This is an automated archive made by the Lemmit Bot.
The original was posted on /r/simracing by /u/SuellioAlmeida on 2023-09-05 17:32:08.
Hey guys! This is the draft of one chapter of my upcoming book that I'll release mid-october about racing technique. This is still not edited and formatted, so it you find any grammar mistakes or typos, ignore them :)
Hope you have a good read and this helps you!
Trail Braking - Make the Car Rotate or Let the Car Rotate?
(Draft from The Motor Racing Book - Car Handling, written by Suellio Almeida)
Trail Braking can feel very differently from car to car when it comes to rotation management. Some cars will feel like adding brakes will make them rotate more, while others will feel like braking while turning will make them rotate less. This happens because as soon as we reach the limit of grip (longitudinal + lateral), the excess braking stress can have a stronger effect on the front or rear tires, depending on the car setup, suspension geometry and brake bias.
Yes, weight transfer will always transfer more load to the front tires, but sometimes the geometry of the chassis or brake bias will make more braking density mean less cornering grip to the front tires, enough to compensate the higher grip from the weight transfer, accounting for a lower total grip difference, causing the car to understeer with more braking pressure.
Let’s recap a few effects of braking while cornering.
First, you have the weight transfer, which increases the load on the front tires and decreases the total load on the rear tires. The effect of weight transfer under braking is more grip on the front tires, and more rotation.
Second, you have the stress caused by the braking itself (creating longitudinal slip). By isolating this stress from the weight transfer, we can have two possible effects on the proneness to rotation. Let’s analyze two extremes, for clarity purposes, which would be 100% brake bias on the front tires and 100% brake bias on the rear tires.
- With 100% brake bias to the front and 0% to the rear, the stress would be so high that it would be very easy to lock up the front tires, making the car go straight. This effect is contrary to the benefits of weight transfer to the front tire grip, and in this extreme example it would completely cancel the extra grip from the weight transfer, making the car understeer even before completely locking, and go completely straight as soon as the two front tires lock completely (if we added even more brakes). Reaching ABS on the front tires first has the same effect of drastically decreasing the amount of rotation the car is capable of doing, causing understeer.
- With 0% brake bias to the front and 100% to the rear, the stress would be totally concentrated on the rear tires, decreasing even more the grip on the rear while allowing the front tires to have all the grip available being used for rotation. In such situation, the car will already rotate more with braking even before locking, and then completely spin as soon as the rear tires lock (if we added even more brakes). Most karts have only rear brakes only, which is why they are so prone to rotation under braking. In this situation, the weight transfer and the braking work together to create a huge difference in lateral grip between front and rear.
Most cars will have the front brake bias at the 50-65%. The higher this number, the more we feel that we let the car rotate with the brake release. The lower the brake bias, the more we feel that we make the car rotate with the brakes. This is a very subtle topic, because the difference between making it rotate and letting it rotate can be very subjective and two different drivers can feel the opposite in the same car, depending on how heavy their braking density tends to be while turning.
Another factor that influences the Make it Rotate versus Let it Rotate sensations is the brake pedal resistance. If the car has brake boosters and you can reach ABS easily with not a lot of brake pressure, then it will almost always feel like you have to release the brakes to Let it Rotate, since it will be very easy to just be over the limit (and most cars have the brake bias towards the front due to safety). But if the brake pedal is very heavy and requires high pressures to work, then you will have a bigger range of pressure to work with on turn-in, which might make it feel like you are Making it Rotate.
It’s important to note that it’s still possible to lock the fronts and understeer mid-corner even if your brake bias is low enough to lock the rears on a straight line. Be careful with this, because if you change the brake bias in order to gain more rotation mid-corner and ignore the effects on a straight line, you might lock the rear tires on initial braking and spin.
Trail braking in stiff cars, like Formula cars, can be tricky. If you trail brake too hard, imagine that you’re “choking” the front tires, as if they can’t breath, and they lock. This can happen even if you have a low brake bias. If this is happening, then Let the Car Rotate with less trail braking density. Let the front tires breathe and they’ll give you more rotation.
Let me know in the comments if this helped you think a bit about it! The book is still not released, but you can join the waiting list on my website, google it :)
