Scamp Axle - Leading vs Trailing

[Harvey]

Roger, you raise some good points. I can't counter 'em, only tell you what & why I did it the way I did. I talked to an engineer at Dexter before finalizing my axle replacement (you're correct, my original axle was an Al-Ko, leading arm, no brakes). The engineer told me that they no longer built 'off the shelf' leading arm axles but had no objection to building a #9 axle in that configuration, with brakes, & it would work as well as trailing arm. I determined I wanted 22.5 degree down start angle, 7" electric brakes, & 5 on 4.5 hubs, as well as 'high In', side mount mounting bracket kit. The side mount kit adds 1/4", the 'high' mount bracket 1", & the portion of the old axle bracket I left welded to the trailer chassis another 1/4", giving me an added 1 1/2" plus whatever the 22.5 degree down start angle would add, depending on my tires (remember, I was sitting at better'n 10 degrees up & the radial automobile tires were occasionally contacting the underside of the wheel well on bumps).

I'd received an e-mail from Trent Fisher at Scamp stating that my original axle should have been a 22.5 degree down start angle. I found that very hard to believe as it was at that time sitting at better than 10 degrees up. Upon reviewing my pix of the old axle & using another forum member's procedure for determining original start angle I believe my old axle started at zero degrees (the square shaft that goes into the axle tube was welded into the torsion arm with opposing corners pretty well in line with the longitudinal axis of the arm).

I ordered the new axle from Dexter through Liberty Trailer in North Little Rock, AR, price $411 including the side mount kit & shipping to Liberty Trailer (I believe they get their stuff through Redneck Trailer Supply). My grandson took my trailer to his shop, set it up on blocks & jack stands, & we began the removal/installation shortly after noon by 'torching' off the old axle at the 'L' of its mounting bracket. After grinding the torch dross off the remainder of the old bracket we used a floor jack to raise the new axle into place (with the mounting kit attached), squared the end of the spindles with the center of the coupler (essentially the same position as the old axle), clamped it in place & 'tack' welded the mounting brackets to the remnant of the old brackets (had to cut & weld onto the trailer chassis 'spacers', from the brackets still attached to the old axle since the new side mount brackets were approx 1 1/4" longer in front than the old bracket remnant). We then unbolted the axle from the side mount kit, lowered it & welded the mounting kit in place. It was then a simple matter to raise the new axle, again square it to the coupler & bolt it to the side mounts. I'm not sure what your source meant by having to modify the mount to fit the drop floor in front?? We had no problem with the Dexter axle or mounts designed & mfg'd as a leading arm. If you take an axle mfg'd as trailing arm & switch the brake assemblies so you can mount it in a leading arm position you may possibly have a problem with the mounts?? If ordered from Dexter as a leading arm I'm pretty sure it'll fit if you've given them correct dimensions. We mounted the new wheels & tires & I was home before 5pm.

I've only towed my trailer a few miles since the axle swap (still working on my interior re-build) but have experienced NO difficulties. Of course I've not made any 'panic' stops but I don't expect any untoward trailer movement.

You'll note that the 'In' side mount brackets are designed so that the axle brackets fit 'inside' them. My measurements were close enough that it was a very snug fit for the axle brackets between the side mounts & the chassis on the 'curb side' of the trailer...

1st pic shows the axle mount from rear curb side. 2nd pic shows same side from forward of the axle. 3d pic shows street side rear & front. Welds are actually better than they look & I got good penetration without 'burning' the stock. Last pic is the old axle showing a better representation of the square shaft welded into the torsion arm, note one corner to corner axis is perpendicular to the arm's longitudinal axis...

[RogerDat]

Harvey,
Thanks for taking the time to post those pictures and write up the process.

I think going off the Dexter catolog they only show trailing arm, the mounting bracket that is on that axle extends further one way than the other. Same as the original but in the opposite direction. Turning a trailing arm around puts the long side where it would run into the dropped section.

Looks like you and your grandson used the side mount bracket kit to successfully work around that. Being a commercial shop there may be a bit of reluctance to get that creative.

He was in Elkhart at Al-Ko plant on other business and took the sketch of my axle measurements with him. When he asked they said they could make it from that or if he wanted he could send them the old axle and they would copy it. I'm guessing if he had made direct contact with Dexter as you did they might well have offered a solution not in the catalog.

