After looking at RD's drawing of radius rod geometry would it be right to think at the start of a problem the top rod would be in tension and the lower in compression?? Bud.
Bud, They are both at rest until you hit a plot hole or accelerate. When you load them they both go into compression with the lower rod resisting the torque produced in the upper and the spring pack.
"plot hole"? oops, pot hole!
Terry,Thanks for the reply but when RD say's put a large pipe wrench on and rotate backwards? Probably everything changes with a shock load? Bud.
Either one would be in compression.
Looking at the load paths, I don't see how anyone could claim the axle would try to fold under on an early one with loads placed at the wheels, as the spindle axis is on the same plane as the upper wishbone. The moment-arm is actually smaller (less bending) with an early wishbone geometry.
The difference would be that a later wishbone would flex to create more caster when it hits a bump, whereas an early one would not flex. The momentary additional caster that occurs under load may help add some stability. I've never driven a car with an early wishbone on a bumpy road, so I don't know if there's any notable difference in steering feel.
All wheel loads go directly in a straight line to the spindle axis, which is well above the "beam" part of the axle. The only way to load the axle in a way that could fold it under is to add brakes (moment arm moves to the contact patch while braking) or to hit something with the axle itself rather than the wheels.
I'm not claiming to be an expert, but suspension design is something I've studied quite a bit for my other hobby, racing.
I live on a very bumpy one lane dirt road that is over a mile from the nearest pavement. I have cars with both wishbone styles and actually find that the early cars with over the axle wishbones seem to handle better on the road than the later style. That said, last year I added auxiliary under the axles braces to all my early cars based on the safety concerns raised on the Forum. I did not notice any difference in handling but assume the safety factor has improved.
That's odd... I was expecting an argument.
See that the spindle axis is on the same plane as the upper wishbone.
It's early Derek... lol
This is NOT an argument but just some info for your analysis since I have the info. I am interested in the discussion on the early wishbone and accessory braces since I have early cars. My son's '16 roadster has an angle iron brace under it and the rest of the front end has been rebushed and is tight.
The factory drawings show that the centerline of the spindle is the highest point of the wheel axis and the wheel stub angles down 3 degrees from that point to provide the camber on the front end. That centerline if extended across the axle from side to side will rest precisely on top of the spring perch-to-axle mating surface. The radius rod center is above that perch-to-axle mating surface by 9/16 of an inch to 19/32 of an inch (allowable tolerance) in early 1915 which is the drawings I have. The 1914 dimension for this is 9/16 with no tolerance window shown (I have that drawing too). So for sake of discussion it is 9/16 above the centerline of the front wheel hubs.
I don't know how this affects this discussion but I thought this info might be helpful since in the pictures the camera angle makes it look like the radius rod and hub are on the same centerline but they are not.
Carry on the discussion since I am interested in this and wish to learn what you have learned.
I get what you are saying that with the spindle and wishbone on the same plane there is little to no leverage under ideal driving conditions. Any time the wheel meets an obstruction while moving may be considered a braking action as it is slowing the forward movement of the car. If this is so, the moment-arm you speak of would move to the contact point where the wheel meets the obstruction until such time as the wheel has cleared it. Therefore, the moment-arm pivot point is constantly moving.
In my view, hitting a deep enough pot hole or rut sideways, say at 1/4 to 1/3 the height of the wheel, could result in a braking action by the front wheel(s) with enough force/leverage to flex the over axle wishbone ends. While it is a seldom occurrence, it is no different then the damage that can be done to a modern car's alignment in similar conditions.
Nope,I was not trying to pick a fight only trying to pick others thoughts! My interest is to look at the actual facts without jumping the gun and BS. Thanks in advance-Bud.
John, can you post the drawings? I'd like to see them. I don't have any early-wishbone cars or parts around here to measure from. I will admit that I'm simply going from pictures, but the moment arm would be very small if it's not exactly 0. It is certainly not 2" as shown in Ralph's illustration.
Dave, there would be no torsional loads or braking action as you described, except the minimal amount of frictional drag in the bearings. The load path would be a straight line from the centerline of the contact patch to the spindle axis. A small bump would produce a nearly vertical load path, whereas a large bump would produce more longitudinal loads on the wishbone and socket. The angle of the load path would determine the what percentage of the load is transferred to the spring or wishbone.
For convenience, here is the drawing again:
Looks like I may need to correct my measurements.
I would like us to get together and make up some semi-standardized tests we all could do.
*Wheels turned left to full lock.
*Chocks in front of right wheel.
*Push car from behind and observe actions of all suspension components; on cars with early and with late wishbones.
