of revs. In fact when running along a bend, considering we should be running more or less at the same speed even using diﬀerent transmission ratios, we will be at higher revs if the rear sprocket has more teeth. This will help avoid too low revs, where torque is insuﬃcient. Also the torque curve narrows (on a graph with speed as the horizontal axis) and increases its peak if the rear sprocket increases in teeth and diameter. This means we will have a stronger push but for a more limited range of revs.
Feelings exiting slow curves were immediately better. No lack of torque and immediate good acceleration. Lap times decreased by 0.3s, which is surely a very valid improvement, indicating that what was gained in acceleration at low speed was not lost at mid or high speed. Maximum revs was 16,365, with an additional 236 revs compared to the previous session.
Mathematically the increase in maximum revs should have been higher (over 350 revs), but most probably the torque over
16,000 revs decreases quite fast so there is anyway a loss in the way revs increase along the straight. Remember it is Torque which aﬀects acceleration and not simply Power.
Third session: 87 teeth
We lastly decided to try to exceed in the transmission ratio, as we wanted to ﬁnd the limit of the engine. This is where the increase in performance (acceleration) at low speed is oﬀset by reduction of performance at mid and high speed.
Obviously with the 87 teeth rear sprocket, acceleration at low revs was very good, but there was no real need for the additional torque compared to the previous session. On the exit of very slow curves in fact power was transmitted to the ground with diﬃculty and I had to control the acceleration working softly on the pedal, which meant not being able to use all the torque available. On the other hand at mid and high speed I had the feeling that maximum torque had already gone and the engine was a little bit stuck, with reduced acceleration.
To conﬁrm this, the best lap time worsened by 0.4s compared to the previous session and 0.1s compared to the ﬁrst session. Max revs reached 16,601, close to the rev limit of the Super Rok, but the increase in maximum revs does not necessarily indicate an improvement in set- up of the engine.
Testing as described conﬁrms that tuning of the engine cannot be done based on the maximum revs obtained along the straight being equal to maximum revs reachable by the engine. What is important is the acceleration at low and medium speed, which are the speeds at which we run on most of the circuit, exiting every corner and in every single acceleration. So maximum revs are always just indicative and can also diﬀer slightly depending on the braking point of the driver.
Since we have been working with increasing the number of teeth on the rear sprocket, which also means greater diameter, a ﬁnal hint is to remember it is best to reduce the number of teeth on the front sprocket when the diameter of the rear sprocket is too high. This helps produce better transmission of power and torque through the chain, but also avoids the chain and sprocket hitting the kerb when cutting through a corner.
For more information on the Predator email