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Old 19 Feb 2007, 12:30   #1
meb
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Pneumatic trail vs mechanical trail

My assumption is that mechanical trail is nothing more than the caster measurement on the ground plane to the center of the tire's contact patch in side view. And, that pneumatic trail is the character of tire deformation due to a given machanical trail? Is this correct? I'm trying to decided upon an optimal caster setting for my car...the fanny dyno isn't always the best method
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Old 19 Feb 2007, 17:36   #2
flavorPacket
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yes it is. there's no good way of modeling contact patch movement (which creates pneumatic trail), but there's a great way of feeling it: by driving.
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Old 19 Feb 2007, 17:58   #3
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Thanks flavorpacket. Are there any assumptions I should make regarding the affects of caster on SAI or scrub as the wheel is turned? I've read too much caster will wash out SAI but Im not sure I really understand how caster affects scrub if a wheel is turned to full lock for example...I can only guess that this is where pneumatic trail comes in?
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Old 20 Feb 2007, 12:30   #4
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Let me ask the question in another way...sometimes I write too quickly.

The self aligning torque established at the contact patch is respnsible for telegraphing grip levels via steering wheel effort. I do not fully understand how (hi values) caster washes this out...perhaps the point projected ahead of the tire is so strong it overrides this aligning torque? And my question above regarding SAI really concentrates on what caster does to SAI and perhaps scrub(if anything) at high steering wheel angles.
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Old 20 Feb 2007, 13:01   #5
ian.stewart
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I think the easy way to visualise what too much castor will do is Imagine the castor angle @45desg moving forward and and turn the wheel on full lock @90degs to forward motion, the contact patch is right on the edge of the tire reducing mechanical grip
This may be over simpilistic but as an basic example I think it works
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Old 21 Feb 2007, 07:39   #6
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Meb what surface(s) are you planning to be racing or driving on? And will you be turning left and right equally or what?.........trikes
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Old 21 Feb 2007, 12:33   #7
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Lime Rock Park and Watkins Glen primarily - two very different tracks. Most of the surfaces thru the turns are concrete...which odly enough is quite gripy when hot and not when cold or wet. LRP has only one left hand turn while Watkins Geln is a roller coaster ride with a good mix of everything - huge elevation changes too.

I should also inform you that this Mini is still my daily driver - a horrid compromise. Everything but caster is adjustable. Stock caster is about 4.5 degrees. Camber is set at 2 deg. neg up front with a hair toe in. Rear camber is set at 1.35 deg neg with about 1/8"toe in. Front toe is a street compromise. The car sits about 15-17mm lower than stock.

Basically, and clarifying further, a little more caster is desired to help offset some loss of pneumatic trail due to static camber changes? That nice self centering torque generated at the contact patch seems to go away with neg camber values above 1 deg. neg. There is less feel at the steering wheel - less effort.

Corner weights are 475lbs for each rear wheel and 852 for each front wheel - if this helps.

Last edited by meb; 21 Feb 2007 at 12:43.
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Old 25 Feb 2007, 15:10   #8
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in the middle of a turn (ie at max lateral acceleration), your contact patch will most likely not be where it is statically. That's called pneumatic trail, and that's one of the largest sources of steering weight. If you want more weight, add kpi, caster or scrub radius. Only an increase in caster, however, will significantly improve the self-centering effect.

You could also try different tires that have different self-aligning torque values (which is what real drivers use to 'feel' the limit). Unfortunately, that kind of data isnt available to normal customers like yourself, so find others with a similar setup to yours and find out what works.

I've been to the glen and lrp many times (my hometown is about 45 minutes south of lakeville), what groups do you go with? I've had the best experiences with scda and pca.
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Old 26 Feb 2007, 09:53   #9
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Ian don't you mean camber? Caster does tip the tyre on it's edge.....trikes
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Old 26 Feb 2007, 11:28   #10
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1 - Mechanical trail is the distance between the geometrical centre of the contact patch and the intersection of the steering axis with the ground.

2 - The pneumatic trail is the self aligning torque divided by the lateral force - i.e. it is the distance between the geometrical centre of the CP and where the force is actually centred.

To calculate the torque about the steering axis you add the mechanical and pnuematic trail figures together and multiply by the lateral force.

