Tire Stiffness & trailer swaying

Stick with the 20, fly PW5 tasks, then you won't ever
land out & won't need to tow at all!

At 12:12 29 September 2003, Graeme Cant wrote:
Tim wrote:
Cheers!

I've decided that the combination isn't the best.

So do I trade the 20 for a PW5? Or do I sell the wife
and get a Mondeo
????

As long as it's stable who cares?
I know people who have walked away from pretty scary
accidents - punctures at speed limit whilst towing
on dual carriageway and the like - when it has been
a sensible, well behaved combination, SUV or otherwise.
Equally, there are combinations which are just an
accident waiting to happen. Once it gets out of control,
the soggy suspension and high C of G just make an SUV
dead easy for the trailer to flip, however heavy it
is (ever watched wrestling?)
Light cars are easily steered by big trailers.

Don't know about in the US, but here in the UK you
could probably put all trailer smashes down to one
of three things:

1. lightweight cars with a large distance betweeen
the rear axle and hitch. Plenty of these around here.
The trailer does the steering especially if it is heavy.

2. Rogue trailers. Most trailers are perfectly OK towed
within the limits. Some change dramatically between
empty and loaded. I remember a ? homebuilt two-seater
trailer which loaded was a nightmare and empty was
almost impossible. I know of trailers which use old
caravan suspension components or car tyres, which are
almost always not stiff enough. Get one of these on
an SUV and it would be interesting.


3. Bad driving.Self explanatory.

Have a nice day

Al

At 16:12 29 September 2003, Bruce Greeff wrote:
Hi Graeme

Lots of prejudices here mate, sorry if I offended.

Maybe it is a SIFOW thing, I prefer not driving trucks.

Again each to his own.

Personal choice and the sensible matching of the tow vehicle to the
trailer is what it is all about.

I should explain that here in California where we are recalling our
Governor one of the over 140 candidates is placing tv ads stating that
if you drive an SUV you support terrorism. This candidate now also
supports the governor who made it legal for illegal immigrants to
obtain a drivers license. Go figure. Other people label SUV drivers as
antisocial. Give me a break. You can be just as 'antisocial' on the
road in a Civic as well as in a Tahoe; or flying a Ventus as well as a
PW5. So some of SUV drivers here are just a bit touchy about all that!

Alex Chappell wrote in message ...
As long as it's stable who cares?
I know people who have walked away from pretty scary
accidents - punctures at speed limit whilst towing
on dual carriageway and the like - when it has been
a sensible, well behaved combination, SUV or otherwise.
Equally, there are combinations which are just an
accident waiting to happen. Once it gets out of control,
the soggy suspension and high C of G just make an SUV
dead easy for the trailer to flip, however heavy it
is (ever watched wrestling?)
Light cars are easily steered by big trailers.

Don't know about in the US, but here in the UK you
could probably put all trailer smashes down to one
of three things:

1. lightweight cars with a large distance betweeen
the rear axle and hitch. Plenty of these around here.
The trailer does the steering especially if it is heavy.

2. Rogue trailers. Most trailers are perfectly OK towed
within the limits. Some change dramatically between
empty and loaded. I remember a ? homebuilt two-seater
trailer which loaded was a nightmare and empty was
almost impossible. I know of trailers which use old
caravan suspension components or car tyres, which are
almost always not stiff enough. Get one of these on
an SUV and it would be interesting.


3. Bad driving.Self explanatory.

Have a nice day

Al

At 16:12 29 September 2003, Bruce Greeff wrote:
Hi Graeme

Lots of prejudices here mate, sorry if I offended.

Maybe it is a SIFOW thing, I prefer not driving trucks.

Again each to his own.

CHRIS WROTE...
Mine is a
nightmare. It is unstable above 50mph. It has a lot of side to side
sway above 50mph if there is any steering input. It is almost
harmonic in nature – it does not dampen out quickly. If feels like
you are driving on Jello.

