Hub lead is used to provide mechanical trail, which is used to set the compromise between being able to feel Mz, vs some self centring and linearity in efforts. Generally the mechanical trail at the ground falls between 0 (2CV) and 30mm. The castor angle needs to be considered when working this out from the side view geometry.

However from your drawing I see that I may have been misled by your use of the word forward and that you actually meant sideways.

If so then that distance, KPO, is set by the required scrub radius, which is a compromise between reducing the pull under braking on rough surfaces, parking efforts brakes on and parking efforts brakes off. And probably some more realistic concerns with packaging a given wheel/tire, turning circle, and a few other things.

Scrub radius varies anywhere between -20 and +100 mm, these days with PAS I'd expect to see +/- 20mm

Cheers

Greg Locock


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cheers for that you were right with your first answer by the look of it, it will have little effect with what I want to do and sounds like there’s not much to gain by changing it either

Was just thinking if I can get good feel and self centring from hub lead then why is it that I need 8.9degs caster and I’m being told to increase this to 9.5degs it seems far too much when other cars such a Nissan Silvia‘s have around 7degs and have no hub lead at all

Corvettes have 3.5 deg, 6 deg also works well. Unfortunately I don't have any real guide to tuning castor angles, whereas mechanical trail has the above mentioned straightforward effects.

Be aware that a production car's suspension is heavily constrained by other requirements, so if it ends up at 5 degrees instead of 7 because that way the alternator fits better, that's what happens.

Cheers

Greg Locock


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I can understand mechanical trail can give you more feel from the front tires but when the car starts to understeer it starts to pull the steering wheel into the corner even more to the point that you have to force the wheel back the other way if it was a bike it would be called wheel flop if this is occurring then I can’t see how this would promote a self centring effect and with a bit of research into mechanical trail I found the scientific understanding of bicycle steering remains incomplete apparently according to"Chapter 8". Bicycling Science (Third edition ed.). Massachusetts Institute of Technology. I don’t know if the same applies to cars

once again thanks for the help

If the front of my car was setup like this car in the diagram i would have no problem agreeing that it would have a self centring effect


the e30 looks to be the same as a bike

As long as trail puts the contact patch behind the steering axis projection, why would there be any difference (other than in magnitude)? Fx will still act to self-center as soon as it isn't acting directly behind the SA. I'm assuming zero scrub radius just to simplify things (and match the RCVD Figure to the bicycle which should not have a nonzero scrub radius to begin with).

Fz would behave opposite in those two diagrams, but for this effect it's the bicycle configuration that would exhibit self centering. The RCVD Figure would have the chassis weight trying to further lower itself once steered out of straight ahead and be anti-self centering as a result.


Norm

Interestingly I have driven a production BMW which had insufficient returnability, and perhaps even a tendency to drive itself into lock. It may be that they do not consider it an issue, our evaluators certainly did.

What the bicycle people call rake we call hub lead, or side view hub offset(we might measure it horizontally, not perpendicular to the castor axis).

From a practical perspective on a tricycle where I copied a mountain bike's geometry, bending the forks forward from the headstem reduces steering feel and increases efforts and increases straight line running stability. So all of that is due to trail not castor.

Cheers

Greg Locock


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GregLocock:
that’s a relief that you have had the same problem with insufficient returnability with BMW I talked to a few BMW workshops and I was told that it was just understeer and that I wasn’t driving the car right and I know this was wrong because I’ve owned and driven a few Toyota ae86's with no hub lead or trail and there was no problem like this
But I must admit that this effect is minimal but still enough to take you off line

NormPeterson:
Thanks for that you’ve cleared up my understanding of this subject I can see now that there are two ways of getting returnability one through trail and the other through gravity pulling it straight

The insufficient returnability of a BMW is, I think, a mater of taste in the steering feel. BMW seems to do things a little out of the norm. I guess that's what makes a BMW a "driving machine".
The combination of lateral offset, caster and trail determine the steering feel and behavior. A designer can play around and get what he wants. You might not agree with his tastes. There have been strange combinations in front end geometry due to designer tastes (and economics, of course), such as negative roll gain and dive instead of anti-dive.

For a bicycle, the combination of caster and trail determine the amount of "wheel flop". Wheel flop determines the stability of a moving, riderless bike and the hands-off steering effectiveness.

