Pneumatic trail vs mechanical trail

[meb]

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

[flavorPacket]

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.

[meb]

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?

[meb]

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.

[ian.stewart]

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
Ian

[trikesrule]

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

[meb]

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.

[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.

[trikesrule]

Ian don't you mean camber? Caster does tip the tyre on it's edge.....trikes

[ubrben]

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

[meb]

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???

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.

[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.

[ubrben]

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.

Ben

[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!

[ubrben]

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

[meb]

Hey Ben,

First, thank you for being patient with my questions, even if I do mix up terms and units. The terms are not yet fluent to me. Your last response made me realize that I have been co-mingling caster and mechanical trail. I agree, trying to anwer ambiguous questions is tuff...especially if the guy asking is really persistent

So I'll give it one more shot. Will positive mechanical trail dilute the affect of pneumatic trail. I understand that pneumatic trail is a function of the tire. However, to my novice brain, mechanical trail imparts another force on the tire???

[flavorPacket]

one does not affect the other, but their sum is directly related to steering weight

pneumatic trail has to do with properties of the tire only.

mechanical trail has to do with steering geometry, which is (largely)independent of the tire

ptrail*lat force + mtrail*lat force + friction = steering weight

so, if you increase mech trial (through an increase of caster etc), a higher percentage of steering weight will come from it, so less overall steering weight will come from pneumatic trail.

Yes, if you add a ton of caster, breakaway may be harder to feel. But honestly, it has WAY more to do with tire properties (in most cases). I personally believe that a change from 4-5 degrees will not significantly change your saturation perception, if you will, because a much higher percentage of your mtrail comes from KPI trail/scrub radius. Do the calculation and see how much your mtrail changes for 1 deg of caster change: i'm guessing not too much. And keep in mind, you have power steering which adds a whole new level to this discussion.

I'd recommend different tires before you change your steering axis.

PS I grew up in Newtown, small world eh?

[ubrben]

Can I give a really crappy answer of "it depends".

Some other factors to consider.

1 - As pnuematic trail drops, the overall lever arm drops
2 - Pnuematic trail drops as lateral force increases

If the force increases proportionally more than the decrease in pnuematic trail, the steering torque will increase. If the pnuematic trail drops proportionally more than the force, then the weight will decrease.

One final point to consider.

3 - It is possible for the pnuematic trail to be negative at high forces/slip angles

My view of point three is that you always was a positive mechnical trail to guarentee that the resultant trail (mechanical + pnuematic) is always positive so you don't get a torque reversal in the steering.

Whether the weight increases or decreases near the limit will depend on tyre properties.

Ben

[meb]

Okay, not we're cooking with hot oil!

flavorpacket - you basically answered my question in this sentence, "so, if you increase mech trial (through an increase of caster etc), a higher percentage of steering weight will come from it, so less overall steering weight will come from pneumatic trail." However, I'm going refine my question based on your sentence. I am concerned with adding too much mech trail because as you wrote, it adds to the over-all weight at the steering wheel...can a point be reached whereby mech trail is so strong - contributes to a huge portion of steering wheel weight - that it essentially doesn't allow pneumatic trails sensations to reach the steering wheel? That is my central concern. I want to add a bit maore cater, but not in lieu of washing out pneumatic trail.

urban,

I actually wondered about this, thanks for identifying!

"My view of point three is that you always was a positive mechnical trail to guarentee that the resultant trail (mechanical + pnuematic) is always positive so you don't get a torque reversal in the steering."
or identifying!

[flavorPacket]

yes, but I'd be surprised if you ever reached it. Like I said, do the calculation and see how much your mtrail really changes with a degree or 2 of caster.

[meb]

Okay, understood. But this will change with tire construction, understood as well.

urban, Flavorpacket, thanks for helping out.

michael