There was a lot said on the *Komet Trailer Fin* causing instability, about a
year ago. I typed in *Komet Trailer* into Google and found a good report by,
Dan Dunkel. He was able to tame the beast by adding turbulators on each side of
his trailer fin. This report is on page 3 of Google, under Komet Trailer.
Hope this helps,

JJ Sinclair

(JJ Sinclair) wrote in message
There was a lot said on the *Komet Trailer Fin* causing instability, about a
year ago. I typed in *Komet Trailer* into Google and found a good report by,
Dan Dunkel. He was able to tame the beast by adding turbulators on each side of
his trailer fin. This report is on page 3 of Google, under Komet Trailer.
Hope this helps,

JJ Sinclair

JJ,

Last year I installed the turbulator strip on my Komet fin. It is
about .75" [19mm] high and about 3" [75mm] back from the front of the
fin. [aluminum extrusion]. It has made a very noticeable difference,
and stablilzed the trailer at higher speeds, especially behind the
disturbed flow of 18wheel trucks. This cured an aerodynamic problem
that felt aerodynamic.

I also did a short test drive with the rear hatch of the Forester
open. This should have significantly altered the airflow between the
tow vehicle and the trailer. This had no impact on the swaying issue.
The swaying issue begins with a steering input, does not appear/feel
to be aerodynamic in nature. [And even if it were the Forester does
not seem to have much steering response on these tires to counteract
the motion].

Also the swaying starts at about 45mph [72km/h] which is low for this
much force to be generated aerodynamically.

Thanks for the feedback,
Chris

Chris wrote
Also the swaying starts at about 45mph

Have you tried adding shock absorbers? Don't jump my sierra, if this has been
covered, I haven't been following along. I had a utility trailer that would
sway so much I was afraid it would dump its load, adding shocks did the trick.


JJ Sinclair

"chris" wrote in message
om...
(JJ Sinclair) wrote in message
There was a lot said on the *Komet Trailer Fin* causing instability,
about a
year ago. I typed in *Komet Trailer* into Google and found a good report
by,
Dan Dunkel. He was able to tame the beast by adding turbulators on each
side of
his trailer fin. This report is on page 3 of Google, under Komet
Trailer.
Hope this helps,

JJ Sinclair

JJ,

Last year I installed the turbulator strip on my Komet fin. It is
about .75" [19mm] high and about 3" [75mm] back from the front of the
fin. [aluminum extrusion]. It has made a very noticeable difference,
and stablilzed the trailer at higher speeds, especially behind the
disturbed flow of 18wheel trucks. This cured an aerodynamic problem
that felt aerodynamic.

I also did a short test drive with the rear hatch of the Forester
open. This should have significantly altered the airflow between the
tow vehicle and the trailer. This had no impact on the swaying issue.
The swaying issue begins with a steering input, does not appear/feel
to be aerodynamic in nature. [And even if it were the Forester does
not seem to have much steering response on these tires to counteract
the motion].

Also the swaying starts at about 45mph [72km/h] which is low for this
much force to be generated aerodynamically.

Thanks for the feedback,
Chris

My Komet trailer has the aluminum spoilers on the fin and it seems to tow
well up to 75MPH - unless there is a sharp steering input like Chris says.
My feeling is that the problem is suspension/tire related. I tow with a
Jeep Grand Cherokee and below 65 MPH the handling is perfect.

I'm pretty sure that the problem is with the Jeep. (Despite remarks to the
contrary, the older Jeep Grand Cherokee (ZJ) is a lightweight among the
current crop of SUV's and not much heavier than the Forester.) I've had
similar but worse problems towing other trailers.

I deliberately started the sway on an empty stretch of interstate highway
and observed that the trailer sways back and forth rotating about the
vertical axis while the Jeep rolls side to side rotating about the
longitudinal axis. In other words the trailer fishtails while the towing
vehicle rolls back and forth. The period of the trailer's sway is the same
as the Jeeps natural roll frequency so there seems to be a destructive
resonance going on. The Jeep rolls in the direction of the travel of the
front of the trailer which suggests that the Jeeps roll center is much lower
than the tow hitch. Lowering the tow ball is not an option for ground
clearance reasons so increasing the Jeep's roll stiffness seems a good way
to go.

So far, I've increased the Jeep's tire pressure up to the point that the
ride is too stiff for comfort and, while it does help, that's not the
solution I want because of the harsh ride. The next try is to replace the
13mm stock rear sway bar with a 25mm diameter ADDCO bar. Since the
handling with the trailer is really quite good below 65 MPH, this is not a
high priority issue with me since slowing down saves a lot of fuel anyway.

