honda's scrub radius
#1
honda's scrub radius
Hello guys I would to know why honda's car are sensitive of scrub radius?
I have several cars, sometimes with a lot of positive scrub radius but they was never so sensitive than my honda on bumpier roads, the car wants to pull or follow ruts in the pavement with some positive scrub radius.
Does it come from the steering or from other think?
Thanks a lot guys.
I have several cars, sometimes with a lot of positive scrub radius but they was never so sensitive than my honda on bumpier roads, the car wants to pull or follow ruts in the pavement with some positive scrub radius.
Does it come from the steering or from other think?
Thanks a lot guys.
#2
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Re: honda's scrub radius (ti pat)
Originally Posted by ti pat
Hello guys I would to know why honda's car are sensitive of scrub radius?
I have several cars, sometimes with a lot of positive scrub radius but they was never so sensitive than my honda on bumpier roads, the car wants to pull or follow ruts in the pavement with some positive scrub radius.
Does it come from the steering or from other think?
I have several cars, sometimes with a lot of positive scrub radius but they was never so sensitive than my honda on bumpier roads, the car wants to pull or follow ruts in the pavement with some positive scrub radius.
Does it come from the steering or from other think?
SR will cause the centre of the contact patch to 'want' to swing around the steering axis when longitudinal forces are acting at the contact patches, so if you have (say) positive SR then under power the front wheels will try to move forward and swing around the steering axis, and move backward around the SA under braking (vice versa if SR is negative).
This is 'torque steer', and it's not really a product of engine torque so much as the tractive forces generated at the contact patches being reacted into the steering from points (the two contact patch centres) that are laterally offset from the points where the steering axes intersect the ground (i.e. the scrub radii), and these forces create a rotational torque force centred around the steering axes trying to ‘steer’ the wheel one way or the other. If these are fed into the steering axes unequally enough side to side (and in effect they often are) then the driver will feel 'torque steer' at the steering wheel.
Unequally split axle torque or contact patch grip (possibly created by torsionally dissimilar axles, or loss of grip at one contact patch) can have an effect and may cause the torque steer effects to be unequal side to side, and this is because the tractive force is unequal at the contact patches. If the SR were effectively always zero on *both* sides then unequally split axle torque (or single contact patch grip loss) wouldn't result in a 'torque steer' that the driver would feel at the steering wheel. This is because with zero SR there is no 'lever arm' (i.e. SR) through which torque steer effects can be multiplied, and the tractive force is always acting from a point that is longitudinally directly in front of the steering axis intersection with the ground (the CP always ‘wants’ to be directly behind this point).
In an ideal world (i.e. perfectly smooth roads) these effects are balanced side to side due to the geometry being equal side to side, but in the real world as the contact patches roll over uneven surfaces the centre of contact patch loading moves across the face of the contact patch (as it traverses bumps / undulations), and thus the effective centre of the contact patch moves laterally relative to the point at which the SA intersects the ground.
This means that the *effective* SR changes over uneven surfaces, so the side to side effective SR becomes momentarily unequal and the forces acting at the contact patches then leverage forces into the steering axes unequally (because the SR is a 'lever arm' that effectively changes length). This creates torque steer effects, whether initiated by braking forces (which create a torque force fed into the steering axis through the SR), or from engine torque with FWD (same as braking force but in reverse).
With zero (or close to zero) nominal SR you can easily get the effective SR becoming positive on one side and negative on the other (over bumps / undulations), so you may get moments when at both front wheels the torque reaction into the steering axes are both causing a torque steer in the same direction (which can also happen if *nominal* SR is significantly positive or negative, as opposed to zero, but it happens less readily because the centre of CP loading has to move more for the effective SR to become pos on one side and neg on the other). When this happens you tend to get quite strong torque steer at the steering wheel because both wheels are ‘pulling’ in the same direction rather than one pulling X in one direction and the other pulling Y in the other direction.
