Tire Stiffness & trailer swaying - resolution

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

"chris" wrote in message
om...
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.

Thanks for your report. I have one question about what you've written. I
understand your conclusions related to tire aspect ratio, but why do you
relate tire speed rating to sidewall stiffness? My understanding was that
the speed rating primarily involved tires capacity to deal with heat. Could
there not also be T or H rated tires with relatively stiff sidewalls?

Thanks.

"mm" wrote in message ...
Thanks for your report. I have one question about what you've written. I
understand your conclusions related to tire aspect ratio, but why do you
relate tire speed rating to sidewall stiffness? My understanding was that
the speed rating primarily involved tires capacity to deal with heat.
Could
there not also be T or H rated tires with relatively stiff sidewalls?
Thanks.
Here is more of my limited understanding:

There are separate ratings for Treadwear, Traction and Temperature.

A different rating system applies to the speed rating. This indicates the
maximum speed the tire was designed for. So it is not a direct measure of
sidewall stiffness. However to survive at higher speeds the sidewall often
has to be stronger, and many tires designed to be driven faster also are
designed for cornering too. As a general rule a H rated tire will have a
stiffer sidewall than S or T, and a V rating is stiffer than H.

The variation between different tire models is probably as important also.
The best description of the qualities I was looking for with my application
were "Cornering Stability" and "Steering Response". [The original tires on
my Forester would barely respond to my steering input when towing].
You can see ratings for each tire model at www.tirerack.com.

See also:
http://www.tirerack.com/tires/tirete...eral/speed.htm
http://www.tirerack.com/tires/tirete...l/sidewall.htm

Also if you had the budget for it, going with a lower profile tire on a
bigger rim and maintaining the original outside diameter would sounds like
promising solution too.
http://www.tirerack.com/tires/tirete...al/plusone.htm


regards,
Chris

In article i68mb.13071$Ec1.1157392@bgtnsc05-
news.ops.worldnet.att.net, says...

Also if you had the budget for it, going with a lower profile tire on a
bigger rim and maintaining the original outside diameter would sounds like
promising solution too.
http://www.tirerack.com/tires/tirete...al/plusone.htm

The most interesting thing to me about Chris's experience is fixing a
trailering problem by changing the tires on the tow vehicle, something
I don't recall hearing about before. Each of the changes he made...

-wider tires
-lower aspect ratio
-higher speed rating
-higher tire pressure
-emphasis on handling and steering response

are factors that tend to reduce the 'slip angle' of a tire (tread
deflection under a side load). This, in turn, reduces the ability of
the trailer to "steer" the towing vehicle.

Even if each change results in only a 5% improvement, the cumulative
effect could be significant, as it was in his case. His suggestion for
bigger diameter rims to allow an even lower aspect ratio tire would be
another good step, and one to consider when buying a new vehicle.
Often, larger diameter rims are an option.

Has anyone tried to improve a trailer by going to larger diameter (or
wider) rims and lower aspect ratio tires? This might yield similar
benefits but at a lower cost.
--
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directly

Eric Greenwell
Richland, WA (USA)

In article ,
Tnet says...
In article i68mb.13071$Ec1.1157392@bgtnsc05-
news.ops.worldnet.att.net, says...

Also if you had the budget for it, going with a lower profile tire on a
bigger rim and maintaining the original outside diameter would sounds like
promising solution too.
http://www.tirerack.com/tires/tirete...al/plusone.htm

The most interesting thing to me about Chris's experience is fixing a
trailering problem by changing the tires on the tow vehicle, something
I don't recall hearing about before. Each of the changes he made...

From an article about the Forester in our newspaper:

"On the road, the ride is very good, and handling is exceptionally
nimble though the 16-inch all-season radials moan and and scream when
you take a corner faster than they'd like, which is anythingg over
about 20 MPH. With different tires, the Forester XT would make a great
daily driver."

So, sounds like they went for the 'bottom line' on the tires.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

"Eric Greenwell" wrote in message
The most interesting thing to me about Chris's experience is fixing a
trailering problem by changing the tires on the tow vehicle, something
I don't recall hearing about before. Each of the changes he made...

So, sounds like they went for the 'bottom line' on the tires.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

Eric.
I think the critical change was the lowering of the hitch point.
I have a Chevy Tahoe with Geolanders on it. I play with sports cars
a bunch and I can't believe how good these tires were at the limit.
When I changed the brakes a bit ago you could clearly see where
I was on the bump stops from cornering. I was suprised to see that much
body roll indicated when the feedback in the seat was so secure.

Besides that, tires usually are reactive to forces, not force generators.
In order for an oscillation to be established, the tires must be excited by
something. A high hitch raises a trailer CG ans contributes mightily to
swaying back and forth. Stiffer tires would be more difficult to excite,
but
they are masking the force, not curing its existance.

Scott Correa

In article , scorrea-removethis-
@socket.net says...
Besides that, tires usually are reactive to forces, not force generators.
In order for an oscillation to be established, the tires must be excited by
something. A high hitch raises a trailer CG

It's hard to see how this can be, since the CG is so close to the
trailer wheels. Raising the hitch 6" would raise the trailer CG less
than 1".

ans contributes mightily to
swaying back and forth.

The explanation that seems more likely to me is the higher hitch lets
the trailer roll the tow vehicle more, which would introduce steering
forces.

Chris also said:

"Other advice that helped included lowering the hitch position and
shortening the hitch to reduce the distance from the ball to the rear
axle."

It sounds like he tested the hitch effect separately from the tire
changes.

Stiffer tires would be more difficult to excite,
but
they are masking the force, not curing its existance.


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directly

Eric Greenwell
Richland, WA (USA)