We often hear about overdrive and underdrive referred to Scalers/Crawlers, but what do they really stand for?
In a previous article, we explained how to calculate the gear ratio of a scaler/crawler by assuming the same front and rear gear ratio, but this sync is not observed at all times.
Let's try to understand the pros and cons of a transmission with different gear ratios in the two axles and how to achieve them.
Wheel kinematics during cornering
We discussed wheel kinematics in cornering in an article about differentials.
It should be remembered that the wheels turn at the same speed as long as the vehicle goes straight, while the following effects are observed when cornering:
if a central differential is present;
if there is a differential in each axle.
Features of a transmission without differentials
Scalers/crawlers are climbing models and have a very special 4WD transmission since they have no differentials.
This feature ensures that no one wheel can lock if the others are spinning, but the limits immediately show up as soon as a turn occurs; the transmission forces the wheels to a different speed than they should have in relation to their path, generating slippage. The slippage counteracts the motion and and the car steers less than expected.
Traction or steering?
It would seem that the vehicle can only perform on a straight lane, but previous discussions suggest that a fair compromise can be achieved by satisfying condition #1, that is, giving the front axle more speed than the rear.
When the wheels are straight, the rear axle will indeed be dragged sacrificing some traction, but effect number 1 will be guaranteed when cornering and the crawler will steer more smoothly.
Overdrive and underdrive
According to kinematics, it makes no difference to talk about overdrive or underdrive; in both cases, the wheels of the front axle spin at a higher speed than those of the rear axle.
Some manufacturers offer the option of having a different ratio out of the central gearbox, where the front and rear shafts are joined. If the gearbox instead provides the same front and rear gear ratios, overdrive can be achieved by changing the bevel gears within the axles or the gears of the portal axles, if any.
Gearbox with same front and rear gear ratios
We talk about overdrive when the effect is achieved by changing the ratio on the front axle, underdrive when operating on the rear axle; the choice depends on the characteristics of the model and the availability of aftermarket parts provided for modification.
How to make the adjustment
Overdrive is achieved when the gear ratio of the front axle is reduced; on the other hand, underdrive is achieved if the gear ratio of the rear axle is increased. In some cases it is possible to further increase the overall ratio difference by installing both overdrive and underdrive.
Bevel gears with different gear ratio
Pros and cons
As we have seen, higher front wheel speed can lead to advantages in cornering and disadvantages on straight paths, which we can summarize as follows:
Pros
The vehicle turns more effectively because the average speed of the front wheels is higher than that of the rear wheels.
In mud or on sandy ground there is a good traction during cornering.
Due to the geometry of the links, the strain on the transmission could generate a slight compression of the suspension and lowering of the center of gravity.
Cons
On soft or muddy ground, the model may bog down or sink especially when proceeding with straight wheels.
On the side slopes, the lighter end of the model tends to slide downhill.
On good-grip terrain, the front wheels will pull the rear wheels stressing the transmission and increasing battery consumption.
How to calculate overdrive
Calculating the overdrive/underdrive ratio is extremely simple. Just do the ratio of the rear to the front gear ratio.
For example, if the reductions in the picture above were used, the overdrive (or underdrive) ratio would be 2.9 : 2.5 = 1.16. In this case we would say that the overdrive is 16%, because the front wheels spin 16% more than the rear wheels.
In conclusion
The following considerations are meant to be illustrative and not exhaustive, but we would like to recommend what we consider to be good practices, at least for those who are new to Crawler class.
For those who use the Crawler walking in natural long, low-tech trails, overdrive/underdrive is poorly suited because of the high battery consumption and additional stress on the transmission.
On the other hand, for those who participate in competitions and use the model on very technical tracks with lots of tight turns, overdrive is needed (or underdrive) to ensure lots of steering and even when climbing, where the steering is also used to scan the ground looking for additional grip.
Overdrive (underdrive) ratio vary depending on the tracks and the preferences of the drivers. For more leisurely uses, one can opt for a value around 10%, while it can go up to 40% for more extreme applications.