How to pick your axle start angle

[angib]

There was a discussion about how to select your torsion axle start angle and this shows how I think it should be done - if you know better, please post a reply!

I have used Dexter Axle information as they have really good information available on their web site, but the same information could be used for almost any make of axle.

The reason to pick a particular axle start angle is to get the right ride height on your trailer. This is shown as dimension 'B' below - the height to the top of the axle mounting bracket, which will usually be the height to the underside of the trailer frame.



The ride height 'B' is made up of two dimensions added together:
1) 'SLR' (static loaded radius), the distance from the ground to the center of the spindle/wheel/tire. As the tire squishes (technical term!) under load, this is less than the normal tire radius.
2) 'H', the height from the spindle to the top of the axle mounting bracket.

The SLR for tires 12" and up is given in this Dexter Wheel/Tire data pdf

The same info is given in this Goodyear Tire data web page and, yup, Dexter and Goodyear do give slightly different answers! Don't worry, I don't believe any of this info is more accurate than to the nearest 1/2 inch.

Most people seem to want to fit trailer tires which are several sizes bigger than they need to be - their tires will not be loaded anywhere near their capacity and so will not be 'squished' as much as these charts show. I think it is sensible to pick a SLR that is part way between the SLR and the normal radius in the tables, to allow for this.

To find out the height 'H', you need to look at the Dexter chart for the axle you will be using - go to the Torflex web page and click on Torflex Product Information. Here is the page for the #9 axle:



In the table, we can look up the dimension 'H' for different start angles. As with the tires, it's likely that the axle won't be loaded to its 'Full Load', so we could pick a figure that's part of the way from 'Full Load' to 'No Load' - for the first line of the chart above, we might pick an 'H' value of 4.0. Don't get caught out by the negative 'H' values in this chart, which mean the spindle is above the top of the mounting bracket.

Add together the 'SLR' and 'H' dimensions you've looked up and see if that gives you the ride height you want. If not, do it again for a different start angle - an Up start angle will give you a lower ride height than a Down start angle.



There is more than one style of axle mounting brackets and which one you chose will affect the ride height. For the Dexter #9, adding a side mount hanger increases the 'H' dimension by 1/4 inch and there are separate 'H' dimensions given for the high profile bracket.

Any comments?

Worked Example

We want to use a 14" wheel, a Dexter #9 axle and we want a ride height of 15" to the underside of the frame.

From the first table, the SLR of a lightly-loaded 205/75R14 will be about 12.5" (a bit more than the SLR in the table of 12.1").

So we want a 'H' dimension of about 2.5", because that's what must be added to the 12.5" SLR dimension to get the 15" ride height we want.

From the second table above, we can estimate these 'H' values for an axle that isn't loaded to its full load:
45deg Down: H = 4"
22.5deg Down: H = 2"
10deg Down: H = 0.5"

So the nearest answer to the 2.5" we want is for the 22.5deg Down axle.

Any comments?

Andrew

[subtearanean]

My comment:

Wow. It almost makes sense to me....after I read it 2 or 3 more times I'll be able to profess mastery. Once again you have reached a new height in my esteem.

This is one part of my (now in a stalled stage) build that had me worried. Figured I'd only get one shot at doing it correctly.

Some of the TorFlex axles are adjustable? With a spline? Can this fine tune the ride height?

[angib]

Some of the TorFlex axles are adjustable? With a spline? Can this fine tune the ride height?

Torflex is Dexter's rubber torsion axle brand and they are not adjustable after manufacture. Neither is a:
- Henschen Axle;
- Al-Ko axle; or
- Reliable axle.

The only one that is adjustable is the Flexiride available from South Western Wheel (and elsewhere?). And beware, it appears that some (all?) small Flexiride half-axles (separate units for left and right wheel) are not adjustable for start angle.

Andrew

[SteveH]

Andrew,

Really great as usual.

Or, you can do like I did...put it together and if the height is wrong, change it.

[madjack]

...once again Andrew...you da man, Thanks...I went ahead and made it sticky too.................................

[grant whipp]

NICE JOB, Andrew!

Lots of technical stuff boiled down to layman terms!

However (and you just KNEW there was going to be a "however" from me, didn't you ... ;-} ;-} ...?), 15 years of practical experience with Dexter tells a slightly different story.

Maybe I missed it, but I didn't see a spec'd Load Rating in your calculations. Load Rating makes a LOT of difference in "squish" factor and the initial "settle" of the ride height after the normal unladed (net) weight of the trailer is placed on the axle. If you put a 1,000# trailer on a 2,000# rated axle, you're not going to get much "squish" and/or "settle". Only spec your Load Rating at about 30-40% more than your spec'd or estimated NET weight.

Here's what I've found using Dexter's #9 side-mount axle, rated at 1,000# under a 750# trailer with 195/75R14 tires:
10º Up Starting Angle (my standard) - 12-13" ground-to-frame clearance.
22.5 Up - 10-11" clearance
0º - 14-15" clearance
10º Down - 16-17"
22.5º Down - 18-19"

A while back, I started on an off-road teardrop to go behind my Ford Ranger 4X4 ... I wanted the trailer to ride at about the same ground clearance as the truck and use the same wheel/tire combo (235/75R15) ... through my calculations and experience with Dexter, I spec'd a #8 side-mount, rated at 1,100# (max for a #8), with a 10º Down Starting Angle. The trailer will net out at about 800#, and SO FAR, my calcs are spot-on ... 18" of ground clearance, straight & level with the rear bumper of the truck. (Unfortunately, the trailer has been put on the back burner - need to take care of the paying customers, first, ya know - and since the truck went out of commission with a severely wounded engine, it will be a while before I can get 'round to completing it ... ;-{ ;-{ ...)

