Castor experiment

[Tim Lewis]

I went out testing (playing) today and messed with castor. I'd like to get other people's read on it to see if what I experienced was normal.

My front tires were rags, so I had a pretty considerable entry to mid-corner push. After I was on the gas the kart would pretty much hold it's line. I started on the minimum castor setting.

I then came in, had a friend set the castor to maximum (twisted an offset bushing 180 deg.) and went right back out. Steering effort went up quite a bit and initially I thought there was _way_ more front grip. After a few laps I realized the increase in effort was fooling me a little.

All in all, I picked up front grip entry to mid-corner and found I could get down to the apex on a couple corners easier. I also noticed that it was easier to get the kart loose during heavy braking/corner entry on a couple corners. On a really fast sweeper that is always flat out, the kart also seemed to roll a little freer (is that a word?).

I probably need to repeat this experiment on something resembling decent rubber, but anyhow, that was what I felt. Is that similar to what others have experienced, or am I out to lunch?

Tim Lewis

[Mark Dismore Jr]

quote:

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Originally posted by Tim Lewis:
I went out testing (playing) today and messed with castor. I'd like to get other people's read on it to see if what I experienced was normal.

My front tires were rags, so I had a pretty considerable entry to mid-corner push. After I was on the gas the kart would pretty much hold it's line. I started on the minimum castor setting.

I then came in, had a friend set the castor to maximum (twisted an offset bushing 180 deg.) and went right back out. Steering effort went up quite a bit and initially I thought there was _way_ more front grip. After a few laps I realized the increase in effort was fooling me a little.

All in all, I picked up front grip entry to mid-corner and found I could get down to the apex on a couple corners easier. I also noticed that it was easier to get the kart loose during heavy braking/corner entry on a couple corners. On a really fast sweeper that is always flat out, the kart also seemed to roll a little freer (is that a word?).

I probably need to repeat this experiment on something resembling decent rubber, but anyhow, that was what I felt. Is that similar to what others have experienced, or am I out to lunch?

Tim Lewis

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Alright! Castor, One of my favorite adjustments!

First off everything you say happened all sounds very correct to me. When you added castor you made the kart transfer weight faster and to a bigger degree. The push was from the rear inside tire staying to planted in the corner and over powering the front. When you increased castor the inside rear tire was lighter then before because the kart was moving weight to the inside front and outside rear tire by making the inside front push harder into the track (cross weight is where this all comes into effect). So you didn't use the back tires in unison as much so the front could grip more, it just helps the kart pivot better. This is the effect of a live rear axle. If you liked the change and are out of adjustment, move your front tires out more this will increase weight jacking to.

Mark

[ October 06, 2001: Message edited by: Mark Dismore Jr ]

[Jimmy Moore]

Set your castor back to normal and move your seat forward an inch.

[SHAYNE SHIPLEY]

1 inch is a huge changein my opinion , try a smaller change with the seat moving forward and see how that is.if need move it again so you can see the handling differences.just 1 of many opinions

[John Learmonth]

quote:

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Originally posted by Tim Lewis:
I went out testing (playing) today and messed with castor. I'd like to get other people's read on it to see if what I experienced was normal.

Tim Lewis

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Tim,
Sounds more or less what I would expect to happen with an increase in castEr (as opposed to castOr, which I would expect to be good for 2 / engine bearings………but that’s enough pedantry!).

Like Mark said, increasing caster increases the mechanical weight jacking effect, which will increase unloading of the IR at turn in, allowing the kart to initiate the turn with less understeer, which is very important (the inside rear won’t ‘push’ the front as much as you turn-in and during the entry phase).

It’s not so much the caster itself that’s helping turn in and early corner entry, but rather the steered front wheel height change caused by the combination of caster angle and scrub radius (the distance from the king pin axis to the centre of the tread as measured at ground level). The greater the caster or scrub radius, the greater the front wheel height change will be. Beware going too far with weight jacking, you’ll discover the meaning of ‘twitchy’!

Keep in mind that relatively large caster angles are a necessary evil on karts (because of the ‘solid’ rear axle and the short wheelbase / track width ratio), and that increasing caster will also increase how much the camber will change as you steer the front wheels. It’s best to keep as much of the front tread widths as possible in contact with the track at all times (less contact; less grip and more wear!), but caster works against this by tilting both front wheels as they turn.

If you have more wear on he inside of the front treads than on the outside then you probably have insufficient static pos’ camber, or too much caster angle (which can sometimes also cause wear on the outer edges of the front treads). I’ve had very good results (good consistent lap times and tyre wear) using less caster, greater scrub radius, and enough static pos’ camber so that camber is 0 with the driver in position.