Bill Daniels

In article . net,
says...
So far, I've increased the Jeep's tire pressure up to the point that the
ride is too stiff for comfort and, while it does help, that's not the
solution I want because of the harsh ride. The next try is to replace the
13mm stock rear sway bar with a 25mm diameter ADDCO bar. Since the
handling with the trailer is really quite good below 65 MPH, this is not a
high priority issue with me since slowing down saves a lot of fuel anyway.

This stiffer bar will tend to move the handling towards oversteer (less
stable in turns) because it increases the load on the outside rear tire
in a turn. Be cautious checking out the handling, and be prepared to
also increase the size of the front sway bar to move the handling
towards understeer (more stable in turns).

--
-------
Eric Greenwell USA

chris wrote:
...This had no impact on the swaying issue.
The swaying issue begins with a steering input, does not appear/feel
to be aerodynamic in nature. [And even if it were the Forester does
not seem to have much steering response on these tires to counteract
the motion].

Also the swaying starts at about 45mph [72km/h] which is low for this
much force to be generated aerodynamically.

I owned a Forester for a short time a few years ago and I think the
steering is either over-assisted or inadequately self-centreing (or
both!). I suppose you'd say the force gradient is too flat. Whatever
the cause, it always seemed to me to be too sensitive to small inputs.
Unless I rested at least one elbow, the car would readily set up a
slight wander from side to side. It was easy to set up a rhythmical
roll which had very little natural damping even with no trailer
attached. It would drift off line easily with a moment's inattention.

The tyres were probably Bridgestones but I can't be sure now. They
certainly weren't low profile - probably 75-80 (this was the unblown 2
litre - not the GT)

I towed a Blanik about 800kms with the Forester. I certainly didn't
have chris's problems - I drove happily at speeds up to 110km/h with no
trailer instability but I remember it wasn't a relaxing drive and winds
did affect it. I recall carefully keeping my elbows on the armrests to
damp the steering. The drawbar load would have been about 60-70kg and I
followed the book on tyre pressures for max load.

My thought would be that the main problem is the trailer but it's
exacerbated by a car with sensitive steering and inadequate roll
damping. Check the drawbar load and suspension of the trailer first.
It's probably cheaper to move the trailer axle back than to change cars!

I'd agree with the guy who said boost the tyre pressure at the rear but
not the front. Stiffer rear roll bar would be useful. The GT version
probably has lower profile tyres and rims which would help - check if it
has a stiffer roll bar.

The Forester steering is similar to a SAAB I once owned. Audi are a bit
like it too. I think largish FWD cars tend to have dead, over-assisted
steering and Subaru is a 4WD which comes from a FWD tradition. 4WDs
which come from the other end - Land Cruiser, Land Rover - seem to have
better weighted steering and are less sensitive and more stable.

GC

Here is the feedback on my trailer towing problem.
I thought I'd make a detailed write up to share what I learned with
others.

I was waiting until I gave a real towing trip as proof that this fix
worked. I recently drove from Atlanta up to the Sequatchie Valley in
TN. This included highway driving as well as crossing Signal Mountain
on a steep winding road. The Forester XT now is a good safe tow
vehicle. The turbocharged engine is really powerful, during hill
climbs I had to downshift not because it was too weak, but because it
wanted to race up the hills.

As I suspected it was indeed the tires at fault. For some reasons
the auto manufacturers insist on pinching pennies and putting on lousy
tires when for ~$20 of cost to them they could impress their customers
with a much higher quality ride.

The original tires were Yokohama Geolander G900 P215/60 R16 94H.
These are really poor quality, I suspect that they are the very
minimum standard of the H speed rating. This is an all season tire.

I replaced them with Toyo Proxes TPT P225/55 R16 95V.
These are much higher quality overall, [as well as the ratings for
treadwear, temperature & traction]. The key was switching from poor
"H" to good "V" rated tires. This gave a much stiffer sidewall.
The tire size is also 10mm wider [225 vs 215 with a 5% shorter
sidewall 55 vs 60]. [2% net reduction in diameter].
The tread compound is also harder, more like a sports car tire.

All these changes eliminated the uncontrolled harmonic
swaying/fishtailing that I experienced when towing my trailer with the
tall tires on the tow vehicle. During normal driving the ride is
stiffer, this is fine with me, the cornering is much improved.


Years ago I read that every force your car generates, acceleration,
braking and cornering etc. is transmitted through the tires - don't
skimp, they are one of the most components of the vehicle.
My advice to others that are experiencing swaying:
Invest in good quality tires.
Upgrade the speed rating to get a stiffer sidewall. [from S or T to H,
from H to V]
Consider shortening the sidewall with a lower ratio.
Increase the tire pressure.

