Tramlining and Tire Design

[edrush]

I have handling problems which I am convinced are related to my tires. I get an incredible amount of steering feedback, tramlining & sensitivity to any groove or rut in the road, a pull to one side depending on the camber of the road, some bump steer, and in spite of the tendency to wander, it also doesn’t want to turn. It takes an amazing amount of steering wheel force to follow a freeway size curve.


The tires are Pirelli P5's (pic below). I don't understand what characteristic of these tires creates this behavior. Is it the tread pattern, or a soft sidewall, or a stiff sidewall, or something about the ply construction? The effect these tires have on my car’s handling really twists my brain. I have read that certain tires do this. Can any of you tire designers enlighten me about what tire characteristics lead to this behavior?

http://www.vividracing.com/catalog/...s/pirelli_cinturatoP5.jpg&sizex=250&sizey=200

 

[CapriRacer]

Since this is an engineering forum, I think it is appropriate to demand a higher level of detail than ordinarily provided.

First, the basics: What tire size are we talking about? What inflation pressure are you using? How do those 2 things compare to what is listed on the vehicle tire placard?

Do you notice the condition ONLY on grooved roads? Has the alignment been checked? What was it and how did that compare to the specs? Please note that I think many published specs produce conditions that may be good for vehicle handling, but not so good for tire wear.

What vehicle are we talking about? Year, make and model, please.

Once we have that info, we can start sorting out what may be going on.

 

[edrush]

I was trying to make a long story short, but here are the details.

The vehicle is a 1995 Subaru Legacy. My reward in life for 25 years of being an engineer. I purchased it with 104k on it, it now has 175k miles on it. The tires are 195/70R14. The standard tire size is 185/75R14, so they are a little oversize. I believe the aluminum wheels are OEM but I am not certain. I have tried various pressures, from 30-35 psi. Knocked them down from 35 psi to 32 psi just last night. It changed the behavior a little, but did not improve the behavior.

I don't remember any serious handling changes when I first got the tires except that they gave a very cushy ride and vague cornering. After some time the handling became so bad I replaced the rack & pinion (rebuilt and maybe just spray painted), control arms, control arm bushings, struts and strut mounts, and ball joints. Both inner and outer tie rods were OK but not new. None of this helped, although I swear that jacking up my car makes it handle better for a short time (1 day - 2 weeks) which has always confused the issue. I did the work myself.

I have had the car aligned 3 times, the first two at tire stores (worthless) and the last time by a dedicated alignment shop. I don't have the results with me. I have also played alignment games myself, but not since the last alignment.

One of the Pirelli's separated on a Sunday and Discount Tire was not open so I had to buy new tires as my dad had fallen down the stairs and I needed to drive to the next state immediately. So I bought 2 Sears Guardsman Plus tires and the handling immediately improved, although it still wasn't perfect. I put the new tires on the front. This was in December of 2011. The Sears Guardsman Plus tires are much, much better in the snow and are dirt cheap. They are also 195/70R14.

I recently rotated my tires so the Pirellis were on the front and I could not believe the change in handling. With the Pirellis on the front I get an incredible amount of steering feedback, tramlining & sensitivity to any groove or rut in the road, a pull to one side depending on the camber of the road, some bump steer, and in spite of the tendency to wander, it also has a tendency to run straight (or almost straight). It takes a lot of steering wheel force to negotiate a freeway sized curve at freeway speed and it does not want to maintain a constant turn radius. I also get a lot of body roll and I suspect undersized (low stiffness) torsion bars. I have checked the torsion bars probably 10 times and they appear fine (visual check mostly, but I have removed the front torsion bar a couple of times and it looked good).

My car is an old beater and it handles poorly. I'm sure if I replaced all suspension components with brand new OEM parts it would improve. But the Pirelli tires really bring out the worst handling traits. It drives acceptably with the Sears tires on the front, and intolerably with the Pirellis on the front.

 

[cibachrome]

I'll take a stab at this for want of anything better to do at the moment. I'll take the approach of what would it take to make a car act that way instead of "why does my car act that way". That's because your symptoms are fairly easy to explain and the exact causes are beyond our reach right now. I'll use technical terms because that's the way it ought to be done on this forum and the Peanut Gallery agrees with this.

You have two sets of tires Pirellis, likely having high cornering stiffness and low aligning moment stiffness (hence a low P-Trail to use a descriptor which I do not care for much). The camber stiffness is usually high because that's the way it works in most Pirelli constructions. The other set are some aftermarket Sears (maybe Michelin clones) that have a low to average cornering stiffness and an average to high aligning moment stiffness (hence a long P-Trail to use that bad word again). Camber stiffness is probably pretty low because that's about all you can do when your sidewall package is made up from recycled pantyhose.

