32 replies to this topic

[meb58]

Yes, still giving myself an occasional theoretical headache.

I began to wonder about how track width affects the possible range of toe settings (Front axle) and how these might affect or be affected by Ackerman...

I also imagine that there are some very interesting and potentially harmful forces acting on the strut if we get into extremes...

In practice I have come to realize that a wider track requires a little more toe in...all else equal. This is a fanny dyno observation, nothing scientific.

Or am I splitting the already thin air...again?

[cheapracer]

Yes, still giving myself an occasional theoretical headache.

I began to wonder about how track width affects the possible range of toe settings (Front axle) and how these might affect or be affected by Ackerman...

I also imagine that there are some very interesting and potentially harmful forces acting on the strut if we get into extremes...

In practice I have come to realize that a wider track requires a little more toe in...all else equal. This is a fanny dyno observation, nothing scientific.

Or am I splitting the already thin air...again?

I never worry about these things too much, different speeds will require different settings.

A wider track should in theory require more ackerman as the inside radius must reduce (which suggests the requirement of more toe out) but if you think for a moment you should change the manufacturers ideal ackerman just because you have widened the track say 2", I would suggest you are overstating it's importance.

Note - unless you mean more toe in to offset the extra deflection forces of a 'rubberised' suspension system.

I believe Greg has an interest in ackerman..........

Edited by cheapracer, 25 September 2009 - 13:31.

[meb58]

Yes, I am concerned about more deflection...but I must admit when I thought about this I was also aware it might be nothing.

I only gave toe in the nudge in my thinking because track is being increased via wheel offset and small spacer combination - re-designing a proper knuckle might prove out a better Ackerman angle. My feeling is that a wider track will require less toe out, but also that more toe out might place more deflective loads on bushings in this context.

[gruntguru]

Yes, I am concerned about more deflection...but I must admit when I thought about this I was also aware it might be nothing.

I only gave toe in the nudge in my thinking because track is being increased via wheel offset and small spacer combination - re-designing a proper knuckle might prove out a better Ackerman angle. My feeling is that a wider track will require less toe out, but also that more toe out might place more deflective loads on bushings in this context.

In terms of the geometrically correct Ackerman angle (the one that avoids tyres scrubbing while manoeuvring slowly in a car park) - increasing track with wheel offset and wheel spacers does not affect the correct Ackerman. On the other hand, if you widen the track by moving the uprights outwards, the Ackerman will be incorrect (too little)

[cheapracer]

In terms of the geometrically correct Ackerman angle (the one that avoids tyres scrubbing while manoeuvring slowly in a car park) - increasing track with wheel offset and wheel spacers does not affect the correct Ackerman. On the other hand, if you widen the track by moving the uprights outwards, the Ackerman will be incorrect (too little)

The wider your track the smaller the inside radius that the inside tyre has to follow and consequently the larger the outside radius the outside tyre has to follow - doesn't matter how the tyre's get to those arcs, only matters that they are there and that more Ack. 'toe out' gain will be required.

All based on a road car of course.

Edited by cheapracer, 26 September 2009 - 07:30.

[Joe Bosworth]

Come on guys. Isn't this all getting too preciously theoretical for the good of everybody?

Keeping this to road car types of performance, ie say 1G max cornering force:

A 600 foot cornering radius turn is going to be taken at a max of 94.8 MPH. Or put another way a corner that you max out at 94.8 MPH has a cornering radius of 600 ft. The track's radius is set by the outer reches of the paving.

Altering the track width of the vehicle by a couple of inches one way or the other is going to make less than 0.05% difference to the radius that any tyre is going to take around that corner!!! Please don't pretend that you can set any toe or ackerman to that kind of accuracy. It just can't happen because we don't have the tools that measure that accurately.

Let's just declare this thread an exercise in mental masturbation.

If anyone wants to know the best toe setting go out on a track and try a couple of settings and see what happens to lap times of anything. Of course the setting that is best at one track may be different than the setting fot the next track.

Regards

PS: Pardon the use of imperial units but you get the picture.

[cheapracer]

Well for a start I did say this - "but if you think for a moment you should change the manufacturers ideal ackerman just because you have widened the track say 2", I would suggest you are overstating it's importance".