In your fast sweeper, if you need to negotiate all or most of this corner with some steering angle applied, and if the ‘G’ force you’re generating is insufficient to keep the inside rear adequately unloaded (which one is the chicken or the egg?!), then the increased mechanical jacking created by more caster will assist the ‘G’ force to keep the IR unloaded through the turn. The likely effect will be less steering angle needed to counter any understeer through the turn, which will create less rolling resistance and result in less ‘bind’ and increased corner and exit speed (but of course there will be a point of diminishing return).

Keep in mind that any change to the position of the eccentric cam adjusters is almost certainly going to alter the toe angle as well as caster, even if you’re just going from max’ to min’ caster or vice versa. Moving from min’ to max’ caster won’t change the KPI / camber angle (which will definitely alter toe), but it will raise or lower the end of the steering arms, which will change the vertical angle of the tie rods.

Assuming the tie rods are perfectly horizontal at min’ caster, then moving to max’ caster will lower the ends of the arms and the outer ends of the tie rods, causing the tie rods to angle down toward the steering arms. The outer ends of the rods will move in arcs, and the effect will be the same as if you shortened the tie rods slightly, toeing out the front end to some (probably relatively small) degree. If the tie rods angle downward then driver weight will have an increased effect on weight induced toe change. The effect will be opposite if the rods angle up toward the arms at min’ caster, ie the rods will become more horizontal at max’ caster (effectively longer), causing toe in.

Increasing caster will also place the steering linkage in more tension (as gravity and weight try to turn the spindles around the increased caster angle), which will cause an increase in weight induced toe change (add extra toe in – and possibly neg camber - with driver weight), but you’d be amazingly lucky if any of these effects perfectly cancelled each other out. Any change to your front end will mean you should at least check for a possible toe (and camber) change. But just because a particular geometry change may not result in a change in another geometry (when no weight is in the kart) doesn’t mean there will be no unplanned change (when you do place the driver’s weight in the seat).

Jimmy’s solution may well work to give a good turn-in / entry, but keep in mind that moving the CG will have a strong effect in all stages of the corner, including mid corner / exit grip and braking grip into the corner, ie if you move the CG forward you’ll decrease braking capacity (assuming only a rear wheel brake).

I suspect (but could be wrong) that many people place their weight ‘too’ far forward in order to get good front grip (to compensate for front geometry deficiencies, or a too rigid chassis, or both). I often hear people recommend 55 to 60% rear weight, but I know for fact that Lewis Hamilton’s mechanic (when Lewis was racing for CRG) recommended 60% as a minimum rear weight, with 65% as a starting point, and 70% as a maximum (keep in mind that this is with very sticky tyres).

[Tim Lewis]

Thanks for the replies so far. It's good to get a bunch of different reads. Looks like what I felt was pretty much what was to be expected.

I could move the seat or weight in general to reduce understeer. Tuning the kart for the tires that were on it really wasn't my goal (trust me the fronts especially were _shot_!). What I was after was a general read on what increasing the castor would do directionally to the handling. With fresh rubber I can move 5 pounds of lead around to tune the handling.

It's funny that John mentioned twitchy, because it was getting to that point on a couple corners. I had to make sure to move my hands slower than usual or I could get a pretty substantial entry loose.

I'll have to give it a few more A-B-A runs before I figure out what is ultimately the fastest setting, and that will probably change track-to-track and day-to-day.

The next time I put my toe-bars on the kart, I'll check how much it changes by spinning the bushing. My guess is that the toe change was small in magnitude compared to the castor.

Thanks for the feed-back, does anyone else have similar things that they've done and want to throw around?

Tim Lewis

P.S. John, I've seen it castor and caster...I got my point across, that's gud enuff for me.

[Paul Kish]

[QUOTE]Originally posted by John Learmonth:
[QB]

"In your fast sweeper, if you need to negotiate all or most of this corner with some steering angle applied, and if the ‘G’ force you’re generating is insufficient to keep the inside rear adequately unloaded (which one is the chicken or the egg?!), then the increased mechanical jacking created by more caster will assist the ‘G’ force to keep the IR unloaded through the turn."

What you just wrote John is to me one of the most difficult things in setup. The difficult thing is do you get the needed additional mechanial ability of the chassis from front end adjustments or do you get the needed additional mechanial ability of the chassis to unload the inside rear from chassis flex. Or do you keep what you have as far as mechanical adjustments go and change how the "G" force is applied to help unload the inside rear. Three different ways to skin the cat. The hard part comes in deciding which way to use for a particular track condition. The problem is the same the kart is tight and you have to unload the inside rear more but you have three choises to use to cure a problem that can be for many different reasons.

