Front Wheels???

[paccorti]

Ok, the curiosity is killing me. Why is my tractor designed with the top of the front tires further out than the bottom of the tires? Kind of like toe-in but, instead of pointing forward, pointing straight down. I've seen this on Kubota's too. Why, why???

Peter

 

[Roy]

This is my guess (just my own deduction). You also notice the same configuration on some small vehicles (my wife use to have a 4WD Suburu wagon that had the same offset). I have always assumed that the reason is for accommodating loads on the front end. When the FEL is loaded, the tires flatten out. If they were not designed as such, they would toe-out when you need traction and control the most, rather than lay flat under loading. As stated, this is a guess.

 

[Bird]

Roy, cars and pickups have independent suspensions that allow the angle of the tire (top to bottom) to change as the load increases. Tractors (at least the ones I'm familiar with) don't, so I think you missed your guess./w3tcompact/icons/smile.gif Trouble is, I don't remember a good explanation myself, but I'm pretty sure it has to do with the steering geometry; to keep it going straight instead of trying to turn itself, less scuffing on turns, or something like that.

Bird

 

[Roy]

Correct, Bird. I failed to consider the differences in suspension. (Good thing I threw it out as only a guess). The steering theory does seem plausible.

Now, I'm also curious as to why, and at what tractor size and weight does the configuration change (maybe that will give us a hint).

 

[Anonymous Poster]

The name for it is "positive camber" , but I'll be damned if I can come up with a reason for it on a rigid , unsuspended axle. Dave.

 

[MJB]

This post will get into some steering geometry theory so I apologize if this is hard to follow. I think that you will find that all vehicles with a steering front axle will have positive or negative camber to a certain degree. I used to work as a mechanic in a shop that did alignments and the camber numbers in the reference book we used varied between about -1 degree to about 4 degrees of camber. This is usually not enough to see with the naked eye. My understanding is that there are at least two reasons for camber. The first relates to steering geometry. If you look at a steering front axle you will notice that the "Kingpin" or steering hinge is inclined at an angle such that the top of the pin is further inboard than the bottom. If you extend an imaginary line downward through the kingpin it should contact the ground in the center of the tires contact patch left to right. This is so that as the steering is turned the tire will pivot on the center point of its contact patch on the ground. This both reduces tire scuffing and requires less effort to steer. If the front end is configured such that the king pin is well inboard of the wheel hub it is necessary to tilt the wheel out at the top so that the bottom moves inward to coincide with the king pin location. The camber angle on compact tractors that I have seen is readily visible because the design of the steering knuckle places the kingpin much further inboard than you would see on a typical road vehicle. The downside of tilting the wheel in or out is that it causes the tire to ride on edge instead of flat. This causes increased tire wear but it is not considered a big deal for slow moving off-road vehicles. The second reason for positive caster is to equalize the load on the wheel bearings. Moving the bottom of the wheel inward transfers load from the outer wheel bearing to the inner one. The kingpin is also tilted so that the top of the pin is further back than the bottom. This causes the imaginary line through the kingpin to contact the ground slightly ahead of the center of the tire contact patch front to back. This is called positive caster and causes the wheel to want to go straight instead of turning left or right. I hope this information is helpful. MJB

 

[DVerbarg]

MJB,

Thanks for the post. Your description was great.

DavidV

 

[paccorti]

MJB,

Wow that was great! Thanks. The level of knowledge on this site is just amazing...

Peter

 

[Rowski]

Peter,

As an auto technician in an independent shop, wheel alignments are one of the very many task I have to perform. MJB explanation is excellent, but can be hard to understand with out the visual aids. I found a link that describes all the angle in an automobile. Toe and camber are the only angles the tractor world is concerned with. I have better visuals out of a Hunter Wheel Alignment text book but my scanner is down. Hope this helps.

http://aligncraft.com/terms/terms.html#Front camber

Derek

 

[paccorti]

Thanks Derek,

On another note. It's pretty impressive how the 4wd power must be transferred to the front spindles (from the front differential). It would appear that the power must make a right angle turn (straight down) and then another turn outwards to the wheel. All of this must also be enclosed in a mechanism that also supports steering! Pretty clever. This would appear to be more complicated than a typical truck 4wd mechanism. The added benefit of the arrangement would appear to be ground clearence.

Peter