Mercedes Vario 814DA handling problems

[ianc]

I've built a bad road camper based on the Vario 814DA. Details of the build are here: http://www.expeditionportal.com/forum/t ... o-laughing!

But I'm having a road handling problem which is proving difficult to diagnose. Apologies in advance for the long and detailed posting.

Background
The camper details are as follows:
-Chassis is a 2002 model 814DA with approximately 230K Kms
-Overall height 3.5m
-Overall length 7.2m
-Wheelbase 3.7m and rear overhang of 2.2m
-Weight: Front axle 2600Kg, Rear Axel 3400Kg
-Original wheels were standard 17.5Inch rims with Michelin 9.5 R17.5 XZT tyres and with duel wheels on the back.
-New wheels are 8.25 x 19.5 wheels (+ 98mm offset) with 285/70 R 19.5 Continental HDR tyres and with just singles on the back
-New Koni adjustable dampers have been fitted and with the damping adjusted by 1.5 turns
-The rubbers in the anti-roll bars have been replaced
-The rest of the suspension is original

The problems
1) On a good smooth road steering inputs feel like they are delayed and then when the steering change is felt, the rear feels like it’s swinging around. This can lead to me over steering and then over compensating. Not a nice feeling. However on long fast curves, the steering is slow to react and the tail feels like it’s swinging a bit, but then the van settles into the corner and can carry good speed.
2) On bumpy or poor road surfaces (we have a lot of this in Ireland  ) the van does not want to track straight. It will tend to change direction without warning even on small things like centre white lines. This is very disconcerting.

Steps taken to try and resolve or at least diagnose the situation
1) I replaced the suspension rubbers on the anti-roll bars. There was no noticeable change
2) I got in touch with Continental and they advised me to change the tyre pressures down to 4.5bar front and rear. They also pointed out that the HDR tyres are “Drive Only” tyres and not designed for steering, but they felt they should be “Ok”. The lowering of the pressure (from about 5.5bar) improved the ride comfort but didn’t help on the two problems
3) Fitted the new Koni dampers. This did help somewhat. But the problems still exist as described.
4) I refitted the original 17.5Inch wheels. This made some difference to the first problem. The steering felt a little quicker to react and the rear didn’t swing quite as much – although there was still tail swing. The big change was that the van was far less troubled by poor or bumpy roads and tracked straight.

These are my thoughts on why changing back to the original wheels helped:
1) The dual rear wheels – This probably would have helped reduce the feeling of the rear tail swing, but I can see how it helped with the feeling of steering or the changes of direction on poor roads
2) The new (custom made) 19.5inch wheels may have a different offset – This may explain the changes of direction on poor roads?
3) The HDR tyres have a much wider contact patch which might be more susceptible to following the poor road surfaces and causing changes of direction?
4) The HDR tyres are designed to be drive only tyres and are not at all suited to steering?

Does anyone have similar experiences and have the magic answer to my handling problems?

[Hatzlibutzli]

Good evening,

Sounds a little bit like play in the steering gear or the stub. The change of the wheel size, the change of the track width or the change of the wheel width should not do so much harm. 230Kkm are some, so I would check those two:

1. How much play do you have at the steering wheel, till the front wheels turn. Powersteering can camouflage some play, so another person has to turn the steering wheel while you watch.
2. Lift a frontwheel and move (rock) the wheel around a horizontal axis. There should be no play at all. Here the organisation, that does the roadworthy certificate has a certain stressplate, that test that play, while the front wheels stand on it.

Greets ... Simon

[ianc]

I've checked the steering mechanism and there is no unwanted movement in it.
The van has also recently gone through it's annual government check and it passed without steering problems.
Also if there was unwanted movement - wouldn't it be noticeable with both wheel types fitted?

[Zentralgestirn]

Hi,

do you have checked the wheel alignment? For me it´s sounds like your toe -in angle is not right. It can be the reason for not driving straight ahead.

Greatings Sebastian

[Veit M]

Hi,

some comments from my side.
You write about a "high" load on the back wheel.
Have you tried to reduce this a little? If you have heavy mass behind the rear axle this mass has a larger lever to turn the car around.

All tyres need some drift angle to create some cornering force (hope thats the right word for that). There might be a difference between different types of tyres.
MPT mostly feels more spongy than trailer tyres.

Also the tyre pressure is a big field to play around, weak pressure makes it more comfortable but may also increase abrasion.
A weak pressure normally also gives not so good cornering force, the tyre will bend more.

On the "good" roads you drove, there are lane grooves? I have a problem with my car in this case because different gauge than big lorries so the car runs around in this grooves.

I have this idea, may be you can whatch it.
If you start into a turn at the beginning the car stays straight. After a short way into the turn the top of the car bends outwards the turn, this gives more pressure/load to the outer tire (on back and front). This will change the force the tyre takes to the road.
In case of front wheel this will increase the force turning the car more into the direction of the turn (due to the toe-in). May be your toe-in is a bit to high for the new tyres/rims.

