Here are the specs: '40 spindles and unbent (at least by me) steering arms, 107 inch repro '32 frame, 47 inch kingpins. I know the geometrically correct situation is to have the ray drawn between the kingpin C/L and the Differential C/L intersect the tie rod end C/L. I currently have the body on the car so checking at this point is kinda suspect. Just wondering if anyone has used this set up without problems. I have sufficient steering arm/tie rod clearance between the hairpins, so I hate to bend the arms unnecessarily. Thanks.
Don't sweat Ackermann. Of all the sins we can commit when building a car, Ackermann is the least of them. In the "perfect world", yes, you want that situation you describe, but tons of cars (my own included) are not dead on in that respect and the go down the road just fine and turn just fine. Caster and toe in are the biggies to worry about, not Ackermann. You will be fine. Don
Thanks Don. I have a reputation of over thinking, over engineering and well, of being a wee bit anal. That's of course not my opinion, just the observation of some others. But I know the Ackerman built into those arms is for a much longer wheelbase and I don't want it to understeer like a 9N in a wet field.
Nah, you are doing it right, wanting to put the very best geometry into your car that you can, but these old cars were never perfect from the factory and anything we do just makes them better. Some people say putting a tie rod out front is an absolute no no, and I understand that reasoning, but my old 23 could not have had Ackermann that was further off from what is ideal if I tried, and it made a lot of 500 mile trips and went right down the road and turned just like it should. Don
Don's right, but if you really want to check it so you can sleep better tonight use a plumb bob and make some marks on the floor and a carpenters chalk line to connect the dots.
Don - When you put the tie rod out front all you have to do is reverse the Ackermann. That being said you measure the distance from the king pin to the yolk on the rear end. Take that measurement and go forwards to a point on the ground in front of the car. Now string a line from that point to the king pin. It should come close to the tie rod as far as intersecting.
No...that may be what someone ends up with...but it certainly isn't a desirable form of Ackerman! "Perfect" Ackerman as described by the string from the kingpin and the centerline of the rear axle is a working baseline for what a car might need. Some cars will need less Ackerman...some more, depending on a lot of factors. MY car, like Don's car, has the tierod out front and the Ackerman totally screwed up. (Not that having the tierod out front automaticly screws up the Ackerman...it doesn't.) On the highway it drives like a dream, but taking a hard turn into a driveway or whatever shows the problem in a big way. Thing is, on the highway, Ackerman effect doesn't even come into play! The wheels are basicly going straight ahead or in a very slight turn. Only in tight quarters does Ackerman become really noticeable. Personally I HATE bad Ackerman and will be doing something over this summer to correct mine...but I also realize that getting it 100% "perfect" isn't nearly as important as just getting it within reason.
Sorry guys. It is almost impossible to have the tie rod in front of the axle and make the Ackerman work even close to right. I did a quick CAD diagram to illustrate what happens when the car is steered to the left with the left wheel turning 40 degrees. Diagram A shows the correct Ackerman, wherein the left wheel (outside on a turn) turns only 34 degrees. Diagram B shows the steering arms reversed to the front. Look at how the outside wheel turns 52 degrees! If it were possible to spread the steering arms in front of the axle, so that they are on the same line from the rear axle, as shown on Diagram C, Ackerman would work. Unfortunately, the steering arms would interfere with the drums, discs, or wheels. In the name of safety, tire life, and reliability, I suggest that we all try to keep the Ackerman principal working right!
Ackerman wont affect straight ahead driving but in a turn incorrect ackerman forces one front tire to slip. The farther it's off the more the slippage. A slipping tire does not have as much traction.
