BMW e30 steering geometry 3

[ke70gt]

Hi there
I’m trying to set up an BMW e30 for drifting and after a good look at the front suspension I noticed the front axle was placed around 50mm forward of the bottom ball joint so that the front wheel has move left to right slightly and forward and backwards due to the 14.4mm scrub radius it has from standard I haven’t seen any other steering arm with the axel so far forward and wanted to know what the theory or reason was behind this before I start modifying it all
cheers
Just a pic of the bmw e30 suspension setup looking forward

This pic shows the later model e36 setup ignore the one on the left it the best pic I could find to help explain it better

 

[GregLocock]

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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[ke70gt]

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

 

[ke70gt]

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

 

[GregLocock]

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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[ke70gt]

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

 

[ke70gt]

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

 

[NormPeterson]

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

 

[GregLocock]

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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[ke70gt]

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

 

[140Airpower]

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.

 

[cibachrome]

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.

 

[140Airpower]

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.

 

[NormPeterson]

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

 

[GregLocock]

"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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[ke70gt]

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.

 

[Tmoose]

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

 

[cibachrome]

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.

 

[GregLocock]

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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[cibachrome]

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.