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Thread: Understeer. Loads of understeer.

  1. #1
    Join Date
    May 2019
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    58

    Default Understeer. Loads of understeer.

    Took the car to the Mississippi Region autocross in Columbus today. Iím not really all that interested in racing this exclusively, since Iím not quite done kicking everyoneís butt in a 90 hp Civic, but I did want to see how it handled at the limit.

    Itís a 1992 with a 1.4 K-series. Toyo R888 all around, 185 front and 205 rear. I had the pressure set at 18 front and 18 rear and by the end of the day it was 9f and 8r. I think optimal is 10f and 12r

    But omg, push. Pushpushpushpushpush. Worse than my stock Saturn sedan. I figure itís the combination of skinnier tires and no weight in the front. Hitting the gas does nothing except make the understeer worse.

    The standard fixes I think apply - more roll stiffness in the rear, less roll stiffness in the front, etc etc. Anyone manage to correct this tendency or is it just a characteristic of the design? If so how did you accomplish it?

    Oh and for future reference, the car is eligible for Heritage Classic Street with 200 tw tires, which has a much more advantageous PAX vs D Modified.

    Yes, and I cracked a rear fender hitting a cone. I was warned :-(. I used to ignore my momís warnings too and she would always say ďthe day will come when you crack a fender on your Caterham and youíll wish youíd listenedĒ. Lol.




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  2. #2
    Join Date
    Feb 2009
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    Its more about no weight in the front than the tires. Your driving technique needs to adjust for this in the first instance - left foot brake taps, quick lifts or full on brakes - get the weight back on the nose for tire grip to make the turn. Trail braking is an essential technique in these cars on track - somewhat irrelevant for for autocrossing though.

    Caterham autocrossers usually have wide tires. One driving relic on this forum that used to autocross had 10 inch wide tires. Not needed for circuit driving since the directional changes are not as abrupt and you can progress the weight balance change around the car to prevent bulk understeer. On a circuit you need to be 100mph plus before high speed understeer starts to reassert itself on the narrow tire cars although it is now like 120mph plus on the wider tire cars.

    A couple of things:
    1) Corner weight - if it is driving better with a passenger than without then you probably need to revisit the set up or only do autocrossing with a passenger. Same rule applies on a circuit - you do feel the difference in set up (or at least I can)
    2) Do an alignment - the fronts should have some toe out which makes direction changes easier.
    3) Tire pressures - you discovered this but at 8/9psi you are risking to take a front tire off a rim one day. This is an area where people have lots of different beliefs. Most UK Caterham circuit racers swear by 18psi cold which equates to 25psi when hot on the Avon rubber Caterham specify. I would not drop below 14psi cold for anything on the Avons. If you are using the Yokohamas that some people on here do with the stiffer sidewalls then you can drop lower since thats the only way to get the tire sidewall to flex. More pressure at the rear will get the rear end (of the car) rotating better around a cone (as you noted)
    4) 13 inch wheels versus the 15 inch wheels - Thicker rubber sidewall flexes more in corners and assists the direction changes. Less rotational mass of the smaller wheel has benefits too.
    5) The good thing is that you will (re)acquire some driving skills that were not needed for modern cars. Finesse and delicacy will better assist weight transference around the car at speed and will get a lot of gains in overall car speed around a course.

    Sadly there is no real cure for high speed understeer other than left foot brake taps or a front wing (Stalkers particularly need this) but even then it may make sense to keep the foot hard down and let the front scrub to achieve the best overall lap time.
    Mike
    2010 Caterham CSR with Cosworth 2.3 Duratec
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  3. #3
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    Just to reiterate Crocís spot-on comments.

    Quote Originally Posted by Croc View Post
    Its more about no weight in the front than the tires. Your driving technique needs to adjust for this in the first instance - left foot brake taps, quick lifts or full on brakes - get the weight back on the nose for tire grip to make the turn. Trail braking is an essential technique in these cars on track. . .

    Yes! Yes! Yes!


    Quote Originally Posted by Croc View Post
    . . .1) Corner weight - if it is driving better with a passenger than without then you probably need to revisit the set up or only do autocrossing with a passenger. Same rule applies on a circuit - you do feel the difference in set up (or at least I can)
    2) Do an alignment - the fronts should have some toe out which makes direction changes easier.somewhat irrelevant for for autocrossing though . . .

