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Forced understeer driving technique?

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  • Forced understeer driving technique?

    Does it work on Caterhams on race tracks?

    An explanation of forced understeer:

    During the braking phase on corner entry, the front tires are turned inwards to near (or maybe just past) the maximum slip angle for the tires. If the rear end starts to slide, the cars rotation increases the front tires slip angle beyond the maximum grip angle, reducing front grip, in turn reducing weight transfer to the front, reducing front grip even a bit more, so the result is a 4 wheel drift instead of a spin.

    I've made a video from a game showing a very exagerrated case of this: forced understeer demo. It's also been reported that the real drivers of the cars of the 1967 F1 cars also used this method for corner entry (Jim Clark stated that he also used forced understeer to recover from spins, turn the wheel inwards, and brake hard to wash out the front end). (Note, never brake while countersteering during oversteer, it just makes the spin worse.).

    Has anyone ever tried this method of corner entry with the Caterham? General idea is that this shold work with most rearward weight bias cars (at least ones without a lot of downforce).[/url]

  • #2
    Re: Forced understeer driving technique?

    Originally posted by jeffareid
    General idea is that this shold work with most rearward weight bias cars (at least ones without a lot of downforce).[/url]
    I don't know the answer to your question but don't most Caterhams have a forward weight bias with that engine sitting way up front?



    • #3

      The Caterham more or less has a 50/50 weight distribution with a very little pendulum effect due to the fact that all the weight (engine, gearbox and fat driver) is located between the axles.

      An example of the opposite would be (older) Audis that for some reason have their engines hanging over the front axle, and, of course, 911:s.

      The Audis go very well in a straight line (autobahn), but have to overcome some mass in order to swing its front rapidly sideways.

      A Porsche with a swinging ass only has one wish, to go backwards.

      Or, as Andre Citroen (the founder of a certain french car company) so aptly put it, about rear engine cars: If you throw a hammer, it is not the shaft that will hit the wall first.

      On the other hand, he also refered to car engines in general as "the guy in the kitchen"....

      End of ramble.

      /Magnus F.


      • #4
        Caution, forced understeer only works when near the limits of traction (at least at the front end). Pegging the wheels inwards when not at the limits is likely to cause a spin. The car needs to be going fast enough and pushing hard enough that turning the wheels inwards causes them to wash out instead of steering more.

        I've also posted this question at blat chat. So far only one possible driver is using it, and this is hearsay from another driver.

        On a tail happy car (either an older 911, or Caterham setup with some oversteer), forced understeer will always work while braking or slowing down. Any loss of traction at the rears will reduce braking force, transitioning weight away from the fronts, which will then plow even more.

        The other case is really depends on the car and setup. When power oversteering out of a corner, you should counter steer, but if you lose the rear end, pegging the wheels inwards may cause the front to wash out saving the spin. Also, if a power oversteer spin starts, lifting the throttle or hitting the brakes will just make the spin worse. By pegging the fronts inwards, you can probably stomp on the brakes and force the front end to wash out. On some cars, just pegging the wheels inwards will save the spin, even without braking, which is the best possible scenario. The car transitions from a rear wheel slide into a 4 wheel plowing drift, and you can lift throttle or use the brakes if the drifting isn't slowing the car down enough.

        On cars where forced understeer does work to recover from spins, it has an advantage over counter-steering and waiting it out. The car can't over correct and start spinning the other way. The fronts are kept turned inwards the entire time until the car recovers. If during recovery the car starts to spin again, the pegged inwards turned fronts will have their slip angle increased, reducing front traction and the car goes right back into a plow.

        Countersteering is inherently unstable, it takes a good driver to manage it. Forced understeer, on the cars it works on, is inherently stable (as long as the fronts are kept past maximum grip slip angle and plowing).


        • #5
          The caterham is in the shop getting engine mods, so I took my 1997 Trans Am WS6 to a local, empty parking lot and did some skid pad type experiments. It was kind of interesting to mess with steering and throttle inputs while circling around at the limits. I'd recommend this to anyone trying to figure out how a car will behave when pushed to the limits. You find out more about the car quicker than you would running laps at a track, and there's no worries about running off track. Unless you try this on a vehicle that could roll from a slide, there's no danger.

          Back to the Trans-Am, it wasn't too hard to put the car into a 4 wheel drift (not good for the tires but it was fun). With this car, while pressing on the throttle to kick out the rear end then lifting, I couldn't tell much, the car pretty much recovered on its own as long as I held the wheel steady with the fronts turned inwards right at the limits. I was definately able to transition into and out of plow with steering inputs, there was a point of maximum grip, and steering inwards past this point would cause understeer, but the car would continue to turn, just not as quickly. Pressing the throttle in forced understeer mode would just cause the car to plow drift more, instead of kicking out the rear end. I'm sure if I stomped on the throttle I could spin the car, but with reasonable throttle inputs, the car was self correcting as long as I kept the fronts turned inwards enough.


          • #6
            don't most Caterhams have a forward weight bias
            With a typical driver in the car, it has a rearward weight bias, but not much, it's almost 50/50, something like 52/48 or 53/47 rear/front.


            • #7
              On corner weights, my car's weight distribution was 51/49 without driver.

              As for the driving theory above, I would argue that you can't learn much from reading people's views on car handling. Every car is different, and handling at the limit will be affected by a wide range of factors. You need to get in and drive, and be careful not to assume that any two cars will handle the same at the limit.

              Driving is art and intuition, not science.

              End of ramble #2.


              • #8
                FWIW mine is with driver 51.9%rear/48.1%front and 53.7%left/46.3%right. I know I should lose a few pounds.

                I agree with Michael about the futile application of paper theory. It might be interesting for professional drivers with a lot of practice. But on the level of somebody who sees a track a few hours a year, come on.....



                • #9
                  Gert, those are pretty ugly numbers.

                  There's not much to be done about the front/rear distribution, but you should be able to improve the left/right distribution (assuming the numbers are without driver). Have you fettled the suspension or is that next year's project?


                  • #10
                    Michael, these numbers are WITH driver and flatfloored as good as I can get it.

                    I agree they are ugly but all I can do about that is to eat and/or drink less. That means maybe I can live with some imbalance 8)



                    • #11
                      Some would say that you have to be unbalanced to drive a seven.

                      I withdraw the "ugly" comment. I assumed the weights were without driver. With driver they are obviously closer to what you need.

                      As you said, you could lose weight but there is another option: gain a passenger. Try rivetting the missus to the passenger seat and feeding her duck liver pate. May seem harsh, but sacrifices must be made in the pursuit of excellence.


                      • #12
                        Thanks again for the ride last Sunday at Sears Gert. See, you WERE faster with me as a passenger.



                        • #13
                          Got my SV back a couple of weeks ago, and went in today for the SB100 check. Part of it was a weigh check, 780lbs rear, 700 front with a driver (the smog check person) in the car.

                          Got my plates a bit later.


                          • #14
                            Getting back to my original question, a driver on blat chat mentioned that he used induced understeer on a mid engine car he drives to keep it under control, but doesn't need to do this with the Caterham.


                            • #15

                              Congradulations on getting your car. I'm a friend of Jon Stokes and I'd love to chat with you about how your engine turned out . My e-mail is [email protected]