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S13 spindle tie rod mount relocation.

 
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assassin10000



Joined: 14 Apr 2006
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Location: Sacramento, CA

PostPosted: Fri Aug 24, 2007 12:59 am    Post subject: S13 spindle tie rod mount relocation. Reply with quote

After glancing at a thread on ZT: http://www.ziptied.com/forums/index.php?topic=11801.msg274781#msg274781

And seeing this:


I'm having thoughts of seeing my local machinist and possible having something similar made. Maybe even a bolt on design.

I was thinking about possibly not only moving the tie rod mounting point down for better bump steer, inwards for more angle & faster response. BUT also outwards for less ackerman... My question is, how much should I move it outwards. I'd be looking to bring it closer to parallel steer. That is, if myself & my friend go through with this (actually, I haven't even discussed it with him, just a random thought at this point).

I really don't see this as being a SUPER high stress part unless in an accident or wreck of some kind. Even then it'd be made out of high grade materials so hopefully the tie rod shaft will still be the weak point of the whole setup Shocked Wink

Andrew
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jmortensen



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PostPosted: Sat Aug 25, 2007 11:03 pm    Post subject: Reply with quote

I'm just curious as to why you would want less Ackermann. I'm guessing you're a drifter if you're wanting more steering angle. Seems to me that you would also want better turn in to fling the ass end around. If you have parallel steer it's not going to hook up as well in front.

I have a 240Z with close to parallel steer and it doesn't turn in very well. I have actually moved the rack back (front steer) to try and get some Ackermann in there. Similarly, I had a friend with a 510 who bent his steer knuckles for Ackermann and his car instantly turned in WAY better than mine with no other changes.

Staniforth has some good info on Ackermann in Competition Car Suspension.
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assassin10000



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PostPosted: Sun Aug 26, 2007 3:00 pm    Post subject: Reply with quote

Yeah, drifters Embarassed

The reason for less ackerman is during a drift the wheel that is normally 'unloaded' (inner) during grip driving becomes the loaded wheel. And what was the loaded wheel (outer) becomes unloaded. And with lots of ackerman you can end up dragging that outer wheel somewhat. With closer to parallel steering it should be easier to throw the car completely sideways and drift/recover easier as you won't have that extra drag creating a point of rotation.

I dunno, there are other variables that may make less ackerman less desireable. But afaik several of the top drifters have altered their suspension designs for less or no ackerman.

Turn in can be compensated for as lots more static camber can be run that with a 'grip car' as we don't care about front grip quite as much. Much of the car control is done through the rear wheels and throttle control with minor corrections done through the front (if done properly).

This isn't even for my car, but a very good friend of mine.

Andrew
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Wiisass
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PostPosted: Fri Sep 14, 2007 10:37 pm    Post subject: Reply with quote

Well my first look at that piece, all I could think was loading nightmare. tie rod loads aren't going to be too high, but it's still an ugly design in that respect. There's like 8 different possible modes of failure just from adding that piece.

But as far as modifying the steering, this is something I have been thinking about for a while and trying to come up with good reasons to change it or to leave it alone. It's just that everything is backwards compared to normal analysis. So the inside wheel is really the outside wheel. So the normal, adding caster adds camber when steering and adding kingpin inclination takes away camber when steering, doesn't apply. It's the opposite. So the additional caster that many people run to get quicker steering return can actually hurt the grip in the front. Kingpin angle is a little harder to change without affecting much else, the only way to change it normally is with camber plates. Camber plates will change both camber and kingpin.

As a preface for this next section, whenever I say inside or outside wheel, I mean for a normal turn. I will try and keep it clear, things just get a little confusing. I will talk about which wheel is loaded and such, so hopefully it's easier to follow.

As for the ackerman, reverse ackerman, parallel steer idea, I'm not really sure what would work best. With ackerman steering the inside wheel turns more than the outside wheel normally. But for drifting, the inside wheel is the loaded wheel and will have more steering angle than the lighter loaded outside wheel. The thing is making an estimate on ackerman is hard without tire data. You want to bias ackerman so depending on the loads on the wheels, both tires are producing the most amount of grip as possible given the load and position conditions.

