David, were any of those bias plies? I have never experienced anything like that except when mixing tire types, and based on when you say that happened, I believe could have been the case. I will say I have seen 2 new tires with 2 old tires trip ABS lights. But that is ultra rare. AWD Volvos would have trouble with only 2 new tires too. IIRC, Volvo used that as an excuse for their AWD unit failures (some kind of vicious coupling or something, I don’t remember anymore). All due to overall diameter differences.
I spent 7 years managing a tire & related service business. A busy shop, we moved around 150 tires a week. It also included a good deal of training from Michelin (we were official M.A.S.T. dealers). It was Michelin who drove home the point about new tires in the front causing oversteer issues. They did demos for us to prove the point. Because, as noted, it seems counter intuitive.
From the Uniroyal website:
If I only purchase two new tires, where should they be mounted?
For front or rear wheel drive vehicles, we recommend mounting the new tires on the rear axle, in order to prevent an oversteer condition. When purchasing a single new tire, it should be paired on the rear axle with the tire having the greatest remaining tread depth. When radial tires are unavoidably used with bias-ply tires on the same vehicle (not recommended), the radial tires must ALWAYS be placed on the rear axle. NEVER mix radial and bias-ply tires on the same axle.
I’ll go further with real life experience. My 82 Trans Am had 16” Goodyear gatorbacks out in the rear. I put Michelin MXXs in the front. Whoo-we, did that create an oversteer issue, especially cold. Those MXX were soft & sticky – the exact opposite of the hard as nails gatorbacks. It took some playing with air pressure and learning to drive in the cold to help that out. I didn’t have enough cash for all 4.
But my training backed up my personal experience.
I understand the theory that front tires do most of the steering & braking. But IMO, if the tires are not good enough for the front, they are not good enough at all. Oversteer is the hardest thing for a regular driver to handle, and I would never consciously create it (again, we are talking about regular, retail customers). And think about it, rain compounds the issue. If the new front tires grab the wet roads better, you are creating even more oversteer since the worn rears will slide that much easier. What’s going to happen with the ½ worn tires up front? Spin in the rain pulling from a light. Ok, so what. Sliding? Welcome to understeer, slow down – ABS makes understeer even less of an issue to Soccer Moms. But oversteer? I hope Mom knows how to drive. Most people don’t (ask Ford & Firestone). Understeer is a compromise I am willing to take. All 4 tires touch the ground, not just the front.
Back to topic….I also agree on tire related pulls. If swapping front tires changed the pull that is a radial pull – a factory defect covered by warranty. That is also why I called out air pressure. Based on what sounds like a unidirectional pull, something is being missed. Something bent, out of align or loose.