I'm quite familiar with BMW steering performance and specifications because of analysis, testing, measurements of components and conversations with it's Development engineers (More so on the E46 5 Series version, though. The principle components of BMW steering performance are the OEM tires and the ZF provided steering system (pump, lines controller, valve and gear. Given the unique characteristics of a BMW Star labeled tire, the front suspension geometry is specified to fill in the connection between the desired tierod loads and load gradients and the desired steering wheel rim forces. Its is NOT done as the arbitrary lore from some 'designer'.

Because the front tire pair's forces peak out after the peak aligning moments, caster is added as necessary to align the peak sideforce with the peak aligning mements. (Nothing unique here, this is usually how all performance cars are arranged). The other gain from caster has to do with what caster effectively produces: Camber change by steer. This is only a low to moderate speed effect though, because high steer angles (and resulting camber changes) at high speed are not viewed with great favor. The downside to caster is an increase in tierod loads from the lateral forces. An undesireable understeer increase results from this. And, its the worst form of understeer change: The nonlinear steering gain change effects from loading the downstream steering system (valve and T-bar), and the upstream components (intermediate shaft isolator, U-joints and spline). To cut back on this effect, reduce plunging of tires while steering (you want to steer the tire, not plunge it sideways), the caster offset (hub lead) is factored in. If a front drive shaft is to be an option, the reduced plunging is an added benefit to the resulting steering feel). This sounds a lot like steady state blather, but the real advantage to the BMW package has to do with the harmonization of the tire transient properies. In order for the Mz transient and the Fy transient to synchronize themselves as the steered tires roll out the input, these forces and moments must be transferred to the tierods in a proper manner. If you look at a plot of normalized (by the steady state values) Fy and Mz as a function of distance traveled, the optimum 'feel' is produced when the two function are coincidental as they approach steady state. Obviously the Mz function has a large impulse at 0+ distance, while Fy has a low or no value. Then Mz drops down to a much lower value. The phasing of these tire functions, combined with the preservation of their coherence with headlevel lateral acceleration is the reason these cars are the most fun to drive. The 5 Series BMW tires are very unique in this respect, with BMW wannabees having close but no cigar properties in their tires. Yes the rim width is a strong player in this response. Some of you may call this phenomenon 'riding on rails'. It actually has some similarity to a wheel flange constraint.

Surprizingly enough, the best way to become a believer of this is to put tires of the same size, brand, and pressure from a domestic BMW wanna-be on a car and gauge your ability as an evaluator. If you can't feel the difference, then you might consider buying and driving a road grader instead of a Bimmer. You are a 'displacement control' type driver, not a 'feel' (moment control type) driver.

There could be legions of engineers designing the steering and suspension geometry, but one of the most important goals is driver experience, a totally subjective quality. What BMW considers the "best" driver experience obviously can not please everyone. I'm sure that even BMW does not believe that anyone not in agreement with their choices should be fit only to drive road graders.

Star for that, cibachrome.

OP may be interested in these other threads (thanks again to cibachrome)

http://www.eng-tips.com/viewthread.cfm?qid=183767

http://www.eng-tips.com/viewthread.cfm?qid=68031



Norm

"Ultimate driving machine" is marketing not engineering. Phase errors in SWT/SWA are error states not preferences.

Having said that I agree with cibachrome, rule of thumb is that 70% of steering and handling improvements are in the tire

Cheers

Greg Locock


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All this info is great thanks it’s been a great help
I was starting to think about how all of this applies to a drift car, back in 2010/2011 everyone was starting to change the Ackerman in the steering to reduce the scrub when I first did this I removed all the Ackerman completely so the inside wheel would not scrub but the problem was that it was very slow to correct but then with a bit of Ackerman put back in not as much as standard it was great to drive and would correct nicely I think having the wheels at different angle pulls on them to get a counter steer effect
The car below would have little hub lead and you can see the outside wheel is not turned as much

this BMW on the other hand has what looks like parallel steering


If I was to put the hub behind the lower ball joint and got the caster and mechanical trail set up right I should be in theory able to run parallel steering with great correction and feel.

Hi Cibachrome,

You said "The principle components of BMW steering performance are the OEM tires and the ZF provided steering system (pump, lines controller, valve and gear."

I just spent a little time on the BMW USA website and the 68 page brochure, and found the tire sizes, but no mention of make and model.
Do you know what they are, and if they are available to mortals?

thanks

Dan T

The BMW 5 Series tires are usually supplied by Continental, Goodyear and Michelin. The way to tell an O.E.M. tire is with the dime sized star symbol on the sidewall. Most dealers are unaware of this. If you ask them for replacements, chances are they will not have the star symbol on them. Only available in Europe, so the best way to get them (as we did) was to get them right out of the car plant. Note that the machinery to make these 'special' fast response tires is kept in Europe by BMW mandate. By fast repsonse, I don't mean cornering stiffness.