So for drifting, reverse ackerman might actually work best. This would allow the outside wheel to turn more than the inside wheel. So the drifting loaded wheel (inside) would be at less of an angle than the lighter loaded wheel. And the lighter loaded wheel would have a higher angle therefore adding an artificial slip angle to the slip angle the tire already has because of the drifting.

Parallel steer could work too, but you might get more grip with a reverse ackerman setup. But this is just a first assumption, I haven't run any numbers or drawn anything out that would confirm that my ideas would definitely work better.

But as far as packaging, reverse ackerman probably won't be able to happen because the tie rod end would probably need to be in the brake rotor to get it right. Parallel steer is possible, but this isn't as easy as making the tie rod end and the lower ball joint make a line parallel to the centerline of the chassis. You also have to consider the position of the rack. You need a 90* angle between the line connecting the lower ball joint and the tie rod end and the line between the tie rod end and the pivot on the rack.

But all of this theory is based on steady state behavior. I'm not sure how this would effect turn in and transitions. Common thought on steering seems to say that ackerman is better for lower speed stuff, but reverse ackerman is better for high speed stuff. So I don't know if reverse ackerman is going to screw up steering feel on turn-in during low speed manuevers or if it won't matter much at all.

You might be able to make some alignment changes to get some feel back or make it better, I don't know. I haven't messed around with this as much as I would like.

Andrew, don't neglect front grip and the importance of it for drifting. As you get going faster, you're going to want as much front grip as possible, so you can push the car even harder.

Also, the more ackerman the system has, the greater angle the loaded wheel will carry during a drift. With parallel steer, assuming the same amount of steering travel, the loaded wheel will be at less of an angle. And even less with reverse ackerman. So I'm not sure how much this would affect the body angle of the car during a drift, but it may limit it or it may increase it due to more available front grip.

What I would do if I were having something made up, I would make it adjustable, so there were a couple different mounting holes available. So you could have one hole that kept close to stock ackerman and one closer to parallel steer and one with reverse ackerman. And then you could test and see how each works out and then have one made with only one hole. Of course, I wouldn't use a design like that because the of what I said about it earlier.

Another thing is, parallel steer is the only one that has actual values associated with it. Ackerman and reverse ackerman can be of varying percentages and would change the way the car feels. I thought I could explain what the numbers mean, but I'm not so sure now. The way I look at it now, and I don't know if this matches the way other suspension guys talk abobut it, is the percentage is the amount the inside wheel turns relative to the outside wheel. So 125% ackerman would have the inside wheel turning 25% more than the outside wheel. 100% ackerman would be parallel steer. And 75% ackerman would have the inside wheel turning 25% less than the outside wheel. And of course you can have it set up for any percentage you want depending on the car setup and the tires and all the other variables that play into this.

Alright, I think that's enough for now, hopefully I didn't miss much and it's all pretty clear.

Tim
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jmortensen



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PostPosted: Mon Sep 17, 2007 5:21 pm    Post subject: Reply with quote

Your statement of how Ackermann works is incorrect.

100% Ackermann in suspension terms is when the wheels turn the correct amount for the radius of the turn you're making. So that means that the inner wheel turns tighter than the outer wheel enough to correct for it's smaller radius in the corner.

100% Ackermann occurs in a rear steer (steering rack behind the crossmember setup) when the lines from the ball joint and tie rod end on each side intersect at the rear axle line. On a front steer setup the rack can be moved back as far as possible and the angle of the knuckles can be changed to affect Ackermann, but I don't yet know of a good way to measure it other than by measuring the angles that the wheels turn at and comparing to a book.

0 Ackermann is parallel steer.

200% Ackermann would be the inside wheel turning twice the difference in angles between the inner and outer wheels in a 100% Ackerman car. The book Competition Car Suspensions has a good section on Ackermann. It was also written later than Tune to Win, which is another oft quoted book which praises anti-Ackermann for formula cars. I think that the tendency towards anti-Ackermann has disappeared in formula cars in recent years, at least that's the impression I got when I read CCS.