These effects tends to be stronger the wider the contact patches are (more distance across which the centre of loading can move), the less rounded the tread profile is, the stiffer the tyre casing is (particularly the sidewalls), and the higher the psi in the tyres. The higher the psi the higher the effective casing stiffness is, and the higher the case stiffness the less easily the tyre will conform to uneven surfaces ('absorbing' load changes across the contact patch face, meaning the centre of loading tends to move less with lower psi and / or less stiff casings) and the more the centre of loading will tend to move across the contact patch face.
Note too that adding neg camber will move the effective contact patch centre by moving the centre of CP loading, i.e. more load on the inner part of the contact patch and less on the outer part. So, when you add significant neg camber the nominal SR remains unchanged, but the effective SR may move from being zero to being negative, or from positive to zero or negative. This will happen more so with wider tread, stiffer case, 'flatter' tread, and higher psi.
So why design steering with low SR if it means that the effective SR can more easily move from pos to neg and create torque steer? Partly because you still get torque steer whether or not the effective SR moves from pos to neg or vice versa on one side re the other (the sum difference in SR is important, not just whether one side becomes pos while one remains neg), but mostly because the greater the SR the more the steering will tend to suffer from 'kickback' when one wheel hits a bigger bump, which creates another very abrupt torque steer effect that may be worse than the torque steer you get from the nominal SR changing from neg to pos or vice versa side to side.
If you significantly increase the wheel offset you will also change the SR (from zero or positive to more positive in most cases, but sometimes from positive to zero or negative). Assuming an increase in positive SR you will probably increase steering ‘kickback’, and if you also have wider tyres with stiffer casings at higher psi then you’ll also tend to increase other torque steer effects (note that kickback is a torque steer effect, not a bump steer effect, that’s a different animal altogether). There can be a number of accumulating problems leading to increases in torque steer.
Also, if caster angle is relatively small (most Hondas don’t seem to have much caster angle) this means there won’t be very much ‘trail’ (longitudinal distance from CP centre to steering axis intersection with the ground), and thus there won’t be all that much self centring action in the steering. This means that any increase in torque steer effects due to changes to nominal and / or effective SR won’t be as ‘masked’ by the self centring action created by caster angle (insofar as caster affects trail). Increasing caster angle may well lessen torque steer affects as felt at the steering wheel.
Does all this blather this answer your question?
#4
Re: honda's scrub radius (johnlear)
Thanks a lot.
I already SR, and what hapen with too much SR, but I don't think about castor. I also found near zero SR on my civic with stock wheels.
and I will see what's happen in the ackerman if a reduce arms steer in order to reduce the torque steer.
I already SR, and what hapen with too much SR, but I don't think about castor. I also found near zero SR on my civic with stock wheels.
and I will see what's happen in the ackerman if a reduce arms steer in order to reduce the torque steer.
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Re: (PatrickGSR94)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by PatrickGSR94 »</TD></TR><TR><TD CLASS="quote">I think you overwhelmed him just a tad bit.... I know I am
for extensive info.</TD></TR></TABLE>
Sorry about that. Once I get some speed up it's hard to stop! I just wish it was easier to explain more succinctly, and diagrams would help...
for extensive info.</TD></TR></TABLE>
Sorry about that. Once I get some speed up it's hard to stop! I just wish it was easier to explain more succinctly, and diagrams would help...
#6
Re: honda's scrub radius
To answer OP's question, handling becomes Very unstable when scrub radius nears 0. This is because it is constantly changing as the car goes over bumps or through ruts. If it's near 0, out will change from negative to positive!
Hondas, and most all front wheel drive cars, have NEGATIVE (not 0) scrub radius. When you install wheels with less offset (most Hondas have +45mm offset) or smaller diameter tires, you increase scrub radius.
My cb7 accord and ef CRX with factory wheels had scrub radius' of about -8 or -10mm with suspension compressed to factory ride height. I'll try to post pics later to help you understand.
Hondas, and most all front wheel drive cars, have NEGATIVE (not 0) scrub radius. When you install wheels with less offset (most Hondas have +45mm offset) or smaller diameter tires, you increase scrub radius.
My cb7 accord and ef CRX with factory wheels had scrub radius' of about -8 or -10mm with suspension compressed to factory ride height. I'll try to post pics later to help you understand.
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