    Enough cant be said about the importance each of these, especially corner weighting. I can tell you from personal experience that proper corner weighting can make a huge difference in your getting through a Ďcornerí quickly, and safely.


    Quote Originally Posted by Croc View Post
    . . .3) Tire pressures . . .

    Proper tire pressures...ranks right up there with corner weighting.


    Quote Originally Posted by Croc View Post
    . . . 4) . . . driving skills that were not needed for modern cars . . . Finesse . . . no real cure for high speed understeer other than left foot brake taps or a front wing (Stalkers particularly need this) . . .
    Iím not saying to put a wing on your car, but adding this in because I do drive a Stalker. And I run with front and rear wings when on track. FWIW, with my current staggered wheel/tire setup, when running on slicks on a damp/slightly wet track (yes, with slicks), my car absolutely pushes through the turns. This not to say that I donít get oversteer as well (Croc can verify ), only that the tendency is for understeer. Example; I did a two-day event at COTA in November. It rained the entire two days and the temperature never got above 50 (proper tire pressure comes back into play here, as well). During those two days, I had more predictability with my slicks and would simply drove (guided) the car through the corners using throttle (more throttle, more push....less throttle, less push).

    in short, pay close attention to what Croc is saying...itís gold.
    Last edited by xcarguy; 06-25-2019 at 08:04 AM.
    Stalker V8, chassis #85

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  4. #4
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    Under what circumstances do you experience understeer? Corner entry? Mid corner? Corner exit? Tight turns? Sweepers?

    dave

  5. #5
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    I have never had any under steer in either of my Caterhams. And never had to do anything to trick the front into gripping. Maybe, in addition to Crocs comments, add more negative camber to the front wheels-I run -2 degrees. And add an adjustable rear anti-roll bar since it adds front grip the stiffer the rear bar gets.

    And I run fatter 15" 225x45 tires on the back, skinnier 13" 185x60 on the front. My first Cat was a "square" set up with 15" 205x45 Toyo RA1's all around (had clams so putting fat front tires on was not an issue).

    Edit: BTW, I am not a fan of Toyo R888's, grip-less wonders IMHO. And/or maybe they are heat cycled or aged out? It helps if they are shaved, but then they are useless in the rain. But significantly grooved tread is the enemy of traction. I just bought a set of R888R's for the rain. They are reputed to be grippier than the R888's. We will see if they are better. Anyhow, if all else fails, maybe try a set of Hoosier R7s on the front? That should fix the grip issue.
    Last edited by Kitcat; 06-23-2019 at 11:29 AM.
    '97 Caterham Super Sprint, 1700 Crossflow-sold
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    '03 Caterham Zetec track car

  6. #6
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    Quote Originally Posted by Kitcat View Post
    I have never had any under steer in either of my Caterhams . . . .
    Mike, Iíve been on track with you. You are the exception. You are the awesome Kitcat.
    Stalker V8, chassis #85

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    Get up in the morning and give the day your very best. You donít get it back.

  7. #7
    Join Date
    Jul 2013
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    Andy 69,

    Welcome to autocross with street tires. I have an old (86-89?) Birkin and have never had a problem not being able to rotate it. The understeer (205/50/15 x 4) is due to modern tires not working with light weight. As already stated, a hard lift or brake should help. Alignment / corner weights ditto. Without being a serious (huge slicks) autocrosser, Mine is almost neutral, rotates easily and is a blast, just not very fast. I have not splurged on AVON ZZS 15", and continue to hear good things about R888R Toyos. Don't know their wear rating. Someone here does... May try them. How old are the tires? If you don't know, they are probably too old.

    Rear fenders don't like cones. Below is a photo of mu cone catcher, just on the off side, I don't hit my side. It is removable. I am only disappointed that it has not sliced a cone off, leaving the base in the square.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  8. #8
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    Quote Originally Posted by Croc View Post
    Its more about no weight in the front than the tires. Your driving technique needs to adjust for this in the first instance - left foot brake taps, quick lifts or full on brakes - get the weight back on the nose for tire grip to make the turn. Trail braking is an essential technique in these cars on track - somewhat irrelevant for for autocrossing though.

    Caterham autocrossers usually have wide tires. One driving relic on this forum that used to autocross had 10 inch wide tires. Not needed for circuit driving since the directional changes are not as abrupt and you can progress the weight balance change around the car to prevent bulk understeer. On a circuit you need to be 100mph plus before high speed understeer starts to reassert itself on the narrow tire cars although it is now like 120mph plus on the wider tire cars.