Obvious fault in the drawing - the text makes statements that are untrue. Any force rearward on the axle would result in the same action from early or late wishbone. Neither one bends easily. There would be no movement in a car with either type of wishbone provided the wishbone ball studs are properly tightened until the spring is completely compressed. Caster cannot be changed by anything short of a collision that permanently damages either type of wishbone.
Your hatred for unmodified Model T's causes you to force your agenda on us.
Most of us have seen the results through vintage pictures of a early wishbone failure. If the placement of the early wishbone on top of the axle is not the problem, them what are the other scenarios that would cause the front axle to roll under the car when there is no evidence of a collision with another car or large object?
Does anyone know of any internal Ford engineering docs that state *why* the change was made? Someone may have done this analysis already - about 95 years ago.
Looking at Ricks drawing, the "fold under" consequence seems entirely logical, if the connection points on the wishbone, and the center of mass icon are correct. On the early wishbones, with both the center of mass and the wishbone connection above the center of the axle, all forces (the one on the wheel, the momentum of the car, the longitudinal forces on the wishbone) will cause a moment in the same direction around the axle center (counter clockwise in this depiction). Which would theoretically result in a folding under of the axle if that moment is enough to overcome the axle's resistance to twist. If you move the wishbone connection down below the center of the axle, you finally have a force to counteract the moment to some degree.
The fact is, early wishbones don't fail unless you are in a collision. Same is true of a late wishbone.
I went back and read Joe Glamb's story in Tin Lizzie by Stern and if we read everything we are told what happned,and what was replaced.Maby speedreading is at fault or the way Joe Galamb guided the story to the radius rod? If the wheels colapesed first as told would any assortment of rods braces have helped after the wheels broke? Bud.
That's my point... Ralph's drawing is incorrect. The upper wishbone connection point is right at or very close to the axle centerline. It is NOT 2" above as it is illustrated.
In Stern's book Joe Galamb tells a story of how Henry Ford forced his driver to go wide open, leading to a collision with a sand bank that collapsed the wishbone. The wishbone didn't cause the accident. Driving too fast caused him to crash the car.
Deep soft sand (or mud or snow) would reduce the amount of caster regardless of wishbone arrangement.
The center of the tire contact patch would move forward and possibly in front of the king-pin axis to create a negative-caster scenario without actually seeing any change in the angle of the axle.
This could have forced the wheels to go full-lock BEFORE the wishbone failure. Wishbone failure would likely be the result rather than the cause of the loss of steering control.
The first accident caused by an early wishbone reported on this Forum was prior to 2002, by Mark Rand in England. He rounded a corner sharply, and the wishbone bent.
I'm pretty sure this is Mark Rand. Terry Horlick messed with the original pic.
I have to eat a bit of crow here, and admit I missed a big piece of the front end dynamics...
There WOULD be torsional loads on the axle while making a sharp turn at higher speeds.
Hitting a bump while driving straight would not change the caster, but going too fast while turning could.
I think there is a lot of hot air and speculation going on here. Go back to simple physics and examine the geometry of the front end. The structure forms a triangle defined by the attachment points: pan socket, spring/frame attachment, and point of contact for the wheel, not the spindle--that becomes the lever point
of. The higher the lever point, the greater the force applied compressing the wishbone. So looking at the early front end in this way, you can see why they occasionally collapse and I for one have not heard of any insidents like this involving the later style. And remember, Royce, even Henry had to admit to the fault and made a change.
I'm waiting for some geometrical analysis showing the early wishbone can't do what is accused. All I see is a lot of bloviating.
Even though some folks post HFM drawings on this public forum - it is a violation of an agreement they signed to NOT PUBLISH the drawings but use them only for their private use. I can give dimensions from the drawings but the copyright on the drawings and info is the property of HFM. Since researching at the archives is my passion, I don't want to endanger my archive priveleges - Sorry - I can't post them for you.
Now I'm confused. The inclination of the front axle is tilted to the front at the bottom resulting in the contact patch of the tire already leading the center of the axle in the vertical plane. From what I been taught, this is positive caster. If by driving through soft sand or mud, the contact patch of the tire is naturally moved forward, wouldn't that increase the positive caster or would the resistance want to push the assembly backwards thus inducing neutral or negative caster?
Bloviating??? Egad! Fap!
All I can say is that the early radius rod was a bad design. Any load pushing on the front end will cause deflection, but the question is, to what degree? More than likely, a fairly insignificant amount under normal driving.
A large force will put a significant moment on the radius rod at the most obvious point, right where it enters the perches. Put a large enough force on the front end and it will snap the wishbone most likely right at that spot, or at least bend it.
On the later design, there will be some deflection as well, but MUCH less, and it will take much more force to cause permanent deflection or failure than the early style.