You replaced the steering gear with a rebuilt unit for some reason (good one I'm sure) but not the pump. You replaced the LCA bushings with some new ones of unknown durometer. You car is a bit roached out with 175,000 miles on it.

With the Pirelli's on the front and the Sears on the rear, you have a high gain vehicle (meaning output to input ratio). It have very low understeer. Small inputs (from the road or the steering wheel excite a large reaction.In closed loop control, Its quite a handful. Adding some new steering gear characteristics to the mix can cause further excitement. If the T-bar in the gear is thick, then you will have more manual steering-like traits. If the valve profile is for a different model or was developed for 'normal' OEM tire characteristics, then you can get all possible combinations of too much or too little steering assist, depending on driving stright or around a motorway exit at .5g lateral, (better at .75g, it my be unstable). Now add some worn wheel bearings and it will tango better than the winners of Dancing with the Cars. Wheel bearings have both steer and camber angle influences, too (Bet you can guess why).

Now swap the tires front to rear. Now you have a fairly low gain car (high understeer), but still with the steering gear performance mismatch. Did I mention that if the pump is shot, you have an almost manual steering car with a huge amount of front compliance steer. With that kind of mileage and low steering friction, it might 'feel' OK until you need to turn it or make a lane change.

Now for the best part. My suggestion? Put the Pirellis' on one side and the Sears gumballs on the other. There may not be much noticable asymmetry until your spouse starts grabbing the doorhandle (or your parking brake lever). The side to side rpm difference may free up a semi-locked (from wear) differentials, and she may steer pretty good. Do it both ways.

Like I said previously, this may not explain YOUR situation but I can take a perfectly 'good' or 'great' car and turn it into your's very easily. We've done this a few times for new kids in the department. The results are eye-opening. We even did this for some government people once ! Who would ever have imaginged that they had discriminating driver skills !

Will Wonders never cease...

 

[CapriRacer]

First, with respect to load carrying capacity, a 195/70R14 is larger than a 185/75R14, BUT it's the reverse relative to diameter.

- BUT - According to Tire Guides, a 1995 Subaru Legacy with 14" wheels came with 185/70R14's inflated to 30 psi front / 29 psi rear. So we need to verify what it actually says on the vehicle tire placard. Also, the wheels are listed in Tire Guides as 5 1/2" so that isn't a problem.

But the pressure - if what Tire Guides says is so, then the larger 195's would need as little as 25/24 psi. Note, I am NOT recommending that, I am just showing my work.

I find it interesting that when you first put on the Pirelli's you didn't notice anything, but it occurred later - which leads me to think you have a problem in the car - something wearing out - which you have yet to identify. The fact that just taking the load off the car improves the situation is further confirmation. Ordinarily, I would suggest shocks, but you've changed those.

I also find it interesting that you put on 2 tires - and they were put on the FRONT. Standard industry advice is to put new tire on the rear. I'm surprised Sears did that!

I've followed what cibachrome said, and he makes sense - except that your car developed the problem AFTER the Pirelli's were out on. I agree that swapping tires back and forth would result in differences.

Some thoughts: So, you rewarded yourself for 25 years of being an engineer by buying a car with 100K on it. I'm thinking of rewarding myself when I retire with a Lotus Super 7. Ah, the differemces in the way people think!

The car is now a beater, and I think you ought to consider throwing simple solutions at it, rather than trying to get at the root cause. My suggestion is just buy 2 more Sears tires and be done with it.

But if the idea of finding a solution gives you a reason to live, then I'd start by looking things over very carefully.

Where I would start is by jacking the car up, then measure the ride height immediately. When the handling gets back to its bas state (0ne day to 2 weeks, like you said), measure the ride height again. This might reveal a shock problem.

You mentioned tie rods - and that might be a place to start as well. I'd also look at wheel bearings.

That's all I can think about for now. Good luck.

 

[edrush]

The door placard says 32 psi front, 30 psi rear. By my simple calculation, this means 1.7 psi lower tire pressure (a 185 should be 5% narrower than a 195). The diameters should be very close.

Ciba, it will take me some time to translate and digest your information, but I appreciate it. I am struggling with the pneumatic trail concept. I can see how deflection would increase as you move aft on the contact patch, but force? Although I have no doubt this is a well established fact.