I had some time to high toe-in, was nice driving even on a arched road, the car runs simply straight but abrasion was incredible high.
After correction abrasion reduces dramatically (i had 8mm instead of 0-4mm) but also the car did not run that straight any more.

In both cases the driving on gravel road was no problem and no influence on toe-in was tangible.

Have a nice day

Veit

[RemcovdV]

Hi,

I am not sure whether your car is a permanent 4x4, but as far as I know, cars with permanent 4x4 should have toe-out instead of toe-in (at least modern Landrovers and my MB 917AF does).

Regards,
Remco

[ianc]

Hi,

do you have checked the wheel alignment? For me it´s sounds like your toe -in angle is not right. It can be the reason for not driving straight ahead.

Greatings Sebastian

Yes - I've had it checked by two different garages. First time by a general garage and then by one who specializes in truck alignment. I wasn't sure the first guys knew what they were doing. There was some very slight adjustment needed the second time - but not enough that it would have been previously impacting the handling.

All the suspension joints are in good order and without play.

[ianc]

Hi,

some comments from my side.
You write about a "high" load on the back wheel.
Have you tried to reduce this a little? If you have heavy mass behind the rear axle this mass has a larger lever to turn the car around.

Although there is a rear overhang of 2.2m, there is not much weight past the rear wheels. It's just the living box which is GRP/Foam/GRP construction and reasonably light. I don't really have an option to reduce this weight

All tyres need some drift angle to create some cornering force (hope thats the right word for that). There might be a difference between different types of tyres.
MPT mostly feels more spongy than trailer tyres.

That's what I'm thinking on my 4th bullet above - The Continental HDR tyres have an unacceptable slip angle

Also the tyre pressure is a big field to play around, weak pressure makes it more comfortable but may also increase abrasion.
A weak pressure normally also gives not so good cornering force, the tyre will bend more.

On the "good" roads you drove, there are lane grooves? I have a problem with my car in this case because different gauge than big lorries so the car runs around in this grooves.

No - the roads were generally just poor.

I have this idea, may be you can whatch it.
If you start into a turn at the beginning the car stays straight. After a short way into the turn the top of the car bends outwards the turn, this gives more pressure/load to the outer tire (on back and front). This will change the force the tyre takes to the road.
In case of front wheel this will increase the force turning the car more into the direction of the turn (due to the toe-in). May be your toe-in is a bit to high for the new tyres/rims.

I had some time to high toe-in, was nice driving even on a arched road, the car runs simply straight but abrasion was incredible high.
After correction abrasion reduces dramatically (i had 8mm instead of 0-4mm) but also the car did not run that straight any more.

In both cases the driving on gravel road was no problem and no influence on toe-in was tangible.

Have a nice day

Veit

As standard the toe-in is -0.5 to +0.5 mm. So effectively there is no toe-in angle. Are you suggesting to try a small negative toe-in?

[Veit M]

A negative toe-in maybe helpful if you have permanent 4x4.
In this case the wheels tending to bend forward by the transmitted power.
For switch on 4x4 where you normally drive only 4x2 keep it a bit positive.

I think the "problem" is caused by overhang in combination with tyres.

Have a nice day

Veit

[Freddy]

2) On bumpy or poor road surfaces (we have a lot of this in Ireland  ) the van does not want to track straight. It will tend to change direction without warning even on small things like centre white lines. This is very disconcerting.

If I understand you right then this might be an effect called "Bump Steer"

http://en.wikipedia.org/wiki/Bump_steer

This occures quite often when you raise the suspension of 4x4 passenger cars (because they have a spring under axle suspension)

Maybe your front suspenion got raised a bit due to you overhang an the extra weight on the rear. This would turn the front suspension a bit softer as well what made the effect worse.
When you added the Konis the suspension got stiffer and reduced the effect.

We used to work against this problem by using a Dropped Pitarm but i don't know the steering geometry of the Varios.

Greetz
Freddy

[moserm]

Hi,

My feeling is that the biggest different is due to the different Rim Offset.
The difference in Rim offsett is appr. 20-25mm i guess (120mm to 98mm).
Due to the Tire Radius Change from 9.5R17,5 to 285/70R19,5 which is only appr. 25mm
this change in Rim Offset is too much for my feeling. I think you should drive the bigger Tire with similar Rim offset at the Steering Axle. Target is that the Scrub Radius is similar (This has influence on the Straight ahead Driving Behaviour).
Due to the Change from Twin to Single Tire your are also loosing Driving Stability.
For Toe I would go to 1,5mm Toe In (even though it is a 4x4).
Than I would go to higher Tyre Pressure at the Rear Axle (7-8Bar) and keep the 4,5 to 5,5 bar at the Front Axle, because I think you need higher Roll Stability on the Rear Axle to get him better to drive, you have to consider that you have done also reduction on the roll stability by change from Twin to Single Tire.

br
Moe