It's all just a matter of simple leverage. Plus, there must be a reason why Henry built the vast majority of Ts with the under-the-axle radius rod!
Being involved for over 50 years with Model T's I have never heard of this as a problem except here by some who don't appear to have anything but an opinion that it should be one.
I have had an above axle wishbone fail, but although the design may be not as good as the later style it never the less does work as it should if it is installed correctly and in good condition. There is usually a reason for such and its not all design.
The later design is a cheaper one to produce and may only have been changed to save money. The later design does not require as precise fitting as the above axle one does.
In the earlier design the fit of the ends into the perch bolt needs to be tight, real tight. Often when restoring builders using old parts put together components which have worn or rusted to an extent that upon fitting they are far from being good enough to do the job properly. If they are loose and you force the axle by trying to tip it back at the top the wishbone ends if loose turn in the perch bolt holes, if they can't turn its extremely difficult to get the axle to tip.
If the ends can turn in the wishbones it can't keep the axle from changing. Maybe pinning the wishbone end to the perch bolt would be a thing to do if you don't want to add an extra brace or you suspect the fit is poor.
When I first got my 1911 Ford it had been damaged in an accident on its first outing after restoration. The road was wet and the T slid into the rear of a car on a wooden bridge. The wishbone bowing up and the axle ended up with negative caster.
The owner heated up the wishbone and bent the axle back to where it should have been. It was driven for a number of years by the two previous owners without problems.
Once when I was out in the car I had to turn into a street which met the street I was in at a fairly steep down hill angle. When I turned into the street at a slow speed the T immediately dived to the other side of the road. Luckily it was a quiet street with no traffic. As I was going slow I was able to stop in a few feet but when I tried to proceed back to the correct side of the road forward I could not steer it, it would dive right or left. The only way I could get to the side of the road was to reverse back which was easily done as I had great castor. I walked a short distance to a friends place and we used a large plumbers wrench to hook onto the axle and bend it back at the top. As the wishbone was soft it bent easily. Upon inquiring with the club members I was told about the previous accident and heating up and softening of the wishbone.
It took a lot of finding to get another wishbone that fitted into the perch bolts properly, once in and tightened up with its nut it should take a great deal of force get the axle to tip. If the connection is loose it can be tipped easily with a 3 foot lever. you would need to ram the front axle into something solid at a good speed to cause it to fail. Doing so with either wishbones would make them bow and the axle assembly would naturally be damaged enough to mess up the steering.
The above wishbone was used for a fair while, the roads were terrible people would have been putting the early wishbone to the test all the time and there does not appear to have been the massive problem some think exists. There were front bearings running short of oil, fuel starvation going up steep hills etc but not anything on failing above axle wishbones persisting in Model T folk law.
I'm putting my money on components of poor condition assembled far short of being good enough to do the job they should be doing being the problem rather than a dangerous design.
On level ground, the extended centerline of the kingpin hits the ground in front of the contact patch. That is positive caster. In the event the front wheel encountered an abrupt incline, say a trailer ramp or sunk deep in sand, the contact patch is now ahead of the extended centerline of the kingpin, giving negative caster. Note this is purely in the geometry and not due to any deformation.
Thanks Hal, I get it know
Would anyone with an early wishbone conduct a test to see how much torque on the axle is required to flex it to 0 caster? I don't have an early axle car, and even if I did, I'm not so sure I'd start bending stuff just for an internet discussion.
Simply for the sake of argument, lets say it's 200ft/lbs (completely arbitrary number, but it seems reasonable)
Using the 9/16" number of how far the axle centerline is below the wishbone according to the original drawings, it would take 4,267 pounds of longitudinal force to produce 200 ft/lbs of torsional load on a 9/16" radius. That's longitudinal force, as in hitting a wall with the front of the tire, or a 15" tall bump. A 7-1/2" tall bump would require 8,533 pounds of force to apply the same force at its linear 45deg vertical angle. 7-1/2" is a pretty big bump... Double that number again for a more reasonable 3-3/4" bump, and you're at 17,066 pounds of force required from a large bump you may see at road-speed on any typical road I've ever been on.
That's quite a bit of force for a 1,600 pound car to create, but shock loads are quite a bit different than sustained loads, and honestly I don't know all the formulas to calculate them. (I'm not an engineer) I simply don't see a pot hole or railroad track being able to bend flex things into a negative-caster scenario, at least not if the parts are in good condition.
A sharp turn is more likely to create enough force to do it. When turning the wheels 20deg, (which is a lot at road speeds) it would take 694.6 pounds of lateral force, or around 0.84 G's... no way there. That's comparable to modern sport-sedan cornering G's. Like stock BMWs and such...
