Mystery steering pull, could it be the tyres??
Mystery steering pull, could it be the tyres??
Second, I hope this is the right forum, couldn't decide whether it would fit better in the tyre / wheel forum...
At the risk of being one of those people who ask less than fascinating 'I have a problem with my car' type questions, I have a problem with my car, that is hopefully intriguing enough to elicit some response...
The car (RHD Honda Accord, with SLA suspension) has a strong tendency to pull left (more like a mission in life to pull left...), that I can only alleviate by running very different caster angles left vs right. I'm currently running 6.5° caster on the left (way more than stock) and about 3° on the right (within stock range), and while the left pull is still slightly there, it's much less severe than with equal caster on both sides.
I am intending to pull even more caster out of the right side to see if I can coax the car into tracking perfectly straight, though it irks me greatly even as it is to be so lopsided (I want my steered camber changes to be equal, and they aren't!).
The mystery arises because the suspension / steering geometries are otherwise all symmetrical to quite close tolerances, i.e.:
• Rear toe zero on both sides (i.e. thrust angle zero)
• Rear camber equal to within 0.1° (-0.8).
• Front camber equal to within 0.1° (-1.5°).
• Included angle equal to within 0.1°.
• Scrub radius equal to within 1.5mm (+8mm and +9.5mm).
• Each spring pair is equal free length (i.e. no sagged springs).
• Chassis is untwisted (tower heights all within the same plain + - 1mm).
• Front anti roll bar has been removed, and rear ARB doesn't appear pre-loaded in any way.
All sub-frames are tight and I can't find any suspect suspension components, all articulated joints / bushes seem to be fine. I've carefully measured the front uprights and both have almost identical relationships between upper / lower ball joints and hub axis (interestingly discovered that the hub is forward of the steering axis), and trail is thus equal to within 0.5mm at equal caster (though of course not with unequal caster). KPI is equal to within 0.05°.
I've tried numerous alignment settings, including asymmetric settings, and only substantially asymmetric caster seems to improve the problem to any appreciable degree (when making any asymmetric change I have equalised the camber and reset toe of course). An exacerbating factor (but surely not causative), might be that I like to use quite high tyre pressures (around 45psi).
The pull isn't caused by road cambering to the left (though this does of course make it worse), the road needs to be visibly cambered right in order for the car to track straight. Steering is also heavier right vs left, more so the more equal the side to side caster is. I have accidentally managed to briefly achieve straight tracking with zero pull, but only with lesser right caster AND both front tyres having significant camber wear off to the right (countering the left pull due to this conicity).
This has been an ongoing issue since I've owned the car, through several sets of tyres, through swapping rims / tyres side to side, and 'flipping' a tyre or tyres on the rim. With the current tyres swapping and flipping merely results in changing the severity of the problem, i.e. either a minor or major left pull, so the current front tyre pair at least seem to have some pull inherent in them (conicity, 'bias steer'..?). But, if the tyres were the root cause then I'd expect that swapping them around etc would result in a pull to the right (surely?), which I've never achieved no matter what I've tried.
I was suspecting unequal power assist left vs right (i.e. leaking seals in the rack hydraulics etc), but have recently removed the PS belt and only achieved heavy steering (which I much prefer to the overly light assist). The fact that it's been a problem with different sets of tyres has had me thinking that it must be some mystery geometry problem, but I'm starting to wonder if it isn't tyre related after all (considering that very careful alignment to symmetrical settings hasn't fixed it, and I can't think of any other geometry that could possibly be the culprit).
Now, to my suspicions about tyres; I've noticed that very many non directional and notionally 'symmetrically' patterned tyres don't have a truly symmetric tread pattern, but have grooves that are more or less diagonally oriented across the width of the tread face. This pattern 'diagonality' doesn't change if the tyres are swapped side to side or flipped on the rim, i.e. the diagonal orientation is always the same no matter how the tyres are fitted, so, all tyres on the car exhibit this tread pattern asymmetry in the same direction...
Could this tread asymmetry of itself cause a pull? Could the tyre manufacturers be deliberately making tyres with an inherent pull in an attempt to compensate for pull caused by road camber??. Could any such tyres be designed to counteract road camber pull where the car is assumed to be LHD and driven on the right hand side of the road? Could such tyres (if they exist) end up being sold into markets where we drive on the 'correct' side of the road, and thus cause the car to hunt for the gutter????
From memory all the tyres I've used have had a somewhat similar 'diagonal' tread pattern...
This problem is irritating purely on the driving level (the pull kills steering sensitivity, and I suspect contributes to tyre wear), but it's also driving me nuts that I've just failed to diagnose the problem, so it's intellectually frustrating as well.
Might I be on the right track thinking it could be tyres??? Are there any other tyre related issues (other than diagonal tread pattern) that might cause a pull and be immune to how the tyre is mounted? Can anybody think of anything else that could be causing this? Any considered or wild theories will be entertained...