Other advice that helped included lowering the hitch position and
shortening the hitch to reduce the distance from the ball to the rear
axle. Tongue weight is critical, 150lbs, around 10% works for me.

ok, I'm done typing, lets go fly!
regards,
Chris



---------------------

Original message
Tire Stiffness & trailer swaying

Several people have had good luck towing a glider with a Subaru
Forester. I do not know why mine is different.
I just bought a 2004 Forester tried to tow with it. Mine is a
nightmare. It is unstable above 50mph. It has a lot of side to side
sway above 50mph if there is any steering input. It is almost
harmonic in nature - it does not dampen out quickly. It feels like
you are driving on Jello. It feels like the problem is much more the
tires than the suspension. Though that is hard to prove.

Even when parked if you push on the hitch with your foot the Forester
will sway side to side [right and left] a lot. You can watch the rim
move right and left in and out of the tire. I think the sidewalls are
just really weak. The tread is probably relatively soft also.

The Forester's tires are Yokohama Geolander G900 P215/60 R16 94H.
Even in normal driving the steering response and cornering are poor.
[Other Foresters may be equipped with 15" tires, and other models of
Geolander tires - rather than the G900].

My Mazda MX-6 [much lower car] has Pirelli P4000 P205/55 R15 87H - it
is stable, and if you push sideways it does not sway side to side
anywhere near the degree that the Forester does. Steering response
and cornering are good.

The Geolander G900 has a tall soft sidewall, it is 4.500" above the
rim rather than 3.625" for the P4000 [24%higher].

My best idea right now is to change to a shorter and stiffer sidewall
tire.
I am considering changing from P215/60 R16 94H
- an "H" rate tire with 60% width/height ratio, to
Bridgestone Turanza LS-V 225/50R16 92V - this is a V rate tire so the
sidewall is stiffer and at a 50 or 55 ratio it is shorter.
This is the V rated version of the tire that some other Forester
owners have [LS-H]. The tread should also be a harder sports car like
compound.



Some of these tires are rated and discussed on www.tirerack.com. It
seems that Steering response and cornering stability are good
expressions of my issue.
On tirerack there are a mass of good comments on the Turanza tire -
though I doubt anyone is towing. As far as I can tell everyone on
"tirerack" hates the geolander 900 like on mine.


Does anyone else have any Experience changing to a shorter V rated
tire for better stability?


My trailer does great and stable to 85+mph behind my Mazda MX-6, so I
do not think it is the trailer. The trailer is a 1979 Komet with a
Mosquito. It weighs ~1800lbs [816kg]. The tongue weight is 153lbs
[69kg]. The Forester's manual says to keep the tongue weight between
8-11% of the trailer. 8%=144lbs [65kg] 11%=198lbs [90kg].
[the max allowed is 200lbs on the tongue] The manual transmission
version of the Forester is rated for 2400lbs [1088kg] towing capacity.


I have already tried raising the tire pressure on the rear tires to
41psi [2.8bar] as recommended by the manual. The trailer tires are
about 40psi.



Since my original write up I have learned the following:
The dealer told me that the 2004 model is the first to be equipped
with the Geolander G900 tires, previous years had a different model
tire. This is the only change to the suspension that he is aware of
between the 2002 and 2003/2004 model.

The Maximum tire pressure on the tires is 44psi, so I upped the rear
pressure to 44psi and the fronts to 41. [for towing the manual says
29psi front, 41psi rear]. I also changed the hitch from a straight
one to a "drop hitch" it is now 1.5" lower than the straight one.
The drop hitch is about 4.0" shorter than the "lift hitch" [bent
receiver hitch] that I originally tried. The hitch ball is on a short
receiver.
A test drive showed that the lower hitch/ball placement and the higher
tire pressures are an improvement. However still not acceptable. I
think the higher pressure stiffened the tires but not enough and the
tread is still too soft.

I have heard another Forester owner has had good luck with 75-100lbs
of tongue weight. An additional test drive was done after I moved
several items out of the front of the trailer to the back of the
Forester. This reduced the tongue weight to 118lbs [down 35lbs from
153]. The swaying seemed worse with the lower tongue weight. [This
follows the logic I have heard many times].

Chris Ruf