From my own experience I can say the following: My car with parallel steering dragged the inside tire BAD. It also plowed horribly at low speeds. My friend's car with 100% Ackermann turned in a hell of a lot better and didn't plow nearly as much. For sure Ackermann wasn't the ONLY factor involved, but when he changed his car to 100% Ackermann without doing anything else it made a pretty big difference.
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Wiisass
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PostPosted: Mon Sep 17, 2007 7:15 pm    Post subject: Reply with quote

Like I said when I mentioned that part, it may not agree with the way most people describe ackerman. So it's just my terminology that doesn't match others, not my understanding. And the numbers don't make any sense if you look at it the way you mentioned. If 200% ackerman means that the inside wheel turns twice as much as the outside wheel, then what would 150% mean? or 100%? If you think about it that way, my terminology makes more sense. At least you know how the wheels are going to turn relative to each other with the way I describe it. With the other way, who knows what 100% ackerman really means? And what numbers do you use for reverse ackerman? Less than 100? Negative numbers? Even though it may very well be the accepted nomenclature, that doesn't mean it's the best way to describe it.

And a certain percentage of ackerman doesn't necessarily work for any radius corner. The amount of ackerman you want depends on the tires and the geometry as well as the types of corners and speeds the car will be going.

Also, the simpler explanation of how to calculate ackerman, i.e. the line from the lower ball joint to the tie rod end and the intersection of the lines from both sides and their relation to the center of the rear axle, doesn't account for the steering rack position. And if you look at it as a simple geometry problem, you will realize how much steering rack position plays into the steering angle.

But all in all, it doesn't matter if it's 100%, 0.003% or whatever, the thing that matters is how much one wheel turns relative to the other.

I just looked through a couple of the books I have to see if they had anything in there that could add to the discussion. But the explanations are pretty basic and don't really get into it. I don't know if it's because it's too complicated to explain on paper because there are too many missing parameters for a general explanation or because it's something that not many people really have a good grasp on and there's no real way to explain how you want the steering setup.

But I don't think there's any good answers to be found thinking about line intersections with the rear axle or what number means what. And without good tire data or at least a good approximation of the way the tire would act, it's hard to do more than design a horse drawn carriage steering setup. Once slip angles get thrown in the mix, everything changes. And depending on different tire curves, the different steering setups could feel totally different with just a tire change.

Also, it's almost impossible to make one steering setup work well for all steering angles. When doing this type of stuff on a race car, a small range of steering angles is usually analyzed. And then with vehicle speeds and corner radii, you can come up with a lateral acceleration and have a good estimate on wheel loads. And then you can look at the tire curves and slip angles and figure out the best guess ackerman for a given track or setup. But even then, it might not be right and could only be good for one radius at a certain speed range and could suffer everywhere else.

Tim
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jmortensen



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PostPosted: Tue Sep 18, 2007 12:57 pm    Post subject: Reply with quote

Quote:
Like I said when I mentioned that part, it may not agree with the way most people describe ackerman. So it's just my terminology that doesn't match others, not my understanding. And the numbers don't make any sense if you look at it the way you mentioned. If 200% ackerman means that the inside wheel turns twice as much as the outside wheel, then what would 150% mean? or 100%? If you think about it that way, my terminology makes more sense. At least you know how the wheels are going to turn relative to each other with the way I describe it. With the other way, who knows what 100% ackerman really means? And what numbers do you use for reverse ackerman? Less than 100? Negative numbers? Even though it may very well be the accepted nomenclature, that doesn't mean it's the best way to describe it.

Not trying to bust your balls here, but if you were in med school and went into surgery and said "Oh, I know YOU call that a heart, but it makes more sense to me to call it a kidney, so I'm going to practice 'open kidney surgery' today", that wouldn't fly.

As to your questions, 100% Ackermann means the wheels follow the theoretical arcs they create in the turn, assuming no slippage. Reverse uses negative values. 200% does not mean that the inside wheel turns twice as much as the outside wheel. It means that the angle difference between the wheels is doubled, vs 100% Ackermann. A diagram of 100% Ackermann is shown on figure 4-5 in the CCS book. It's an asymptotic relationship between the radius of the corner and the amount of change in the wheels. A 10 foot radius needs about 11.5 degrees of wheel angle difference between the inside and outside wheel. A 20 foot radius requires 4 degrees of change. A 40 foot radius requires one degree of change, and 100 ft requires maybe 1/4 degree. 200% would be 23 degrees at 10', 8 degrees at 20', 2 degrees at 40' radius, 1/2 degree at 100 ft.

