Maybe I got too much time on hand but I was just thinking that wouldn't tire diameter affect real toe-in/toe-out measurements. I'm ***uming here that measurements are taken from the outer diameter (front and back) at spindle height. Any thoughts ???
No not really. As you stated perfect toe measurements are taken from spindle center lines. The spindle center doesn't care where it's ground height is on toe. Toe is set according to camber, camber defines the toe settings so its the only real variable.
I don't think he meant ground height, as a larger tire will need a bit more toe, as measured in, say, 8ths of an inch, than a smaller one, to get the same amount in degrees. In the real world, though, you'd be talking a miniscule amount of difference.
No ground height does not matter, but distance from spindle to outer diameter of tire does. If you set 24" tires @ 1/8 toe in, measured at the outside diameter of the tire, then put 30" tires on and measure for toe again at the outside diameter, you will have more than 1/8" toe in. I have always wondered the same thing. Toe measurements should, I guess, be spec'ed in degrees.
The modern way is in degrees. When I align old cars with inches specified I use an online calculator to convert spec to degrees. That takes into account the tire diameter. Half a degree of toe in on a huge tire would be more in inches than on a small tire. Sent from my iPhone using The H.A.M.B. mobile app
You example states exactly what I was driving at. Measurements in degrees would be more accurate. For specs given in inches, they should be specified at a certain tire diameter. But then again as was previously mentioned, any difference would most likely be minor.
Let's say for arguments sake that camber is 0° in both wheels. So we have a tire standing straight up and down. Now on this camber reading we set toe in at 1/16" of an inch. Regardless of tire height it's still set at a 1/16" of toe in. Now as stated above, if you go from a 24" tire to a 36" tire you will see a small amount of change but this is dependent on location of the tie rod end. BUT most vehicles only have a limited range of tire diameter ranges as the fender wells and suspension dictate what can be stuck on the car. So in most average cases your probably looking at a max of 2 to 3" difference in what will actually fit in the car at which rate the amount of change in toe is a non issue. Even if the car came with 15" rims and you install 20" rims the actual heights of the tires will probably still fall with in the 2 to 3" outside diameter max.
Your over complicating a simple setting. Toe in or out is measured from back to front of tire at the time of the alignment. So in simple terms. A 1/16" of toe is a 1/16" on a 20" tire and a 1/16" is still a 1/16" on a 36" tire. The only time this will change is if YOU the customer have it aligned with a 20" tire then go home and then install 36" tall tires only then will the plane change that the measurements where taken on. Also degrees vs. Inches is a mute point, it doesn't matter. Too many variables are involved. Tire width, suspension set up, road surface, wet or dry, toe is not a constant. It's always changing to some small degree, tie rod flex, turning, every time you go thru a bump, turning a corner, anything can change and flex toe and does constantly. Accelerating, braking affect toe. Toe is the biggest wear reading on a suspension but it's the simplest to understand and correct.
A given standard toe setting is just a beginning point for a car. Every car is a bit different than the next, so each might need adjusting beyond the standard toe setting. If everybody says an I beam axle Ford needs 1/8" toe in, that's a good start, but driving it and experimenting may show it handles better with 3/16". Or maybe even 1/8" out. Who knows until they try it?
When it comes to actual tire wear, toe is only dependent on 1 thing and 1 thing only, camber. Camber decides toe. Toe a car out a 1/2" and it will handle better. But at the expense of tire wear. It all depends on what your after. If you want maximum tire life then toe is set according to camber and nothing else.
All the alignment specs in the Motors manuals and shop manuals expect that you are running the wheels and tires that the car came with from the factory.
If camber decides toe, it would make sense that toe should be specified in degrees verse linear measurement. Or maybe due to difficulty measuring or setting toe in degrees, the conversion from degrees to linear has already been done for us.
I'd say your right I suspect back in the old days when owners actually worked on their cars it was easier to specify toe in inches because everyone had a measuring tape and adjustable in a drive way to some degree where as camber setting took special tools to accomplish and the old straight axles didn't move much on camber thru driving and old country roads anyway. We still have all the old tooling for bending straight axles on the old cars and even for twisting them end to end in the car for caster. The caster bending can get a little scary when one of the short 10 ton rams try to kick out. We still do quite a few. We do semi trucks the same as far as bending for camber but use double 110 ton, yes 110, rams. When that goes wrong you find out how fast a man really is. We call it the frame man shuffle.
Ooffah --- I've experienced a broken chain being pulled by a 10 ton ram and that was hairy enough. But 110 ton ? The noise alone would make me have to change my underwear.
I still do mine the old school way that I learned in the early 70's. With wheels and tires not always being true I jack up the car and use a scribe while I spin the wheel and make a line in the center of the tire all the way around. With the car on the ground I use a tape measured and measure line to line front and back of the tires. And set my toe in 1/16" increments as specified. Works great and the ones I have had checked at a shop are within 1/16" Easy to do at home and it's been very reliable. Cliff Ramsdell
Seems to me ,the toe in will only be dependent on the size of the tire ground patch and not on tire dia.. This tells me changing tire size, should minimlly alter the toe in setting.
It is pretty much white mans magic....caster speeds or slows steering, toe speeds or slows steering and camber effects the suspension changes. Tire size has little or nothing to do with it, it is about spindle and king pin geometry. http://www.gomog.com/allmorgan/CAMBER_CASTOR_TOE.html
We do it the same way, railroad chalk, a tire scribe and toe bar. We use this method because it doesn't care how out of round the tires or wheel may be. If the scribe Mark touches when you spin the tire you have a perfect circle to measure toe off of.
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