Secondly - NASCAR are working in the regions of 0.01 degrees of change in toe and Akermann gain settings to suit various conditions and tracks.

[carlt]

Well for a start I did say this - "but if you think for a moment you should change the manufacturers ideal ackerman just because you have widened the track say 2", I would suggest you are overstating it's importance".

Secondly - NASCAR are working in the regions of 0.01 degrees of change in toe and Akermann gain settings to suit various conditions and tracks.

after all the years maybe they've got nothing left to play with , or spend their sponsors money on

[Greg Locock]

0.2 degrees of total toe gives an entirely measurable change in cornering performance. I think FatBoy suggested that using toe as "Fake Ackerman" was a good way of defining what your ideal Ackerman curve was for a specific circuit. Couple of gotchas-many racers use zero ackerman on the basis that life is too short, others use negative ackerman to increase rotation on entry. Finally the optimum ackerman for turning circle is NOT 100%, with modern tires, it is more like 70%.

Incidentally if you are trying to optimise ackerman/toe for cornering then you need to consider weight transfer, a heavily loaded tire will develop peak grip at a greater slip angle than a lightly loaded tire, perhaps explaining the zero ackerman choice.

[McGuire]

0.2 degrees of total toe gives an entirely measurable change in cornering performance. I think FatBoy suggested that using toe as "Fake Ackerman" was a good way of defining what your ideal Ackerman curve was for a specific circuit. Couple of gotchas-many racers use zero ackerman on the basis that life is too short, others use negative ackerman to increase rotation on entry. Finally the optimum ackerman for turning circle is NOT 100%, with modern tires, it is more like 70%.

Incidentally if you are trying to optimise ackerman/toe for cornering then you need to consider weight transfer, a heavily loaded tire will develop peak grip at a greater slip angle than a lightly loaded tire, perhaps explaining the zero ackerman choice.

excellent post.

Here is a sensible discussion from back when it was still popular here:

http://forums.autosp...w...kerman&st=0

[cheapracer]

after all the years maybe NASCAR Guys got nothing left to play with , or spend their sponsors money on

Mac would know more than me but I seem to remember that when some of the racers started to change to rack and pinion steering systems many years ago that they had to resolve steering issues and noticed that there was various improvements ie: Ackermann, to be made along the way.

[gruntguru]

The wider your track the smaller the inside radius that the inside tyre has to follow and consequently the larger the outside radius the outside tyre has to follow - doesn't matter how the tyre's get to those arcs, only matters that they are there and that more Ack. 'toe out' gain will be required.

All based on a road car of course.

No. If you simply change rim offset. The Ackerman will remain correct. (Draw yourself a little sketch)

[gruntguru]

Couple of gotchas-many racers use zero ackerman on the basis that life is too short, others use negative ackerman to increase rotation on entry.

Dirt track cars are often designed with zero Ackerman since the wheels are often straight or turned "out" while cornering anyway.

[gruntguru]

Come on guys. Isn't this all getting too preciously theoretical for the good of everybody?

I think in a "Technical" forum it pays to be technically correct. Sure we need the rules-of-thumb and racers adages, but the correct underlying science should also be presented wherever possible - whether it has a significant effect on racecar speed or not.

[Greg Locock]

No. If you simply change rim offset. The Ackerman will remain correct. (Draw yourself a little sketch)

And then rub it out and draw it properly. The Ackermann error (posh term for the delta in front wheel angle) /is/ related to track, if the track was zero you wouldn't need any ackerman error.

Reimpell and Stoll has the best treatment of Ackermann and turning circles.

I just found this.

http://www.racing-ca...20Ackerman4.doc

Looks like he's got lots of books, and he's got tires that develop max grip at lower slip angles if they are heavily loaded! That's unusual, compared with 'my' tires.

Here's the pac plots for a typical production tire, as you can see this one develops peak grip at increasing slip angles with INCREASING vertical load.

http://greglocock.webs.com/

Incidentally in my previous post i should have said excess Ackermann for increasing rotation, not negative. Negative Ackermann is also used deliberately on some cars.

Edited by Greg Locock, 27 September 2009 - 07:47.