Quote:
Also, the simpler explanation of how to calculate ackerman, i.e. the line from the lower ball joint to the tie rod end and the intersection of the lines from both sides and their relation to the center of the rear axle, doesn't account for the steering rack position. And if you look at it as a simple geometry problem, you will realize how much steering rack position plays into the steering angle.

True, but this example is usually used in cars with steering boxes and rear steer and everything is in line. The CCS book talks about the difficulty in measuring when you have a front steer setup and the tie rods aren't straight, but it seems pretty hard to get an accurate measurement without actually just measuring at the wheels. I'm sure it's possible, but since I'm mathematically challenged I'll leave that to someone else to figure out.

Quote:
Also, it's almost impossible to make one steering setup work well for all steering angles. When doing this type of stuff on a race car, a small range of steering angles is usually analyzed. And then with vehicle speeds and corner radii, you can come up with a lateral acceleration and have a good estimate on wheel loads. And then you can look at the tire curves and slip angles and figure out the best guess ackerman for a given track or setup. But even then, it might not be right and could only be good for one radius at a certain speed range and could suffer everywhere else.

The affect of Ackermann is felt exponentially more when the steering angle increases. It is greatly diminished at higher speeds when the wheels do not turn as far. I think this is what the CCS book was getting at with their description of Ackermann vs anti-Ackermann. There is not a very large difference in the wheel angularity and so very little effect in high speed corners even with a lot of Ackermann, but in the slower bits it makes a big difference.

In the "let's make it mathematically perfect" world, yes there might be a slight detriment in a high speed corner, but practically speaking there isn't a whole heck of a lot of difference one way or the other.

In terms of cars similar to ours, and skipping the F1 or CART comparisons, I've found that lots of 240Z racers who don't have Ackermann built in will just run a buttload of toe out, like 1/4", in the front. This does basically the same thing as having Ackermann, but creates slightly more drag on turns and straightaways where it isn't needed. So having Ackermann would allow the toe setting to be reduced, but the effect on the front end's grip to be retained. Again, in a perfect world there might be a set amount of offset between the wheels that would be the "perfect" amount depending on the vehicle speed and the sharpness of the corner, but the same could be said about camber, caster, and every other parameter of our cars that is relatively "fixed".

I would imagine that one way to get good turn in and better steering when sideways might be to run anti-Ackermann in combination with toe out. This would tend to cause the wheels to be more parallel at high angularity, but might still allow for some decent turn in, which is the part I think you might potentially be giving up in getting rid of the Ackermann.
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Wiisass
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PostPosted: Wed Sep 19, 2007 12:20 am    Post subject: Reply with quote

Well I think your analogy is a little off. I'm not calling different parts of the car different names. I'm not calling the steering system, the braking system. So I'm still going to use my numbers to mean what I want them to, I just won't call it the Ackerman percentage. I also could've sworn I saw an equation in some suspension book, that would've calculated ackerman the same way I'm talking about it. Or it might have just been a discussion I had with a friend and we came to the conclusion that the classis description of ackerman isn't valid when talking about anything that runs on tires.

I just don't like the way that ackerman percentage is expressed. It doesn't seem to tell you much about how the steering will behave unless you know the geometry of the car. So it's kind of like expressing camber with relation to kingpin angle, it's perfectly valid with enough information, but without it, you don't know what is really going on.

I also have problems with the method of coming up with this number. Without taking into consideration the steering rack position, the numbers don't mean anything at all. It's like taking out a huge variable and not even thinking about it.

The only way in my opinion to determine ackerman or toe out with steering or whatever you want to call it at this point, is either physically measured or computer simulation. You could also work the geometry by hand, but if you're doing that, well, good luck. Any other method is a poor approximation at best.

As far as common recommendations, Carroll Smith amended his thoughts on ackerman from Tune to Win in Engineer to Win. He mentions that some of the original thoughts were from a while ago and tires have changed greatly since then. Although Engineer to Win was published in 1984, so tires have changed even more since then. So like I said before, in order to make the best estimation on paper, good tire data is really needed. As well as track data so you can know turn radii and estimate cornering loads.

