Ackerman angle

Wooden Rodder · Post by **Wooden Rodder** » Fri Dec 26, 2008 4:33 pm

Thanks guys, When i've finished playing with the back i'm going to try a commy power rack[just to keep it interesting

] if that don't look like it will work i'll go bact to std box. i'll keep posting developments

Retro Boy · Post by **Retro Boy** » Fri Dec 26, 2008 4:39 pm

Falcon grill is an interesting look!
Cheers

Chris5.7ltr · Post by **Chris5.7ltr** » Sat Dec 27, 2008 9:52 am

Hey guys I found this site that explains Ackerman steering, its for I-beams but I would thing it would be the same for IFS/steering racks??

http://www.nationaltbucketalliance.com/ ... kerman.asp

Brett.C · Post by **Brett.C** » Sat Dec 27, 2008 11:51 am

That's a good basic explanation of the principle. Of course in reality it is much more complex than that but back in the day of Mr Ackerman the pneumatic tyre and high speed means of travel had not yet been invented.
It does get way too esoteric for those of us that even don't know the meaning of that word.

I think the last paragraph sums up the situation rather well.
Well, old Joe is not alone on this deal, he's got lots of buddies with the same problem and they haven't done a thing about it. So what happens? Well they just go cruising along…of course they stop by the tire store a little more often than Joe and leave some of their hard earned. And they have to pay a little closer attention to where their missile is headed when all of the guys are out for a cruise and find that great little road with all of the curves that just beg for a guy to open it up just a tad….a tad? Yah right!

Most people wouldn't give a dam about tyre wear on a hotrod, most tyres would perish well before wearing out (front ones at least), but its the transient handling issue (initial understeer) that can make the difference between a pleasant and not so pleasant weekend drive.

A good rule of thumb that I have read is 2-3 deg of toe out at 20 deg turn. I've managed to scrounge about 2 deg out of my less than desirable setup and it has made a huge difference to the handling qualities and feel of the car.

Pep · Post by **Pep** » Sat Dec 27, 2008 12:02 pm

Yes...exact same for IFS. If you note, to get the correct Ackerman on front steer requires the steering arms to be moved outward a hell of a lot. Half the problem is most of us like 15" rims so you can't get the arms furher than the rim. The only way is to either use very short steering arms, which equates to heavier steering, or large diameter rims...not my style

You will also probably need tie rod extensions once you move the arms out as far as you can (need). That is the draw back with front steering. It ends up as a compromise...but, it is a better option in many instances if sump clearance doesn't allow for rear steering.

Warpspeed · Post by **Warpspeed** » Sat Dec 27, 2008 12:04 pm

Yes indeed !
Bump steer and Ackerman if properly done can make a world of difference to how a vehicle feels and steers, as well as to tire wear, handling, and stability.

If the front wheels are steering themselves in odd directions, or competing with each other instead of helping each other, the results are never good.

An interesting study is to watch a large twin steer truck go around a very sharp corner. All four front wheels steer to plainly visible very different angles.

Brett.C · Post by **Brett.C** » Sat Dec 27, 2008 1:47 pm

Pep wrote:Yes...exact same for IFS. If you note, to get the correct Ackerman on front steer requires the steering arms to be moved outward a hell of a lot.

As I alluded earlier, if you cannot move the steering arms out enough then you may still get it reasonably close with a combination of having the arms as far outward as possible and the rack mounted as far back as possible.

It would take a bit of trial and error when setting the whole thing up in the first place, but it may still be possible to get the 2 to 3 deg toe out at 20 deg turn that I was talking about.

If all else fails then a bit of static toe out may also do the trick.
I ran my car like that for several years before getting around to a fix.
The difference between static toe in and toe out was amazing, and now with proper ackerman angles its even better.

Pep · Post by **Pep** » Sat Dec 27, 2008 4:44 pm

Hey Brett...with the HG front end that Wooden Rodder pictured before, I would have guessed even with the original steering box, the drag link would have been approximately in the same position as the rack is now. The angle of the tie rods would also be the same as it is now. That would explain why the steering arms don't seem to have enough angle for correct Ackerman. Is the angle of the tie rods compensationg for this?

