The build of the drop/belly tank car is moving forward...The owner wants the front axle narrowed 4 inches and the rear axle left stock width. The axles are 47 Ford front and rear, the wheelbase will be 112 inches. I believe 47 Ford wheelbase is around 114 inches. Question, will reducing front axle width 4 inches affect the Ackerman enough to work about? This car will be a toy and not raced and only driven short distances ....Thanks
Yes, not hard to check. Draw it up and see, slight twerk on the steering arms should correct it. Ackerman becomes increasingly more important the further the wheels are from straight ahead, and not very important the closer the wheels are to straight.
Greetings! Don't worry about it: Think about all the pickups you see on the road, some short bed, some long bed, some extended cab, all use the same spindle with a different wheelbase. If Ackerman was that critical, they would all have a dedicated spindle. Another thing: Before you narrow anything make sure the front and rear axles will end up having the same track width. If the front is narrower than the rear the front tires will "follow" the rut while the rear tires will "hook" the rut pulling the rear of the car sideways. Lots of fun at triple-digit speeds.
.....On the axle width, I've swapped later model axles in AD Chevy PU trucks and wound up with the rear axle being 2 inches wider than the front with no difference in handling... And the car I'm building has a V8-60....Maybe see 80 going down hill Anyways, I did a mockup with the stock front axle and the Ackerman meets at 114 inch behind the front axle.....47 Ford Wheelbase, imagine that. Narrow the axle 4 inches and the lines meet at about 108 inches...The build will have 112 wheelbase so the Ackerman will likely be ok without bending the steering arms...
I wouldnt worry about it. Not only does front axle width change ackerma (rear width has nothing to do with it), but wheelbase does. Think about how many model As and model Ts are running a much shorter wheelbase running STOCK 37-48 axles. If it really bothers you, measure and heat/bend the steering arms out that tiny bit.
Maybe. Not positive about front narrower than rear, but the opposite certainly has no ill effect, or millions of Citroëns would be lining the ditches of the world. Cosmo
Probably not a problem on a street car, but I can tell you for a fact that it will put you on your lid in an off road racer on a heavily rutted course.
Ackerman geometry is used to make the front wheels describe the correct arc when the vehicle is turning so that the tires don't scrub. If you are turning that much in a belly tank you are not going fast enough or are in the process of crashing. Roo
By the way, there are lots of factory pickups trucks on the highway with different tracks front to back.
Maybe its the driver... http://www.youtube.com/watch?v=d9M-5CPEtr0 http://www.youtube.com/watch?v=gaHuTvVioLo
Yes the front track is normally wider on pickups. Heavy truck manufacturers have always had ackerman issues with different wheelbases. They have five or six ackerman arms for a given model with multiple wheelbases that get extreme. Draw a line on the pavement extending out from the centerline of the rear axle on each side. Turn the front wheels against the locks right and or left. Draw a right angle (90degrees) line from the center of each front wheel back and crossing the rear axle line. You should get a single focal point of lines on each side. If the front lines do not converge to a focal point, you will scrub the treads off. A change of ackerman arms or modified arm will be required. Good luck
To bad the ackerman arms on light vehicles are not removeable independent of the spindles like heavy trucks.
Ackerman may well be an important factor with HD trucks with a greater GVW. But I found that with a smaller medium duty , (F600) the steering arms are the same for 134, 154,172,194, and 200 plus WB . I checked on this because my own truck had been stretched from 154 to 172. It can be an important factor at speeds above normal road speeds.
One factor that never gets a mention is the length of the arm. With different length wheelbases the arms are of a different length to maintain the correct ratio. I learnt about Ackerman a long time ago and have seen many diagrams that differ from the centre of the rear axle to 3/4 from the front axle so I believe there is a little bit of flexibility. I dont believe different track widths will have an affect on this as its on the centre lines. JW
The ackerman angle has a degree of importance. As the steering rotates the inside wheel turns at a sharper angle than the outside wheel to compensate for the difference in turning circle radius. This prevents the wheel/tire form scuffing as it turns and improves the tirewear and handling. Simple backyard mechanics design is that a line drawn from the spindle king pin centers thru the steering arm pivot centers should converge approximately at the center of the rear axle. This gives the steering arms the necessary differential in turning degrees to match the radius of the turn and eliminate tire scuff. It is a wheel base function not a track function.
Dick, If you narrow the track, without changing the steering arm angle, on the same wheelbase, you will move point of convergence forward, right?
The rear centre line of the axle is the same and what about the Morgan 3 wheelers. If you narrow the front track then they should be adjusted . JW
Back to the story about 10's of millions of PU trucks on the road with wheelbase difference of up 20 inches or more and supposedly using all the same spindles and steering arms for all wheelbases... Is it true? Hollander book? If so that's something to think about......Kinda shits up the Ackerman theory... The car I'm building is not a race car and won't be driven on rutted roads at speed. And living in snow country and liking off road driving and Jeeps, weraving all over the road is nothing new
I know Chevys in the 30's both Trucks and Passenger cars were set 23 degrees on the inside when the outside wheel was at 20 degrees. 47 HD Trucks were 20 outside 23+-2 inside. 57 Chevy was 18 deg 10 min inside, 20 outside. Calculating Ackerman Percentage is as follows. % Ackerman = (Angle Inside Wheel - Angle Outside Wheel)/Angle Inside Wheel for 100% Ackerman Where the inside wheel angle for 100% Ackerman is: Tan-1(WB/(WB/TAN(Angle outside wheel) - Front Track)) - Angle of outside wheel So it's a little easier to go with Toe-out on turn IMO. A Static measurement does not allow for slip angle. Under load in a corner the outside wheel will have more slip angle than the inside. A bias ply tire will develop a lit more slip angle than a radial at the same cornering load. So lets say we are in a hard low speed turn, and the outside front tire is overcoming 400 pounds of cornering force and the inner is overcoming 200. We have the RF Wheel at a 15 degree angle and the inside is at 17. Because of slip angle the tire is steering at 7 degrees, the inside is at 14 degrees. With radials the outside is steering at 12, and the inside is at 15.5 So while it is easy to see the ackermann effect when the triangles are drawn, real life is much more complex. So instead of going crazy, a street rod should be fine with 2-3 degrees of toe out angle spread at 20 degrees steering angle. 20 inside and 18 outside seems like a sweet spot.
Haven't you ever noticed how some trucks eat tires and others don't? I had a GMC ex cab short bed that ran down the road like a leather sofa on wheels and I got Super wear on the tires. My buddies Crew cab long bed ate thru three sets to my one. My SWB k5 blazer ate tires faster than any suburban ever could. I see two vans pretty regularly. One is a extended Wheel base and the other isn't. The extended one eats tires. So yeah, there's bazillions of different platforms and there is a difference in tire wear. The thing is, the fox is guarding the chicken coupe and they proclaim the expected wear and life expectancy of the tires- therefore it's reasonable because they said so.
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