That's a good point. Aftermarket tires aren't necessarily made in the same factories as the original production tires, even if they have the same model name. The OEM signoff for moving tires between factories for OEM fitment is much more elaborate than it used to be, and I doubt a tire manufacturer alone would bother if it's intent was cost saving, which is the most likely reason.

Cheers

Greg Locock


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A sneaky way to get a good set of BMW tires is to buy 'new-car-take-offs' from a dealer who has been asked to install some oversized or aftermarket wheels and/or tires. The new owner probably has no clue of what they are giving up, the dealer makes some cash and you have a set of 'star' tires. (Try to get the GDY's or Conti's). You want the 'summer' tire construction, not the 'winter' provision. Tell a local dealer to Be On The Lookout for a gullible (Oops, I mean uncaring) buyer.

Hi cibachrome,

A while back you said "The way to tell an O.E.M. (BMW) tire is with the dime sized star symbol on the sidewall."

Well, this past weekend I was Cleaning up the garage nether regions and found a couple of tires given to me by a buddy about 10 years ago. One was a Brandy new 70 series Michelin MX something or other on a steel wheel. I still recall he said one of them was a spare off some departed BMW car. Lo and behold there is a star molded in the sidewall. I study tire sidewalls from time to time and can't recall ever seeing that symbol before. I guess I know why.

Still has a paint stripe around the tread, full of air, and a quick inspection revealed nary a sign of cracking in tread grooves or on sidwalls.


thanks

Dan T

• http://files.engineering.com/getfile.aspx?folder=594f7cfd-7322-42a1-80c3-66

So we seem to be saying that BMW base their steering geometry design to a large extent on a particular tyre construction (presumably developed with/for them) marked with a star, but made by several manufacturers. They then send these cars out in large numbers to parts of the world where those tyres are not available and no-one, not even the dealers, knows the significance of the star-marked tyres? How is that supposed to help with the "ultimate driving experience" (or even basic safety) once you've worn out the OE fitment tyres? Doesn't sound that smart to me.......

This notwithstanding I have learned alot from this thread and appreciate the knowledge shared.

Nick

That is the big ugly truth of steering and handling. The tire is the most important determinant of steering. The car is optimised around at most 3 tires, and you as the consumer have no way of deciding what replacement tire to use. Even worse than that, in 3 years time, when you replace your factory fit tires with identical named and sized ones from a tire retailer, you won't even be fitting the same construction and compound in some cases (this may not be as common as it was).

I can think of no other safety related item where this would be tolerated.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

Some Background information on German cars:

Rear wheel driven cars like BMW, Mercedes and Porsche drive with very high castor angles. Typically BMW is around 7°, Porsche around 11° and Mercedes in between. In order to not arrive at immense mechanical trail values the steering axis is having at wheelcenter an elevated castor-offset (side view) of up to 15mm. Going for a pointed forward damper tube on the BMW is usually done for 2 reasons: a) to maximize the overlap between damper piston and strut housing increasing thus the camber stiffness and 2)the extra inclination of the damper increases the caster in jounce travel. Caster creates in a steering motion on the outside wheel more negative camber reducing understeer. It is a brilliant way to reduce understeer and works in effect only when needed (steering = cornering). This allows suspensions to have relatively low jounce motion camber gain values which is important for having a non-road-copying car at 250 kph on the german highway. To some extend the low speed non returnability that has been mentioned here so much is also due to a compromise versus high speed stability. Porsche has never had complaining customers on lacking returnability in a parking lot but boy they have had their part of angry customers on high speed handling .... and when BMW is referring to be the ultimate driving machine they could not care less about low speed returnability (even if it is an error state, they just do not care, just as porsche brakes do always squeal, the Brand DNA is strong enough to cope with "unimportant issues" like that ...).

Now to drifting, as far as I can see on the pictures the cars have been modified quite heavily. I could be wrong but the nissan skyline seems to have a steering gear in front of the wheel center line with a trackrod fixing at the upright behind the wheel center line. This would create all the clearance necessary for the full lock inner and outer wheel positions and most likely also enough design area for more or less parallel steering. In case of a BMW you would probably get yourself a LHD or RHD (if your car is LHD) steering gear, custom made left and right uprights and some specially formed track rods and off you go ....

Cheers

Dynatune, www.dynatune-xl.com