[Rodney Ebersole]

The amount of ackermann will also influence the amount of caster that is needed to unload the inside rear.

[Vince Cochran]

Tim let me tell you about my little boo-boo at a track with a wide range of corner styles, tight back to long sweepers. My kart is very stiff, has upper and lower eccentric pills, and I'm usually faster using low caster(around 8 degrees). At 8 degrees the kart is overall really free to were the kart "slightly" pushes in some corners and is "slightly" loose in others. Mainly a slight push off in tight bank corners and loose in the long sweeper. Well my boo-boo was I had changed my lower eccentric pills and later forgot about it. It was a cold day and I had increase the upper pills before going to the track. The bottom line was I ended up with 20 degrees when I hit the track. The results were: 1)The kart was still balanced front to rear. 2)I was literally piling rubber off all the tires. 3)The kart was planted so hard in the banked turns the 2-cycle actually bogged down a little in the turns, but never bicycled. 4)The long sweeper was astonishing quick, fast, and stuck like glue. I change caster all the time to change how well the kart sticks to the track. Regular sticky days 6-8 degrees, colder days 10-12, rain 16+. I wish I could adjuster caster while racing without changing toe-in, it would be wonderful.

[Phillip Schroder]

Tim:

I have found an inexpensive way to experiment with adjustments. Buy four cheap bathroom scales - use the analog type not the digital (about $5 each). Calibrate them so they all read the same. Place a scale under each tire. Sit in the kart and have someone read the scales. With the stearing pointing straight the LR and RR should read the same as should the LF and RF. Approx. 57% of the weight should be in the rear and 43% in the front (see the Tech Hit Squad article on this). Now incrementally turn the wheel. Note how the weight distribution changes - here is why you don't want the digital scales as most of these will show a weight and then you have to unload them before they will weigh again. You can then make some front-end adjustments and follow this procedure again to see how the weight distribution changes. Of course this is only a static test and will not substitute for on-track testing but it will give you a good understanding of how the adjustments affect wheight transfer so that when you do get some track time you can make better use of it.

Phil Schroder

[ October 15, 2001: Message edited by: Phillip Schroder ]

[Mark Dismore Jr]

quote:

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Originally posted by Phillip Schroder:
Tim:

I have found an inexpensive way to experiment with adjustments. Buy four cheap bathroom scales - use the analog type not the digital (about $5 each). Calibrate them so they all read the same. Place a scale under each tire. Sit in the kart and have someone read the scales. With the stearing pointing straight the LR and RR should read the same as should the LF and RF. Approx. 57% of the weight should be in the rear and 43% in the front (see the Tech Hit Squad article on this). Now incrementally turn the wheel. Note how the weight distribution changes - here is why you don't want the digital scales as most of these will show a weight and then you have to unload them before they will weigh again. You can then make some front-end adjustments and follow this procedure again to see how the weight distribution changes. Of course this is only a static test and will not substitute for on-track testing but it will give you a good understanding of how the adjustments affect wheight transfer so that when you do get some track time you can make better use of it.

Phil Schroder

[ October 15, 2001: Message edited by: Phillip Schroder ]

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I've played around with this on our Longacre scales with no problem. I'm not sure why you had a problem, mine read fine.

Mark

[Mike Goebel]

quote:

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Originally posted by Mark Dismore Jr:


I've played around with this on our Longacre scales with no problem. I'm not sure why you had a problem, mine read fine.

Mark

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He means digital bathroom scales.

Mike G.

[Mark Dismore Jr]

quote:

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Originally posted by Mike Goebel:



He means digital bathroom scales.

Mike G.

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Wow, I was confused! I get it now.

[Roger Miller]

If using bathroom scales it helps to preload them with some amount of weight, like bricks, to bring the readings up to around 100 or so minimum per wheel. Otherwise you can be somewhat inaccurrate at the lower weight readings. Also, ensure they are level across the top plane of the four wheels. So, an arrangement of bricks. like 2 layers of 2 or 3, then a layer of plywood tops for each scale (not one big sheet) then match up the readings, say at 50 or 60, or whatever, then weigh the kart and driver and experiment from there. Playing cards or masonite are good leveling materials, they do not compress.

[Scott McFadden]

one thing to think about when playing with weight and front end is the effect of widening the front track on weight distribution.

When looking for front end grip I prefer to widen the front track. The larger scrub radius means that the line between the inside front tire and the outside rear tire is slightly farther back (the "fulcrum" moves toward the rear) which makes the inside rear lighter on turn-in.

The advantage to this is it allows you to run less castor and/or keep your seat or ballast farther aft. In a shifter especially this will improve exit grip which tends to be a good thing.