Without that kind of information, testing is the best bet. I read on another forum that a way to estimate what ackerman is needed with track testing is to tune the car using toe. So you would adjust toe out until the car felt good for some sections and then adjust toe in so the car felt good in other sections. And then figure out a way that you can get both where you need it. But this is very basic and may not work that well, so I can't say that I suggest it, but I did think it was interesting.

But for a road race car, depending on the tires, it seems that more toe out with steering is better than less up to a point. I still think it would be nice to have an adjustable ackerman setup on the car and be able to spend some time testing it on the track. But unfortantely, this isn't always possible and can be expensive testing.

It could also be tested on the skidpad by messing with toe. Just set up a couple different radii circles, which could be done concentrically and start with whatever your steering setup is now and set toe to zero. Then run the circles and measure lateral acceleration as well as driver feedback and feel, you could also take a look at tire temps to see if it's scrubbing too much. And then start messing with toe. If you end up with a certain toe out setting making the car feel a lot better and be faster, then you need more ackerman. If you add some toe in and the car feels better than you need less ackerman. It's not perfect, but it should give you a good idea of where things stand.

Of course, it would just work for steady state. So some turn in tuning might be needed after ackerman adjustments are made. But if you can get the space to do this, it could be worth it. If anyone does try it, measure steering angle at both wheels before you start making any adjustments. So you won't have an ackerman percentage, but depending on increments, you'll knowo what the inside and outside steering angle is relative to each other.

But now you've got me started thinking about a way to make an adjustable ackerman setup for my cars. Whether it's outboard or finer adjustments by moving the rack forwards or backwards. I don't know yet, but it's now on my long list of things to design and test.

Tim
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jmortensen



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PostPosted: Wed Sep 19, 2007 11:29 am    Post subject: Reply with quote

You're right that was a sucky analogy. You got the point though, that what you're talking about isn't really Ackermann.

Ackermann might have some role in steady state cornering, so I suppose it could be tested on a skidpad, but the real benefit seems to be in the initial turn in. I don't know how you could really test that unless you tested the g's produced at the beginning of a given turn with different settings. I think it's easier just to look at overall lap times on a known track since you can't really dial it in for every turn though.
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Wiisass
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PostPosted: Thu Sep 20, 2007 12:49 am    Post subject: Reply with quote

It may not be the true Ackerman percentage, but it tells you more and is a lot more useful.

Ackerman is more important for steady state. You get the main benefit of ackerman steering when slip angles occur at all the wheels and move the actual center of the turn forward from the rear axle. This is where it becomes important, especially in high lateral acceleration, smaller radius turns.

It does happen whenever the wheel is turned, so the skidpad testing would be part of an iterative process. And like I said, you might need to make some adjustments to get the car to turn in better after dialing in ackerman on the skidpad.

But to say that ackerman might have "some" role in steady state cornering is a huge understatement. And just comparing lap times would be expensive and inefficient testing. The best testing eliminates as many variables as possible. So with skidpad testing to dial in ackerman can allow you to eliminate a lot of other variables, as long as your springs are setup properly. You will eliminate the damper from the equation which will have a huge effect on transient turn in. It also helps to take the driver out of the equation which can be a very good thing. Drivers, by nature, are inconsistent to some degree, some more than others. So having a driver that isn't a robot or a machine can change lap times by appreciable amount which would be enough to cancel out any loss or gain from full course testing. The simpler the testing, the more effective usually.

And I can't remember who actually recommended the skidpad testing idea to my suspension team when we were at school, I think it was either Terry Satchel or Bill Milliken or both. So it's not just some crazy idea that I came up with one day.

Tim
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PostPosted: Thu Sep 20, 2007 1:46 pm    Post subject: Reply with quote

Once the weight transfers the angle of the inside wheel becomes less important, so I'll stand by the statement that it is more important for turn in and has much less effect in steady state cornering. That makes logical sense to me and has been my own experience when autoxing.

In fact I just looked it up in CCS to see what Staniforth said. Here it is:
"It is of the greatest value in the slowest, tighest corners, becoming of less importance and smaller magnitude the faster and more wide open the corner, as G-forces increase and weight transfer moves to the outer wheel."