One last thing: unless there has been a DRASTIC change in the rubber specs on Dexter axles, there should be NO DIFFERENCE in the ride quality of equally load-rated axles between the 10º Up and 10º Down Starting Angles, and only SLIGHT differences in the 22.5º Start Angles (with a 22.5º or more Up angle, there is a marked reduction in the trailing arms ability to make full use of the available/useable arc travel of the spindle). It has been my experience that the factory makes minor load-rating adjustments during production to compensate for these radical angles.

This, of course, has been MY experience with Dexter Torflex axles over the last 15 years. This qualifying disclaimer, however, must be made for obvious reasons: YOUR experience may differ ...

GOOD LUCK, Everyone! and, in the meantime ...

CHEERS!

Grant

[Chris C]

WOW! You are definately our resident engineer and we all thank you. Don't know why I'd never seen that on their site, as often as I've been there. Good information..........and I'll be keeping it for sure! Thanks again.

[angib]

Post deleted

[remarquian]

Andrew,

A bit more info:

The Flexiride half torsion axles rated at 1400lb and above (1400, 2000, and 3500 lb) use the cartridge mechanizm and are adjustible. The 550 and 935 lb are not, they use the British style (whatever that may mean).

Here is a PDF I got from UCF America that describes their half torsion axles.

rob

[mikeschn]

Andrew,

Good information... I printed it out and asked Chell to read it to me on the way up to Saginaw this afternoon... but she refused... something about getting car sick while reading. So I had to wait till I came back home to read it.

You did a good job translating that into laymans terms. I think anyone can set the height of their trailer with that info.

I like spreadsheets, and am considering turning your information into a quick and dirty spreadsheet. It should be pretty straight forward... wouldn't that be;
sin h=opp/hyp +cl of torsion rod to top of mounting bracket.

Mike...

[grant whipp]

... The point I was trying to make about axles with Up start angles is that they get towards the point where they have no compliance to big bumps. A 22.5deg Up start angle axle is going to be close to 45deg Up under load ... at this angle, the spindle is moving forwards just as much as its moving upwards, which is fine for a purely vertical trailer movement. But if you get a severe bump with some backward component (for example, climbing up a kerb stone), the force is in line with the axle arm and so it cannot deflect.

I think I said that ... ;-} ;-} ... "with a 22.5º or more Up angle, there is a marked reduction in the trailing arms ability to make full use of the available/useable arc travel of the spindle" ... you just put it in more technical terms.

Believe me, Andrew (and All), I'm not trying to argue or dispute, but once again, practical experience (at least in MY case) hasn't borne that out ... precisely. If you want a low riding trailer and you've determined that an axle with a 22.5º Up Starting Angle will give you the ride height you want, all I'm saying is that you shouldn't worry about the axle performing adaquately for it's intended purpose ... providing you've chosen the proper load rated series of axle.

Quite a while back, I put a 4' x 8' utility-type box on one of my standard teardrop chassis ... this one with a 22.5º Up #9 Load Rated at 1,000#. Five or six times a year I will load that trailer up with 1,500-1,800# of firewood, sand or gravel, or construction debris and take off down the road. It has performed flawlessly and as expected in all situations I have put it in, including several slow and deliberate treks across some fields and ranchlands. I NEVER worry about whether that axle is up to the task!

Something we need to keep in mind while we are talking about torsion axles is maximum load rating for the series axle you are considering. For example, Dexter's #9 has a maximum available load rating of 2,500# (if my memory serves correctly) ... while it is available in load ratings as low as 1,000#, this lower-rated choice has all the benefits of the toughness and durability and construction materials of a 2,500# rated axle. If you put a 750# net teardrop on a 1,000# rated #9 Dexter (and I do on a routine basis), that axle will be virtually indestructable at any chosen trailing arm starting angle. Can it be damaged or experience failure? Oh, sure! But it will be the result of consistant ABUSE (or maybe an accident!), not normal and intended use.

Enough of my babbling ... let's get those teardrops a rollin'! Let's just not try to make rocket science out of it, O.K. ... ;-} ;-} ...? (No offense intended or implied!)

CHEERS!

Grant

[doug hodder]

VIVA SPRINGS!!! Doug

[angib]

Thanks, Grant and your point is taken - I've edited that part out of my first post and deleted the second one altogether.

Andrew

[alaska teardrop]

Andrew - May I suggest a top view of your drawing? The Dexter literature shows the minimum overhang relating to the construction of the axle. However, some sales companies don't always show that the dimension from the hub face to the inside of the swing arm is 6.2 inches. Therefore, the hub to hub measurement must be at least 6.2x2+the frame/body width (including side panels/trim) for the swing arm to clear. Fred

[angib]

Andrew - May I suggest a top view of your drawing?

Whoah there, Fred - that's good info for the How To Pick Your Axle Width thread that I encourage you to write.....

Andrew