You can see in the future how computer controlled individual wheels will happen, we already have non-connected power steering, why not control them separately?
morrisman1,
Mar 2, 7:41am
are you sure? Im relating it a little to effective crank angle, the angle between the conrod and the Big-end to main bearing lever arm. When the piston is about 40% down this is 90 degrees, and you get a large movement of the conrod (tie rod) for a small movement of the crank (hub rotating on steering axis). alternatively as the piston reaches BDC, the conrod and crank are starting to line up and a small movement of the conrod (tie rod) makes a large movement of the crank (hub steering). This is what is happening on my inside wheel: the tie rod is starting to line up with the steering arm, resulting in a larger rotation of steering axis, although it would take more force to achieve it as the leverage is inferior.
forgive me if Im wrong, Im just trying to understand how this all works.
kazbanz,
Mar 2, 8:21am
MM--I get that you are wanting to work this out to work it out. BUT for the purposes of setting the car up I feel you might be overthinking things initially. Why not go back to the really simple stuff first before mucking around with more complicated setup issues? You Im guessing are trying to set up specifically for teratonga. Given theres only the one real right hand turn and you have heavy sideways scrub on your left tyre id be tempted to play a little with tyre pressures and camber a tiny bit. If you can do laps with just one single change and see what it does to the tyres it would be usefull. hey sorry mon you are /have probably already done this stuff. -But Ive noticed over the years a lot of guys get themselves tied in knots with their setups and the real world solution was something really simple
see how the rack on the race car is relatively more rearward of the tie rod ends than in the stock geometry. We never had this issue when using stock lower control arms, but had other issues of outside tyre wear.
morrisman1,
Mar 2, 8:38am
Thanks kaz, but the graining on the tyre shows significant sideways scuffing while the outside tyre does not. This shows to me that during turns they aren't turning at the appropriate angles. The outside tyre with all the weight is not going to scuff, so the poor inside tyre will, and with camber all that scuffing gets done on the inside half of the tread. Its not a problem with camber, as that wouldn't create the strong sideways graining, and its not wheelspin on inside wheel as it has a plate LSD and the graining would show differently if that is the case (as we had before the plate LSD went in)
kazbanz,
Mar 2, 9:02am
MM all good dude--sometimes ya get a eureka moment Um but isn't it the inside wheel doing all the work Sideways load will be heaviest on that inside tyre I would have thought-with the outside tyre being just along for the ride. if its sideways scrubbing the inside half rather than the entire tyre to me it seems that logically under heaviest loading (mid corner) then ithat wheel needs a bit more toe in so you have full contact patch working. -Mate-to be clear im just spitballing here -In my head picturing you mid corner on T1 (given turn numbers haven't changed since I was last there)
bill-robinson,
Mar 2, 9:03am
had a look at your photos and wondered have you checked bump steer since you put the castor on?
morrisman1,
Mar 2, 9:06am
yup, lowered the rack 10mm to compensate.
mechnificent,
Mar 2, 9:27am
You need to consider the rack movement rather than the rod end. If the rod and arm are at right angles, a short movement of the RACK will cause quite a large rotation of the arm/hub. If the rod and arm are more in line, 20 degrees say over OR under right angle, then the same amount of RACK movement will cause a lesser amount of arm rotation.
Another thing we should clarify is. Is the rack in front or behind the wheels on this thing? Just to be sure all the brains out there are discussing the same thing.
bill-robinson,
Mar 2, 9:32am
yes, but did you check it?
mechnificent,
Mar 2, 9:33am
Ok. I saw the photos and the steering is in front, so some of what's been said about arm angles is wrong. They aren't converging on the centre of the back diff.
And I see that on full left the arm and rod are over centre. that is dangerous as hell MM.