I agree with you on the testing procedures and effectiveness, but again since I believe that Ackermann is more important in transitions and initial turn in, it becomes harder to accurately test a parameter that mostly affects the car only at turn in, which is why I'd probably look at lap times. On a skidpad pulling over a G I don't think it will make a big difference. Maybe if you did a 30' diameter skidpad...
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PostPosted: Fri Sep 21, 2007 12:34 am    Post subject: Reply with quote

The inside wheel does become less important, but it depends on how the car is setup to know how less important it becomes. Two evenly loaded tires will produce more lateral force than one fully loaded tire due to the way a tire works. So evening out the loads on the tires as much as possible is important. And because of this Ackerman is very important to steady state cornering. Look at all the ackerman examples when they incorporate slip angles, it's all after the transient part of the turn.

Based on Carroll Smith's testing notes, he says that increased ackerman helped reduce understeer through all phases of the corner and in both high and low speed corners. He can explain why as well as anyone else and couldn't come up with good enough reasoning to justify why other than the fact that it worked for him.

So it really depends on the tires and the amount of lateral load transfer in the front. It's still a very complex problem because it is hard to really focus a test on. I still think skidpad testing will get you in a very good range of where you want to be and then things can be adjusted to improve turn in if needed. Maybe even a symmetric figure 8 could do better to test, but it would be harder to easily adjust by just messing with toe.

A quick question, have you done any road course stuff with this or just autox? If just autox, then I can see where you are coming from. More ackerman is especially important for tighter stuff. Were any of the courses that you drove on open enough to really get to high-g steady state cornering or was it tight to the point that the car was never really able to settle. If you've done any road course stuff, how different did things feel there versus the autox course? I think our discussion is close to converging, we're just coming from different perspectives and experience making it hard to agree.

Take a look at this:
http://www.racing-car-technology.com.au/Steering%20Ackerman4.doc

I read it a little while ago, but remember thinking it was pretty decent or at least it was decent enough to save into my collection of technical resources for future reference. I just never got a chance to upload onto the website.

Tim
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PostPosted: Fri Sep 21, 2007 1:24 am    Post subject: Reply with quote

Good article! I hadn't honestly thought about the drag on the inside tire helping the car to rotate. Now that I think about it, it reminds me of a guy I knew about 15 years ago who raced a dirt track roundy round 4 banger. He had done something to disable the brakes on the right front corner so when he got to the end of the straight he'd hit the brakes and only the left caliper would work. The car would instantly fling sideways. He was found out after about 3 races, apparently they'd seen that one before...

Anyway, to answer your other question, I have only limited experience with changing Ackermann. I've wanted to do it to my own car because of the experience of one of my friends who I autoxed with for years. He had a 510 (rear steer, drag link setup) and he had his steer knuckles bent for Ackermann. With that one change he instantly started blowing my doors off at the autox. We did some mid size tracks like Streets of Willow, and I had enough straightaway speed that I'd beat him on the straights, and big tracks were no comparison. But Ackermann gave him such an advantage at the autox that I could never quite catch back up to him. It's a piss poor comparison of autox vs the big track, because I had so much more motor, but I did ride with him and drive his car directly before and after the steer knuckle change and the difference was pretty amazing.

Later I found out that I could increase the Ackermann by moving the rack back and I did that as part of my going on 4 year long project of improving my car. I haven't driven it since the change, nor have I been able to measure the change that I've made yet. The angles on the tie rods changed pretty significantly from stock so I know that it will make a difference, just not sure how much. It was theorized by someone who knew better than me that I wouldn't be able to get 100%, but I figured whatever I got would be an improvement and I needed to mess with the tie rods and the rack placement for other reasons anyway, so what the hell.
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PostPosted: Fri Sep 28, 2007 1:56 pm    Post subject: Reply with quote

Interesting stuff on Ackermann for dirt track cars here:
http://www.woodwardsteering.com/Cat05/Cat05%20PDF%2063-79.pdf

I would figure that this would relate to drifters fairly well. They seem to indicate that Ackermann is preferred even when sliding, and that it would be more stable when coming out of a slide.
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