[gruntguru]

And then rub it out and draw it properly. The Ackerman error (posh term for the delta in front wheel angle) /is/ related to track, if the track was zero you wouldn't need any ackerman error.

Perhaps you should re-read my post. I agree that track changes change Ackerman error. What I contend is, if a car is set up with 100% Ackerman, you can change wheel offset as much as you like and it will still have 100% Ackerman.

In essence Ackerman error depends on distance between KP axes - not the actual track.

Edited by gruntguru, 27 September 2009 - 07:19.

[Tony Matthews]

Perhaps you should re-read my post. I agree that track changes change Ackerman error. What I contend is, if a car is set up with 100% Ackerman, you can change wheel offset as much as you like and it will still have 100% Ackerman.

In essence Ackerman error depends on distance between KP axes - not the actual track.

Because the wheel/tyre, instead of pivoting on a point at the centre of the contact patch, is moving in a small arc about that point? The radius of the arc depending on the off-set... That's a question, too.

[gruntguru]

Because the wheel/tyre, instead of pivoting on a point at the centre of the contact patch, is moving in a small arc about that point? The radius of the arc depending on the off-set... That's a question, too.

Yes. A Billy-cart doesn't need Ackerman because the distance between King-pin axes is zero no matter how wide you make the track.

[McGuire]

The Sunbeam Tiger ran a huge amount of anti aka reverse Ackerman. Don't know how much but it is visible to the naked eye when you turn the wheels out near full lock. I have no idea why they did it -- could be they were stuck with it. The effect on handling is very evident in slow turns (screech) and in backing up (shudder). Kits are available to correct it. Back in the day, Doane Spencer developed a fix that involved swapping the spindles left/right and using steering arms from some other British junk, MG maybe.

Sprint cars and midgets often use reverse Ackerman... but then they run at huge yaw angles with the LF tire skipping along on the ground, barely loaded.

[Greg Locock]

Ah yes, that's the right way round. Track - 2xScrub radius is the important parameter, that's the kp to kp measurement.

[cheapracer]

from some other British junk,

ouch!

It wasn't just the Tiger with reverse Ack. either, there was one other also with a modern kit to correct it, name eludes me at the moment.

[Ericoz]

Ackerman debate has been around since the horse and cart! The only thing you have to worry about is, can you modify it easily on your car? I bet that the Monaco setting is far different to the Monza setting!
I would build an assorted range of steering components to give me the full range of adjustment. If you try and calculate this on paper, you will get no where, because it has to take the track and conditions into account. That is why the driver has to work with the Data Tech and his engineer, in the end, it is the drivers call.

[meb58]

Because the wheel/tyre, instead of pivoting on a point at the centre of the contact patch, is moving in a small arc about that point? The radius of the arc depending on the off-set... That's a question, too.

This is what I was thinking about...SAI/King pin do not change if I add spacers or change wheel offset...I began to wonder about that little arc the wheel follows; for a given Ackerman and track width, toe offers us something, toe in or out. If we keep the same Ackerman and toe but make the track wider what do we gain or lose from the perspective of 'that' given toe setting?



This may be more clear...like many of us I have experiemented with toe settings until blue in the face. I think toe - front and rear - has the abilty to make a car feel unltra stable or down right nervous. My question isn't about experimenting on a track - I've done that - it is a theoretical question about about how a given toe angle is affected by increasing track width...I don't know my rig's Ackerman angle but if toe were say .10 deg toe in, we expect toe to do a certain thing for driving stability, tire wear etc. If we increase track width by say 22mm per wheel - Mac Strut - what might we expect from the same toe setting? Or, what might we do to retain the same general driving feel?

I know I can go experiment, but the fanny dyno often tells a lie...

[Greg Locock]

I don't think you can usefully generalise. In just one steady state corner, you have the following interactions: Ackerman, static toe, roll steer (deg/deg)*roll gain(deg/g), compliance steer - so your two different suspensions are likely to vary at least two of those. Then add in camber thrust which is likely to be different for two different suspensions. etc etc

[McGuire]

Mac would know more than me but I seem to remember that when some of the racers started to change to rack and pinion steering systems many years ago that they had to resolve steering issues and noticed that there was various improvements ie: Ackermann, to be made along the way.

