Braking technique with front brakes

(Max) #1

I’m driving a Rotax DD2 which has front brakes using a brake bias balance bar, like the KZ2 shifter karts. I’ve read a lot about braking technique using rear brake only and I wonder if a different technique is favourable when using front brakes? Obviously you can brake later, but the front brakes make trailbraking troublesome. In cars I was quite comfortable with trailbraking, but not so much in the kart. Karts work quite different, but the basic technique should have some similarities.

One problem is that it is quite easy to lock up the fronts when the front jacking sets in at turn in and another that it feels like the braking fronts seem to somehow work against the rotation at turn in. This might be related to other setup issues, but I can’t make trailbraking work very well.

Am I using to much front brake bias, maybe? I guess that it would be a good idea to have enough rear bias to be able to rotate the kart a bit into the corner.

(Nik Goodfellow) #2

Hi Max,

Yes the technique is different. I’m sure you’ve read elsewhere that for the rear only brakes you stamp on the brakes at first and modulate the brake to limit lock up. The maximum weight on the brakes wheels (the rear) is as you start braking, after that weight transfers to the front and you have to modulate the brake to limit lock up.

A 4 wheel braking system like a KZ is different in that as you brake more weight is transfered to the front, where your most powerful brakes are. So you can in fact press on the brake harder and harder and harder until you reach the limit of the tires grip, which is increased due to the weight transfer.

So short answer, brake, then brake harder.

As for the trail braking issue, i’m having trouble understanding the comment[quote=“Max, post:1, topic:877”]
braking fronts seem to somehow work against the rotation at turn in.

Could you expand on this? I don’t know if you mean the outside wheel locks? Which actually makes sense with a kart because you unload the outside front wheel on turn in.

For any vehicle, the tires have a limited amount of grip that you can use. Braking uses some of this grip and so does turning. So in its simplest form you need to release the brake at the rate you turning the steering wheel. Like so.

How trail braking in its simplest form doesn’t take into account a racing line change. If you’ve come straight from a rear brake kart you tend to drive a wider entry line to when you have the benefit of gears and front brakes. For fronts braking you often want to take what i call the motorcycle racing line. If we take a hairpin as an example, we want to make the racing line look like a V and do all our turning on the apex, this gives you the maximum opportunity to use the brakes in a close to straight line. Rear brake only karts tend to drive a hairpin in a U shape. This is because they have limited power bad so need to maintain apex speed (to stay in the RPM). Geared karts don’t have this limitation.

Taking the corner below, a geared kart should take the double apex line because they can brake much later and use the front brakes to the maximum in the extended straighter entry. A rear brake only kart will take the green line, they will be slower in, but have a higher apex speed. The red line is a non committal line lol.

(Max) #3

Thanks for the full reply! And the second graph with explanation was very interesting. The part about how to press the brake pedal is quite useful, since it differs a bit from using rear brakes, as you say.

I do understand the basic idea of the traction circle, grip and trail braking, but compared to a car, it becomes more complex in a kart with its front geometry. But then I reckon that, although you brake better in a straight line with rear brakes as well, front brakes are even more sensitive to turning the wheel under braking? So is trailbraking not a good idea with front brakes?

What I tried to say with “braking fronts seem to somehow work against the rotation at turn in.” was when trailbraking, the front wheels producing a braking force, seem to glue the kart to the ground and make it really reluctant to lift the inner rear and start to turn. I haven’t driven a rear brake competition kart, but I assume that it would feel quite different in this situation. Then there would also be the opportunity to slightly lock up and get a small slide into the turn, which I guess would be beneficial. To gain that opportunity, I would have to set my brake bias more to the rear, but the. I would be losing some of the straight line braking performance. Is that a bad idea?

The thing that I failed to mention is that the DD2 only has two gears and a bit more than 30 hp, which makes it a lot less extreme compared to a KZ when it comes to lines, although it is obviously different than a kart with no gearbox. In many instances you neeed to use a line quite like to green one, since you don’t have that really low gear to use.

(Nik Goodfellow) #4

Yep, I’ve driven one too, they don’t provide as much engine braking (as a kz) but they are very similar in acceleration, it is really only at the top end that the KZ has the legs on the DD2 (i have raced on a fast circuit with both at the same time). You still generally take the shallow line i described.

Brake bias should generally be set slight more to the front, what I have done in the past is spend a couple of laps on my own braking as hard as i can in the middle of the straights (obviously check behind you) to see which wheels lock up first and adjusting the brake bias accordingly. Beyond that it is personal comfort. What you are feeling is the limit of grip because of the combine braking and turning (its understeering). Most road cars are deliberately tuned to do this because its considered better to understeer then to oversteer in an emergency braking situation. If this is the case you have a few options, move the brake bias more to the rear, take a shallower entry and lift off the brake to turn (but do a lot of turning on the apex) or brake earlier. You can do some tuning to, like moving the front width out or increasing caster, but this will just take grip away from braking.

The braking in a straight is also a good opportunity to practice the braking technique of pressing it harder. The first time you do it right you’'ll almost come out of the seat. It is quite impressive.

(Max) #5

Thanks for some really good tips. Most karts only have rear brake, so most of what’s written is only applicable for this setup.

(Nik Goodfellow) #6

No problem. Let us know how it goes.

(Ryon Beachner) #7

From what you describe I’d definitely back out the bias

Any time you’re turning and apply brakes in a shifter you become very susceptible to lock ups as your front tires are now not only being slowed at the same rate, but rolling at different speeds.

That said, the general technique should be to get all your braking and any necessary gear changes completed in a straight line. Do note this doesn’t mean it has to be before the corner, you can always change your approach into the corner to allow later braking.

(Max) #8

I think this is a driving technique issue, but I’m a bit curious about the setup variables you mentioned, affecting braking grip. Can you explain that further, because I haven’t seen much about that elsewhere? How can I improve the braking grip? I assume that it will be sacrificing some of the overall setup, though.