    A couple of things:
    1) Corner weight - if it is driving better with a passenger than without then you probably need to revisit the set up or only do autocrossing with a passenger. Same rule applies on a circuit - you do feel the difference in set up (or at least I can)
    2) Do an alignment - the fronts should have some toe out which makes direction changes easier.
    3) Tire pressures - you discovered this but at 8/9psi you are risking to take a front tire off a rim one day. This is an area where people have lots of different beliefs. Most UK Caterham circuit racers swear by 18psi cold which equates to 25psi when hot on the Avon rubber Caterham specify. I would not drop below 14psi cold for anything on the Avons. If you are using the Yokohamas that some people on here do with the stiffer sidewalls then you can drop lower since thats the only way to get the tire sidewall to flex. More pressure at the rear will get the rear end (of the car) rotating better around a cone (as you noted)
    4) 13 inch wheels versus the 15 inch wheels - Thicker rubber sidewall flexes more in corners and assists the direction changes. Less rotational mass of the smaller wheel has benefits too.
    5) The good thing is that you will (re)acquire some driving skills that were not needed for modern cars. Finesse and delicacy will better assist weight transference around the car at speed and will get a lot of gains in overall car speed around a course.

    Sadly there is no real cure for high speed understeer other than left foot brake taps or a front wing (Stalkers particularly need this) but even then it may make sense to keep the foot hard down and let the front scrub to achieve the best overall lap time.
    Thanks for the input. I was quite a bit faster today than yesterday after some practice.

    I'm no stranger to autocross. I've been driving a first gen Honda CRX since 2012. I can say, this car drives a lot like a FWD with respect to the understeer, usually a RWD does not like trail braking, but this car loves it. My problem was mostly with exit understeer. To cure that I ended up entering faster and trail braking much deeper into the corner than I would otherwise do. But that usually meant being pretty slow at the exit, and since the K series loves to rev, I was usually out of the power band in second by the time I was back on the gas.

    I have not set the corner weights yet, but it feels like they are pretty close. If they are more than a few % off, you will notice it will turn much quicker in one direction and much slower in the other. I figure if I'm serious I'll break out the plumb bob and model the suspension like I did with the Honda. The adjustments I made ended up changing the car so much I had to go down 100# on the rear springs to tighten it up.

    I bumped the fronts to 10 and adjusted them after each run to keep them there, set the rear at 21 and left it. That made the rear a little more loose, I think.

  9. #9
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    May 2019
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    Quote Originally Posted by Kitcat View Post
    I have never had any under steer in either of my Caterhams. And never had to do anything to trick the front into gripping. Maybe, in addition to Crocs comments, add more negative camber to the front wheels-I run -2 degrees. And add an adjustable rear anti-roll bar since it adds front grip the stiffer the rear bar gets.

    And I run fatter 15" 225x45 tires on the back, skinnier 13" 185x60 on the front. My first Cat was a "square" set up with 15" 205x45 Toyo RA1's all around (had clams so putting fat front tires on was not an issue).

    Edit: BTW, I am not a fan of Toyo R888's, grip-less wonders IMHO. And/or maybe they are heat cycled or aged out? It helps if they are shaved, but then they are useless in the rain. But significantly grooved tread is the enemy of traction. I just bought a set of R888R's for the rain. They are reputed to be grippier than the R888's. We will see if they are better. Anyhow, if all else fails, maybe try a set of Hoosier R7s on the front? That should fix the grip issue.
    These are relatively new. but Im not a huge fan of Toyo in general either. Funny thing is Rivals are a much grippier tire at double the treadwear (TW number shenanigans notwithstanding). I don't think you can get them in 13, though. But the tires and the windshield are the only things keeping me out of Heritage Classic with its .79 PAX vs the .9 in D Mod.

    Ditto on the A7s. I ran them on the Honda. I prefer them over the Avons I currently have but I can buy an entire set of them for the price of one A7.

  10. #10
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    May 2019
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    Quote Originally Posted by Davemk1 View Post
    Under what circumstances do you experience understeer? Corner entry? Mid corner? Corner exit? Tight turns? Sweepers?

    dave

    exit.

    Today I was experiencing a little bit of mid corner oversteer in a sweeper, but that was at a pretty high speed.

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