That will lock up the steering at full lock one day. believe me, and others here that have experienced it. It does happen.
morrisman1,
Mar 2, 9:49am
The steering rack is up against the firewall.
mechnificent,
Mar 2, 9:50am
Ha. so much for my eyes then.
mechnificent,
Mar 2, 9:54am
You see how the rod and arm are going over centre on full lock. I reckon that's your problem. The wheel gets to a certain point then it's flipping that arm over through centre and into that over centre position, with too much lock on.
mechnificent,
Mar 2, 9:57am
If the rod was pulling the arm, and no other forces were in play, the rod and the arm would end up in a straight line and then go no further no matter how hard the rod pulled. But it's not. the weight and inertia of the hub when you swing the wheel for the photo is enough to send it over center, and into too much lock. When you are cornering the same thing is happening.
morrisman1,
Mar 2, 9:59am
Yup, definetely a problem. The only thing bringing it back from full lock would be self centring force, so that needs addressed. But do you think that the over-centring would be the cause of the scrubbing at low lock? is overcentring a result of the angles between the tie rod and steering arm being too large, causing both the inside wheel turnign too much at low steering angles, and over centring at large steering angles? I love this stuff - learning heaps.
mechnificent,
Mar 2, 10:08am
Well. Since you've moved the rack there is another factor at work. A factor that will effect ackerman.
If the rack was right in line with the rods it would have ackerman entirely as a result of the cast arms angles. Since the rack is aft, as the steering turns one arm is going to come into the right angle better and the other is going to be disadvantaged. Dropping it has had a similar effect.
As to the question about partial lock. I'd suspect that the same effects as full lock will be occuring, just not to the same extreem degree. So the inner wheel will still be getting to much lock compared to the outer.
morrisman1,
Mar 2, 10:12am
The rack dropped the same amount that the tie rod ends did when the caster was added.
mechnificent,
Mar 2, 10:19am
I suspect that you have two different factors causing ackerman principle. One is the standard cast angle between arm and rod. The other is because you've moved the rack. I suspect that the only way now to get the toe-in, and toe-in on turns back to normal is going to be to bend the cast arms. Without putting the rack back in the original place, or bending thae arms, you are always going to have to compromise between having toe-in in the straight ahead correct, and toe-in on turns incorrect, or having straight ahead toe-in incorrect and toe-in on turns correct.
The answer might be to have the toe-in on turns correct by steering rods and arms, and compensate for the incorrect toe-in in the straight ahead position with camber.
morrisman1,
Mar 2, 10:24am
From what I understand, up/down movement of the rack changes bump steer characteristics, I know this for fact, whereas forward/aft movement of the rack changes ackerman angles.
I have effectively moved the rack rearwards 20mm by moving the lower ball joint forward 20mm, and I think this was a mistake now as we are seeing the results. My next step will be to put factory arms back in and re-align, see how it goes like that.
I don't want to bend the steering arms, although this would probably fix it, but it is too permanent for my liking.
mechnificent,
Mar 2, 10:33am
Yeah, and hard to do.
That is a lot of caster you've put in by the sound of it. That could change the kingpin inclination/camber requirements as well.
It's complex.
Do you pick the brains of the other guys at the track ? Someone might have been through this before. Perhaps there is a racing forum somewhere where they are all discussing this, and the answer is already out there.
It sounds like a bit more trial and error to me, and some compromising is going to be in order. "Humouring", as my builder mate calls it.
snoopy221,
Mar 2, 12:07pm
20 mill alloy spacing blocks on your rack mounting bushes may be a solution m m And yes moving the rack back has increased ackerman probably by about four or more degrees inner wheel turning angle.
morrisman1,
Mar 2, 2:04pm
Spacing the rack forward would be a good way of keeping the caster and fixing hte geometry, but its not hte easiest thing to move forward. The mounts are built into the subframe and there is material in the way of where the tie rods would end up
bill-robinson,
May 22, 11:45pm
i have been thinking about this so i went back and re read your original post, correct me if i am wrong, but are you asking to improve tyre life on a competition car? if so, that is easy to cure
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