Ben
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Old 26 Feb 2007, 15:41   #11
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Flavorpacket,

I signed up with Ian a few years back -SCDA, but never made the events due to work. For the last two years I've been driving with PDA with an event or two with BMWCCA and Fastracks. I have not attended an event with PCA, but the folks at Farnbacherloles in Danbury have performed some setup work on my car and insist I join the club and meet new people. So I'll attend at least one event this year...perhaps we'll catch up there? Would be nice. I work in Bethel.

urban,

I've read that formula in Milliken. I keep think of ways to narrow my question as I go thru the turns on the track. And perhaps the following is clearest...pneumatic trail, from what I understand, is true fidelity. Meaning, this absolutely telegraphs saturation. But some folks add in lots of caster in an effort to increase straight tracking. I understand that high caster values dilute pneumatic trail and therefore saturation. I'm aware enough to know that my thinking is narrowly focused...there are many other potential factors that can make my statement true and not so true???

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Originally Posted by flavorPacket
in the middle of a turn (ie at max lateral acceleration), your contact patch will most likely not be where it is statically. That's called pneumatic trail, and that's one of the largest sources of steering weight. If you want more weight, add kpi, caster or scrub radius. Only an increase in caster, however, will significantly improve the self-centering effect.

You could also try different tires that have different self-aligning torque values (which is what real drivers use to 'feel' the limit). Unfortunately, that kind of data isnt available to normal customers like yourself, so find others with a similar setup to yours and find out what works.

I've been to the glen and lrp many times (my hometown is about 45 minutes south of lakeville), what groups do you go with? I've had the best experiences with scda and pca.
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Old 26 Feb 2007, 16:58   #12
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urban,

This sentence above is really a question "I understand that high caster values dilute pneumatic trail and therefore saturation."

Do high caster values dilute pneumatic trail or just the feeling at the steering wheel...or is this one and the same? I can see a scenario in my brain where pneumatic trail occur regardless of caster, but that the tingly sensations at the steering wheel ar some how filtered out.
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Old 27 Feb 2007, 06:38   #13
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Quote:
Originally Posted by meb
urban,

This sentence above is really a question "I understand that high caster values dilute pneumatic trail and therefore saturation."

Do high caster values dilute pneumatic trail or just the feeling at the steering wheel...or is this one and the same? I can see a scenario in my brain where pneumatic trail occur regardless of caster, but that the tingly sensations at the steering wheel ar some how filtered out.
You use very subjective language. The pnuematic trail is a property of the tyre and has nothing to do with the caster angle. You can also have zero caster and positive mechanical trail.

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Old 27 Feb 2007, 13:42   #14
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I'm in a design based business, lots of subjectivity...and not.

Like a caster wheel on a shopping cart; positive mechancial trail, but no caster. Understood. And I also understand that pneumatic trail is a property of the tire; way back when bias ply tires provided all the mechanical trail. However, is it true that potentially strong self centering forces with high caster angles will wash out or over-power the 'feeling' we receive at the steering wheel due to pneumatic trail?

Steering wheel torque or effort can be measured. As a tire saturates this torque becomes lighter. This information is quite important to the driver.

Two scenerios, same tire, all else equal; no mechanical trail in the first. The second, 4-5 degrees of mechanical trail. In which scenario is the driver likely to receive better information regarding tire saturation?

urban, I don't mean to beat a dead horse, but I want to make sure I understand absolutely. Thanks!
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Old 28 Feb 2007, 08:43   #15
ubrben
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Quote:
Originally Posted by meb
I'm in a design based business, lots of subjectivity...and not.

Like a caster wheel on a shopping cart; positive mechancial trail, but no caster. Understood. And I also understand that pneumatic trail is a property of the tire; way back when bias ply tires provided all the mechanical trail. However, is it true that potentially strong self centering forces with high caster angles will wash out or over-power the 'feeling' we receive at the steering wheel due to pneumatic trail?

Steering wheel torque or effort can be measured. As a tire saturates this torque becomes lighter. This information is quite important to the driver.

Two scenerios, same tire, all else equal; no mechanical trail in the first. The second, 4-5 degrees of mechanical trail. In which scenario is the driver likely to receive better information regarding tire saturation?

urban, I don't mean to beat a dead horse, but I want to make sure I understand absolutely. Thanks!
Mechanical trail is measured in linear units not degrees. mm or inchs

I'm not dodging the question, but the way you framed it highlighted the lack of clarity in these sorts of debates. I don't doubt that you know that mechanical trail is a linear measurement, but why then identify it by an angular one?

Ben
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