Brett.C · Post by **Brett.C** » Sat Dec 27, 2008 5:19 pm

Don't forget pep that the HK-G FE is a swept design. That is the lower control arms are mounted at an angle. I would therefore "assume" that the original tie rods would follow this same angle. I don't know, my mind has difficulty in imagining this in 3 dimensions but I have noted that the inner steering knuckles also follow this line. Col may have more info on this.

One word of warning on mounting the rack such the tie rods are not parallel with the axle line. This can cause an unproportional change in steering ratio, and I'm guessing due to my own experience, its more responsive closer to center.

I've found some good online books on the subject.
This one I use to own before I lent to some prick who never brought it back. Go to page 118
http://books.google.com.au/books?id=smK ... #PPA118,M1
Worth reading the other stuff in this book.

Go to page 33
http://books.google.com.au/books?id=9b1 ... lQSWruHwCw

And this talks more about ackerman and rack positioning
http://www.mech.uwa.edu.au/courses/MMS3 ... matics.pdf

Wooden Rodder · Post by **Wooden Rodder** » Sat Dec 27, 2008 5:44 pm

Pep the torana rack is in the front , the drag link was at the back

Oldcol · Post by **Oldcol** » Sat Dec 27, 2008 6:39 pm

Pep · Post by **Pep** » Sat Dec 27, 2008 9:02 pm

Wooden Rodder wrote:Pep the torana rack is in the front , the drag link was at the back

Sorry..I'm not up on all Holden stuff...I know there was a change in steering set ups around that time. What steering arms did you use when you converted to rack?

PeterR · Post by **PeterR** » Sat Dec 27, 2008 9:27 pm

There is a post I wrote some time ago followed by a rather long Ackermann description (as is his way) by my pal Wilson in the following topic. http://www.ozrodders.com/board/viewtopi ... highlight=

To summarise his explanation. The classic drawings of all wheels tracking perfectly on their respective arcs is very simple to analyse but rarely an accurate representation of the real situation.

Whenever a side load is applied to a rolling wheel, the tyre skids slightly in the direction away from the force and the wheel has to be steered into the force to maintain line. This behaviour is evident when driving on a straight road with a side wind. When a vehicle negotiates a curve all wheels will experience a side load and accordingly will be subject to sideslip making the classic Ackermann diagrams at best an approximation.

As enjenjo has indicated, truck makers do not produce different steering arms for every tray length; similarly car makers do not prescribe different steering arms for long wheelbase variants. A contributor has referred to twin steer trucks, and his observation that the second set of wheels steer through a smaller angle than the front set. This is correct, however even though the second set turn through a smaller angle, they still have the same steering arms as the front set, so there is no attempt to optimise Ackermann for each axle.

The diagram we see so often with the lines through the steering arms meeting at the rear axle is not an absolute, but over the years it has proven to be a reliable guide.

Any constructor suggesting that as Ackermann geometry can not made exact it can be completely ignored, -does so at their peril. The tyre squeal in car parks will be irritating and the rapid tyre wear expensive, but the moment of truth will come when taking a bend (probably a right hander) on a wet road and the steering suddenly goes light as the front tyres lose grip and the vehicle just sails straight ahead.

Fordi6 · Post by **Fordi6** » Sat Dec 27, 2008 9:59 pm

PeterR wrote: As enjenjo has indicated, truck makers do not produce different steering arms for every tray length;

Glad that was mentioned,, had me wondering

rx4ord · Post by **rx4ord** » Sat Dec 27, 2008 10:33 pm

Yep that is right. A shitload of trucks have had their wheelbases altered by bush and country mechanics. They seem to get the job done pretty well without reading Mr Ackermans book. This question is a bit like the piece of string thing. Before the Ackerman boy had his idea all manufacturers must have been in a real quandry. Funny how people drove cars and set world speed records and drove speedway without someone to tell them how to do it right. Dave

Ozrodders.com

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