Obviously I was a fool to replace all those components on this beater car, so yes, this has become an obsession. It really chaps my hide that I can not figure out what the problem is. It should be obvious! Very obvious. I have spent more hours than I care to count trying to figure it out. Although I admit I keep repeating the same, relatively simple and crude diagnosis procedures. Expecting a different result is foolish.

Note: I only replaced the front struts and strut mounts. I was surprised at how good a shape the old struts were in. They had lost some or most of the gas pressure but still had full dampening, or at least seemed to when I moved them by hand.

I will likely buy two more Sears tires and unload the car. But I still have the obsession. I agree, something in the suspension must have worn or changed since I got the Pirelli tires. I am now considering the more obscure potential problems:

a) loose cross member. I did have the clutch replaced at approximately 145k. I have seen no evidence of cross member movement (but haven't put a torque wrench on anything).

b) broken motor or transmission mount which allows things to move

c) wheel bearings

About 40-50k miles ago a tire shop discovered both of my front wheel bearings were loose while rotating tires. I took the car home, torqued the nuts, and then drove it on the freeway and checked the temperature. I considered it fixed (although I did worry about the bearings for a few thousand miles). This was questionable judgment I know. The weird thing was one of the nuts had not been punched (to prevent backing out), but the other one had. So how did the wheel bearing with the punched nut get loose? Either the nut slipped past the punch (seems very unlikely) or the bearing had been changed and the new one had not been pressed completely in and then moved while driving. The car did not have any serious bad habits at this time, even with loose wheel bearings.

The wheel bearing problem was all very strange. I would think a damaged wheel bearing would suffer catastrophic failure in a short time. And I would think any bearing loose enough to affect driving behavior would fail quickly. I have checked my wheel bearings many times since by simply grabbing the top and bottom of the tire and trying to wiggle it.

Unfortunately I am in a temporary living situation and have no tools. They are in storage.

I have also driven a 2010 Forerunner with 7K miles on it to work. On one short section of grooved cement freeway it actually drove worse that my Subaru.

The intelligent solution would be for me to see a psychiatrist and eliminate my obsession. But for now I am trying to understand the effect of tires because the Pirelli's behave very differently than the Sears tires.

Regarding my "reward", my objective was to live within my means. I need to increase my means.

 

[CapriRacer]

Ed,

You've quoted the pressure from the vehicle tire placard but not the tire size. This now becomes important in that we may be looking at a change in footprint size that is affecting things.

 

[edrush]

I was mistaken about tire size. The size on the door placard is 185/70R14. The tires on the car are 195/70R14.

My alignment is (or was a month ago) front toe +.0625" with 0 camber both sides, caster is +3 deg/+3.5 deg. Rear toe is 0, rear camber is -.5 deg both sides.

It seems to be getting worse.

 

[GregLocock]

OK, and what are the factory specs for alignment?




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[edrush]

And what exactly are you trying to say Mr. Locock? Just say it.

Factory specs:
front toe: +/- .12"
front camber: -0 deg 5'

rear toe: +/- .12"
rear camber: -0 deg 55"

I took the car back to the alignment shop. They poked and prodded and checked the alignment. Couldn't find anything wrong except one rear strut which is leaking. They put more pressure in the rear tires (the Sears tires) and told me to try it. No significant change.

From a practical aspect the Pirelli P5 tires are the problem. Maybe these tires have design or manufacturing flaw and the tires are degrading. Even though one tire separated this theory seems unlikely.

The more likely explanations are a loose component which no one can find, or maybe a steering gear design mismatch with these tires and as the tires have worn they have become more responsive and which has really made the mismatch apparent. The biggest problem is with rutted roads. Rutted roads really bring out the worst in this car/tire combo. Grooved cement (with rain grooves) doesn't have much effect. Or maybe UDOT is getting better at cutting the grooves straight.

Ciba's idea to put the different tires left and right is certainly intriguing.

Tapered roller bearings seem virtually foolproof. Unless you can feel that they are loose by wiggling the wheel (top and bottom) I can't see how bearings could affect driving/steering behavior. Agreed?

 

[GregLocock]

Your measurements seemed a bit too close to 0 all round to me but if that is the spec you should be OK. A bit of toe in at the back would be my gut feel.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[pennswoodsed]

First of 2 things ,you answered the question. difference between antique design p5s and sears bobos construction and steering characteristics cause sensitivity.
Second is the worn chassis with several new parts is not going to steer like a (Porsche?) ,car has 175k and you are not committed to blowing lots of money on it ,therapy or a new hobby,girlfriend might be cheaper.
Regards,Ed