Go ahead and laugh but they still run GM Saginaw steering boxes unto this day. Some years back there was a big turnover as everyone went from "rear steer" (linkage behind the spindle, Ford origin) to "front steer" (linkage ahead of spindle, GM origin). The amount of Ackerman run at any particular time appeared to depend mainly on driver preference. That's how I read it anyway. Things are probably different with the CoT.

[meb58]

Greg,

I understand, it's complex...would I be right by suggesting that basically toe in or out does what toe in or out does for minor alterations to track? ...another generlization perhaps.

[Catalina Park]

Go ahead and laugh but they still run GM Saginaw steering boxes unto this day. Some years back there was a big turnover as everyone went from "rear steer" (linkage behind the spindle, Ford origin) to "front steer" (linkage ahead of spindle, GM origin). The amount of Ackerman run at any particular time appeared to depend mainly on driver preference. That's how I read it anyway. Things are probably different with the CoT.

Would that be because the Saginaw box has the advantage of easily adjusting the power steering effort with a different torsion bar?

[McGuire]

Would that be because the Saginaw box has the advantage of easily adjusting the power steering effort with a different torsion bar?

Yep. Also an infinite variety of spool valves, flow control valves, and gearsets from 12:1 to 22:1. The box they use is based on the Saginaw (now Delphi) 600 series steering gear used in millions and millions of GM vehicles.

[Tenmantaylor]

http://autorepair.ab..._principle.html

Nice little explanation for those who are mostly baffled.

Remember learning about this when I raced 1/10th off road electric buggies when I was 12 IIRC we used to induce increased toe out on the inner wheel (more ackerman). Seemed to help in tight turns.

[Catalina Park]

Yep. Also an infinite variety of spool valves, flow control valves, and gearsets from 12:1 to 22:1. The box they use is based on the Saginaw (now Delphi) 600 series steering gear used in millions and millions of GM vehicles.

We had a similar box in Australia fitted to Holdens. It was made by Bishop and it featured a variable ratio "hourglass worm" gear. They were a very good box on a road car.

[Greg Locock]

We had a similar box in Australia fitted to Holdens. It was made by Bishop and it featured a variable ratio "hourglass worm" gear. They were a very good box on a road car.

Bishop also designed the VR rack in current Falcons. A very cluey company.

[johnny yuma]

Zero-offset steering is a rarely met ideal start point for geometry which will avoid bump steer,excessive tyre wear,heavy steering effort and bad loadings.Widening the track by fitting wider wheels offset outwards,and adding spacers,will be all bad except for the extra grip the wider tyres should give.If you also go for toe-out,in my experience on street cars,the car will be a real handful on irregular road surfaces at not-very-high speeds.The road traffic authorities are not completely stupid in their strict limits on how far from standard you may widen your track.Perhaps on a billiard smooth racetrack,in a car with severely limited suspension travel,you can get away with radical departures from design geometry,but it can sometimes pan out that less redical changes are just as quick in lap time achievement,even if the car does not "look" as aggressive.And a good thing too because it is the driver of the car who is the victim of aggression if the car is all over the place like a m** w***** s**t.

[meb58]

I don't thin I have ever used zero offset...but this brings up a puzzle. I assume that there are program goals - from a suspension theory perspective - that make the A list because they are thought to add to either steering feel or performance. One of those to my way of thinking is scrub radius - which can be quickly altered by changing track width. I understand only some of the reasons why most front wheel drivers use positive offset vs negative offset. Aside from the one front wheel losing traction notion - I do not dispute this - why not inlcude neg offset on a front driver?

I had but two goals when considering track changes; adding more feel and potentially more grip. I am sure I have given up something but I am not experienced enough to know what that might be - the car feels better, easier to drive faster. Grip is noticably better and feel did improve...as the front tires saturate I tend to get a clearer message from the steering wheel.

I know tire construction can change scrub as the sideall deforms in a turn...making what might have been a marginally negative offset go positive. I do not detect any vague squirmyness which i would associate with a tire crossing three boudaries; neg to zero to pos and back as the wheel straightens out. DOT R compounds - Toyo RA1 - purchased for longer life, smaller kick in the wallet.