Measure Sag of DM T@B AlKo Torsion Axle ?

MikeT

Do DM T@B AlKoTorsion Axles ever sag ? The 2004 T@B I am thinking about purchasing has mostly been parked and has probably traveled only a few thousand miles. I want to make sure the torsion axle has not sagged. See the picture below. With the trailer level and unloaded, there is 2.5 inches of exposed sidewall on the tire. This is with the original 16 inch alloy wheels and new 205-55R16 tires.

Another measurement is the tennis ball test. Reaching up to the top inside of the tire with my arm under the trailer, I can place a tennis ball with a tight fit between the top of tire and the inside of the plastic wheelwell. The diameter of a tennis ball is about 2.6 inches so the clearance from top of tire to wheelwell is about 2.5 inches.

If anyone with a DM tab and 16 inch alloy wheels could measure the exposed sidewall or try the tennis ball test, it would help me figure out if this axle is OK.

Thanks, Mike

Geo

@MikeT,  Our's measures 2.75" 
I had noticed some years ago that the different year DM T@B's rode at different heights, with some being quite noticeable between high and low.  It may only be an issue with how AL-CO set up the axles.

I do not think you need to be concerned.  I am not familiar with any reports of failing rubber in the torsion axle.

MikeT

@Geo Thank you for the information. The 2004 TaB AlKo axle label lists the model number as B850-3:



A European web site had data on this axle:
BC: dimension A 1250mm (=49.21 inches, this is outside distance across the chassis frame longitudinal members)
OHF: dimension C 1790mm (=70.47 inches, this is distance between the brake drum faces)
Wheel connection 112x5
Offset (ET) 27-33mm
Swing arm position 25 degrees (this is downward angle of swing arm with no load)
Axle profile 80mm
TECHNOLOGY
Model: EURO1
Axle type: B 850-3
Axle load: Single axle 800kg
Wheel brake: 1637
Suspension: Hexagonal rubber suspension
Swing arm length: 160mm
Wheel bearing: Compact bearing
Rim centre hole: Min. 57mm

I suspect that all of the DM TaB production from 2004-2009 used the same B850-3 axle because the GVWR rating of 1939 lbs did not change through those years. The deflection and damping function of the axle relies on elasticity of the 3 rubber cylinders inside the hexagonal axle tube. As the rubber ages, it loses elasticity and starts to permanently deform from round into an oval cross-section. The suspension height decreases and the suspension becomes stiffer (less swing arm angular deflection for a given load change). I took off the wheel and checked the unloaded swing arm angle - it was about 20 degrees, 5 degrees less than the specification. Probably the loaded swing arm angle has also decreased a few degrees, and that is why the suspension height on this 2004 TaB is less than it was when new.
Below is a picture of this 2004 TaB followed by pictures of newer DM TaBs showing the location of the wheel center relative to the trailer bottom edge.

This 2004 Wheel:

2005 Wheel:

2008 Wheel:

2009 Wheel:


Unlike a utility trailer, the TaB axle is loaded all the time to at least 80% of its max rating. Unless the chassis is jacked up to unload the suspension during months when the TaB is not in use, the rubber cylinders in the torsion axle are always under heavy compression. Eventually they will lose some elasticity, causing the suspension height to decrease and the ride to become harsher. Degradation of suspension function is gradual over a period of years, and will vary with individual loading, environmental and road travel conditions. I found estimates of rubber torsion axle suspension life to be 10-25 years. Most commenters said that the worst condition for a torsion axle is to leave the trailer parked for years with wheels on the ground.

The 2004 TaB that I am considering was parked in a campground for much of its life. The 18 year old suspension still works but is unlikely to have the ride quality it had when new. After enough years the rubber tubes will become inelastic and the swing arms will no longer move up when road shock loads are applied. At that point the axle has to be replaced or repaired. In the UK there is a company with specialized equipment to renew an AlKo axle by pulling it apart, installing new rubber cylinders, and pressing everything back together. I havent found a similar company in the US.

Since owners dont normally ride in TaBs, would they know if the suspension had become too stiff due to ageing of the rubber cylinders ? Perhaps the wheel center location (moves up as suspension ages) is an indication of the ride quality and remaining life ?
Thanks, Mike

Denny16

You do know there is a shock absorber modification available for DM TaBs with this AL-KO axle that will help avoid sagging and increase suspension action a little?

Cheers 

MikeT

@Denny16 Shock absorber function is dependent on the suspension having appropriate deflection when the load changes. The purpose of a shock absorber is to dampen suspension oscillations. If the rubber tubes inside the axle have become inelastic due to ageing, the swing arm wont move much and the shocks will have decreased effect. On cars, some gas charged shock absorbers do increase suspension height. I dont know how much pressure the AlKo shocks exert, but it would have to be several hundred pounds to make much difference in the ride height.
Thanks, Mike

Denny16

OK, thanks, and it was an option I thought might help, as other owners of older DM TaBs felt adding the shock option helped their trailers ride better.
 Cheers 

MikeT

@Denny16 I agree with you, shock absorbers would help. I found a technical discussion of rubber torsion suspensions. It pointed out that the rubber cylinders only provide suspension damping over a limited frequency range. Adding shock absorbers expands the frequency range over which oscillation damping is provided.
Thanks, Mike

Geo

@MikeT,  Excellent info.  The pics do show exactly what I have seen and wondered about.
Soooooo - do we put our little DM T@B's up on blocks in the off season ?
Thanks. . .

MikeT

@Geo If the TaB will not be used for several months, unloading the suspension by elevating the chassis might increase the suspension life. The problem I had was that the frame jacking point bracket under the axle is the same place that a jack stand or block should be located. I suppose the jack could be offset slightly to the rear along the frame so that a jack stand or block could be positioned. Offsetting the jack to the front would be dangerous because it might be in front of the center of gravity and the trailer could tilt over backwards.

I found an Alko chassis handbook that recommends jacking up the trailer if it is not used for a few months. The handbook says the corner steadies should be lowered after the block/jack stand is placed. Anyone using this procedure must remember to raise the steadies before lowering the trailer at the end of storage. Otherwise the steadies and floor structure could be damaged.

Thanks, Mike