I just replaced all the front suspension parts of my 48 chevy coupe with NOS parts. What a difference to 60 year old worn bushings and shocks! I read somewhere that for radial tires the toe-in alignment should be more straight and the camber closer to ZERO ? That seems to be more true for modern design suspensions. I have more control and the curves are easier to hold when going with the shop manual specs. Is that the compromise when going to radials on an original 40s suspension? A better ride will wear the tire edges sooner, or you preserve the tires but handling is poorer?
Lets put on the thinking caps for a minute. You can evolve the front end settings, but I'm simply unaware of just how they would "change". Radials have more flex and sidewall bulge. I went from bias ply to radials in '73. I don't think I'd do bias ply again unless it was purely for points at a show (many here like/prefer bias.) Toe-in: I would be surprised to find that having less toe-in would serve the changeover. A frontwheel drive car requires neutral or toe-out(?) but not an old school front end. So you are simply trying to keep the toe set, so that the car doesn't "chase" or follow ridges in the pavement. The "pre-load"(amount of toe) tends to align the tires, and the car maintains a stable direction. You can try minor adjustments, but know that once you pass into min/no toe, it's going to wander on you! Camber and caster can be adjusted, and you might try a Search for some good tips.
I keep reading online that angles for toe-in and camber should be closest to zero degrees for least tire wear and best handling. That's probably true for modern front wheel drive and suspensions, because I get a little too much road wander and it's harder to initiate a turn and to hold a curve. The settings that give the best handling is what I'm going for in the end. Plus I don't have faith that modern alignment shops really understand 40s suspensions with radial tires. At best, they can only adjust by the book and less from real driving experience. I guess the edges of radials will wear out relatively soon, but then handle better and wear less, as they gradually get a rounder profile like bias plys.
On most rear wheel drive cars, try placing your open palm down on the top of the FRONT tire, and draw it outward, away from the radiator. The inside edge of each tread "row" will feel sharp and the outside edge will feel rounded. Think of the two front tires w/loads of toe-in, plowing across aparking lot. The amount of sharp edge is an indicator of how much toe-in, along with other alignment factors and tire compound. Usually when running at a sports track w/turns, you place some chalk marks up the outside of the tire to determine when you've got the tire pressure right, for cornering footprint. Tire pressures are varied in order to find good footprint-handling, ride, wear and mileage/efficiency. When playing around w/tire pressures, too much means low rolling resistance, but high wear in the center...and none on the edges. Kind of opposite to the concern you had mentioned. I suspect that your car will benefit from having a "stance" that sets the front end up at a level height so that the geometry works as designed. Then there's tire/wheel size, offset and a host of factors that influence where the geometry (ex: turning) reaches compromising angles. Properly set-up and aligned, I found that the front radials wear well and last 40+K miles. No compromise reqd. if your geometry is OK.
Thanks for all the tech insights. Not suprising how critical tires, suspension and steering adjustments are, when there is only such little contact with the road. Every little adjustment can make a big difference obviously and driving habits and the type of road just add to it all. It seems that with the old suspension and rear wheel drive one has to sacrifice somewhat on radial tire life, in order to get best handling possible. I rather get a little less life from the tires than having less control, especially when going at freeway speeds for hours in heavier traffic.
The purpose of toe-in is to compensate for wear and ordinary 'give' in your steering components. Slight toe-in plus wear and give equals your wheels going straight ahead. Without any toe-in, your wheels would toe out from the forces applied.
One thing to remember when comparing different era specifications is the roads that those specs were engineered for. Roads in the 40s were mostly high crowned 2 lanes designed to shed water. Todays highways are much closer to being flat than the roads that the old car was made for. If you drive the old country roads then you might want to use the 1940s specs. If you drive the 4 and 6 lanes like me you might want the tires closer to straight up.
You are right about the road's profile. I have read in an old post about giving a little extra angle of camber to the passenger side because of the dropping curve near the shoulder on most roads. I just got back from the hardware store and picked up an affordable angle locator and will adjust the camber just a little straighter than stock and the toe in will be whatever feels best, regardless how much it measures. What I like the most about old cars and trucks is that they talk to you and tell you exactly what works best for them. I just wasn't sure if there are any settings that are ideal for radials. I guess only my car will know for sure in the end.
Seems that I started w/about 1/16th for toe-in, "slight" amount of negative camber (in degrees) and as much caster as the hybrid frontend would permit. The steering returned to center easily, and steered very well when backing up, which came as a bonus. Corvair susp., V8 Vega coils, Chevelle spindles, and MG rack & pinion. Nope, there were no settings, so you come to trust your instincts and measure carefully. Favorite tools were a 6' level w/two framing squares clamped to it for toe, and a bubble machinist's level for the camber. This was in '70 and the car was fully fendered, hopefully to appease the purists.
Radials are not new technology! Michelin has been making nothing but radials for 100+ years. There should be no change from factory specifications if your car has been rebuilt to stock.
That's the underlying question - does suspension, steering and drivetrain design determine ideal alignment specs, or is it the type of tire? It seems both matter. Probably best to start with original alignment specs, regardless what tires and then just fine tuning from there....