At lower speeds near full lock though, you cut that number in half for a 40deg turn. Can a Model T pull a 0.42 lateral-g? I doubt it, but I'm not going to hang an accelerometer on my car and drive it at its limit just to prove one way or another.
I'll have to re-check some of my math tomorrow, when I have AutoCAD at my fingertips... I've become a bit dependent on it for geometry involving circles. Without the most important number though, that being the torque required to flex the axle to 0 caster, all the rest of the math is just a fun little exercise.
No way a bump is going to make it go towards zero caster. Any force upwards on the wheel is going to tend to try to rotate the axle towards positive caster. I don't think 200 foot pounds would have any effect at all in any direction.
Have you seen the Ford / KR Wilson axle / wishbone adjusting tool? It's a big heavy piece of forged steel about 6 feet long shaped like an "F".
The only force that would tend to make the axle go towards zero caster would be a front end collision, which should be something you would normally try and avoid.
Glad to see you're thinking it through, Derek, rather than shooting from the lip. Loosen a nut on the end of the early wishbone, and watch the numbers change drastically.
Why do you think a T won't do a half G lateral?
Speaking of shooting from the lip,where are those pictures??
What pix, Bud?
If you mean the write-up I had on my old website, be patient. The ISP went titsup and I lost it. I have to re-load the page on another site, of which I have four. Haven't decided which ones I'll keep for long.
Half a G at near full-lock? Seems unreasonably fast, even for me. (and I'll admit to driving faster than I should)
I suspect it may have been a bigger problem on the roads of 100 years ago, with mud, sand, rocks, etc. Adding some slip-angle into the equation, the numbers change quite a bit when you start applying shock loads, such as hitting a rock with the side of the wheel. I just don't see that happening on todays roads though... at least not on any roads I drive on.
With good parts, properly installed and maintained, I just don't think it's quite the problem it's made out to be here...
Peter posed, "I have had an above axle wishbone fail, but although the design may be not as good as the later style it never the less does work as it should if it is installed correctly and in good condition.."
"..The later design does not require as precise fitting as the above axle one does."
"In the earlier design the fit of the ends into the perch bolt needs to be tight, real tight. Often when restoring builders using old parts put together components which have worn or rusted to an extent that upon fitting they are far from being good enough to do the job properly. If they are loose and you force the axle by trying to tip it back at the top the wishbone ends if loose turn in the perch bolt holes, if they can't turn its extremely difficult to get the axle to tip."
"If the ends can turn in the wishbones it can't keep the axle from changing. Maybe pinning the wishbone end to the perch bolt would be a thing to do if you don't want to add an extra brace or you suspect the fit is poor."
That all looks like a pretty complete condemnation of the early perch, to me.
"...design may be not as good as the later style..."
"...if it is installed correctly and in good condition.."
"...fit of the ends into the perch bolt needs to be tight, real tight.."
"..Maybe pinning the wishbone end to the perch bolt..
In support of the other theory, the later style wishbone and spring perch arrangement requires less machining operations and has fewer parts than the earlier design. That would make it cheaper to make and install. The savings of two nuts and two cotter pins per axle assembly turns into a quite a profit when you're making a couple million cars per year.
It was not an either/or decision. It was an AND decision. "We improve safety and we save money."
The factor that seemingly nobody takes into account in any of this is the front spring. In fact, the spring plays a huge roll in any analysis of front axle/wishbone distortion because it also adds a large degree of support to the axle assembly.
In the case of the lower wishbone, the spring adds support above the axle, while the wishbone gives support below.
In the case of the high wishbone, the spring gives support above the axle as does the wishbone. Derek suggests that because the point of support given by the high wishbone lies on, or close to, the spindle centerline, there can be only a small moment arm acting to distort the geometry. However, if we consider that the wishbone has a higher degree of flex, or elasticity, than the spring, (considering the elasticity of the spring only in the fore & aft directions), then the most rigid support member in this assembly is the spring. That being the case, the spring attaches at some point decidedly higher than the axis of the spindle shafts and supplies a much more effective moment arm.
I'm not saying that any of this is detrimental to every day, normal driving. But I would take it into account when abnormal forces are applied to the front end, e.g. hard cornering, soft ground, road hazards, otherwise minor collisions or, as Royce might add, high speed driving.
"I'll have to re-check some of my math tomorrow, when I have AutoCAD at my fingertips... I've become a bit dependent on it for geometry involving circles."
I knew a couple numbers were suspect when I posted that, but I was off a bit more than I thought. Like I said, I'm far too AutoCAD dependent when geometry involves circles.
45deg load path occurs at 4-3/8" not 7-1/2"
22.5deg occurs at 1-1/8" not 3-3/4"