My Stude had a camaro clip. The ackerman will never be right. It drove and turn well, it just kept cupping the tread on the front tires. Something I learned to live with. Bill
You really don't have to be perfect but close is good. Yup,backword ackerman works going in a strait line,even off strait just a little if your going slowenough,but your getting in to prob at much speed in at as little as 15*,.The guys that say how great there super wrong backword front steering is,just have not got in a big prob yet,so they been lucky,like not hitting water or gravel in a trun they had to take a bit too fast ect. As anyone can see if they look at the engineering of what happens when it's backword,in drawing "B" by Irish jr =one wheel starts trying to over steer the other=now nether wheel works well in the turn,you are now driving with less then even one front tire to steer with % droped off to under 50% of steering,=very dangerist. To really understand why you can get away with ackerman being off of ideal to some amount,you need to under stand tire slip angle and load at speed [ not just rolling slowly]. A front tire rolls strait foreword to its center line in a turn tell a side force over comes it's hold to the road to some amount/so a combo of how much side load an down load makes it start to have a slip angle of roll that now dose not follow centerline of tire// the out side tire in a turn gets more load=has more slipangle then inside tire=so perfect ackerman at that point dose not line up right on those engineering lines,but can be a little off them,depending on load slip angle[ so less ackerman is better as speed goes up in a turn. But it may take near the speed of light to make backword ackerman come even close to working in a turn.[yes a little joking] I been designing n building racecars for near 50years,if you like taking foolish moves,just keep backword ackerman like you don't know better,more good luck to you,it may not bit you in the ass. In the end,the closer you can get to not being full backwork like in that drawing "B",the better off you are!
My '40 Ford has a '69 Camaro front clip with a 4" longer wheelbase than on the Camaro. With bias ply tires back in the early 80's I wore the outside of the tires badly (plus I pushed it hard around corners)....not so bad now with radials. I just now realized that the change in wheelbase and the big backspace wheels were probably the cause. DUH! Thanks.
To bad I an't perfect,maybe your not going to understand it,even if I did fix,how ever if you already understand it's all good an you wasted you time being a spell cop, I don't think its nessasary to fix spelling ,,were not in school,we are just putting info out to help others. Sorry if your a english techer,but this is building rods an that info can be read fine by anyone not hung up on that extra school crap. I don't spell well,but good enough at 72 it an't gona get better,get over it,thanks If you have a real "Q" that needs "A",about front end design,be happy to give the low down ,but you may have to sound out a few words,100% be the right info,I'm retired ,a design engineer,designed an built many winning racecars,just have some fun looking in my albums if ya like.
Dana, I read your first reply just fine - I was so captivated by the content I never noticed one error. Thanks for sharing your insight.
So, help me to understand all those front steer cars with the tie rods in front that came from the factory that way, like Chevelle's. I know that they are IFS, but the principal is the same, right? How do they get the Ackerman right on those?
For beam/kingpin axles ;The thing that gets in the way of fixing mod to front steer ackerman is were the stock brake backing plate is = bacicely if you move tierod end as close as you can get to the backing plate { by bending or making a new arm] you get better then backword,but still off some,yet a lot better then doing nothing an you gain a lot of corner control back in turns/ also to add more fix,you can make kind of a pocket letting tierod end go even more outword partly in to backing plate,coming closer yet to what is a good working ackerman although not 100% prefect is a lot safer[ but I don't find that a must mod.. Now as for newer cars the diskbrakes often have much more room to put the arm in the right place,even more so if they designed the car for front steer to start with. They are not limited to a strait beam axle kingpin design space wise. Remeber that on the newer cars with "A"frames [IFS],the ball joints are set up an down wider apart then a kingpin type spindel also,makes a bit more room to work with as well, so that is designed in from the get go.
Gm ignored the Ackerman theory through the years on a lot of cars and trucks. How many Chevelle's or 4x4 trucks do you see with the front tires leaning way over when they are turned sharply? Its especially apparent in the cars if they are raised or lowered in the front just a little. The cost was for the consumer to bear in the form of replacing cupped tires or tires with worn out edges, often blamed on poor suspension and alignment upkeep. Wasn't no skin of GM's butt. Gene
Try not to mix up caster with ackerman,there not the same thing and nether is camber,they do interact,but each dose def stuff. All big factory cars have ackerman designed into them after about 1910's
Thanks Dana Barlow, I never thought about the I beam limiting travel like that. I can see where the offset ball joints on a IFS would allow more travel.
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