I think your old axle in that picture might possibly have a few more degrees of up angle than even mine. You know they could mount a plate behind the axle with marks showing axle true "age" based on what mark the arm lines up with. Ours would both be at or near the mark labeled "holy crap this is still on the road?".

It is good to hear that brakes on a leading arm at 22.5 degree down are not an out and out no-no. If it was I don't think Dexter and Al-Ko would both be willing to make them.

If he can get me a direct copy of the original at a good price I will be thrilled, and I bet it will make him more comfortable with installing it. As I told my wife even if it costs a bit more to get exact copy made not paying for custom install labor will offset that increase in the part cost. As long as the bottom line stays close to the same it's all good.

[Brian B-P]

Originally Posted by RogerDat

I know an old rule of thumb was 2/3 of the braking capacity was always supplied by the front wheels of a car or motorcylce because when stopping the weight shifts forward and bears down on those tires. Same as with a leading arm trailer axle. Rear wheel was losing weight (traction) during a stop which seems more like a trailing arm would act.

That's right for a vehicle with front and rear axles, but it has nothing to do with suspension geometry, and the trailer only has one axle (or a set of axles acting like one in the case of a tandem) - not front and rear axles.

The corresponding situation in a trailer is that during braking load (the vertical force) shifts forward to the hitch, and thus away from the axle... regardless of whether the axle is on leading arms, trailing arms, or leaf springs.

Weight doesn't actually shift (because weight is the force of gravity on a mass, and neither gravity nor the mass are changing): the front axle or hitch must carry more load and the rear less, to produce a torque (or moment, or force couple) which to resist the effect of the braking force being applied down at ground level instead of up at the centre of mass.

[Brian B-P]

Originally Posted by RogerDat

... a different axle manufacturer Al-Ko.

Talked to someone he knows there. Was told they could manufacture axle as true leading arm and with brackets to match the install based on his sketches.

I don't think there will be anything specifically leading about the Al-Ko axle - they just have shorter standard brackets which will more likely fit, so a bracket modification might not be necessary. It might also have toe-in set to an angle desirable for leading arm use - that would be useful.

There are bracket options from various manufacturers which are not generally offered to the public and not shown in the catalog, to accommodate the specific needs of OEMs. The U-Haul travel trailers had an example of this, which causes difficulty for later replacement. A trailer manufacturer (even a small one) might have access to one of these options which might make the Scamp installation easier... great if there is and you have the contact.

Originally Posted by RogerDat

I know there is some internet debate over Al-Ko vs Dexter but there is also a pretty good chance the original axle is Al-Ko.

There is little functional difference between a Dexter Torflex and the standard North American Al-Ko rubber axle (the Al-Ko Euro products are different); I'm sure either will work fine. The Al-Ko six-sided outer tube and three-lobed inner tube are probably an advantage, but may not make a useful difference. Different brands may have different arm lengths (Flexiride, for instance, is much shorter than Torflex in this size range), which would affect mounting location.

[Brian B-P]

Originally Posted by RogerDat

I think going off the Dexter catolog they only show trailing arm, the mounting bracket that is on that axle extends further one way than the other.

The leading axle should look exactly the same - just change the "direction of travel" arrow on the diagram (if it had one). The bracket is longer in one direction, not because the rear side should be longer, but because the bracket should be where the load is, and that's where the hub is.

Imagine a trying to balance a load on one point on the bracket. That point would be right over the centre of the hub... six inches from the middle of the cross tube. A sensible bracket for this type of suspension is much longer on the side toward the hub than the other, whether that direction is forward or rearward.

Al-Ko tends to make their brackets nearly symmetric, rather than longer on the side that should be longer. I think I know the reason, but it doesn't matter - the Torflex bracket is right the way it is, and shouldn't be any different if you turn the unit around.

[RogerDat]

Originally Posted by Brian B-P

That's right for a vehicle with front and rear axles, but it has nothing to do with suspension geometry, and the trailer only has one axle (or a set of axles acting like one in the case of a tandem) - not front and rear axles.

The corresponding situation in a trailer is that during braking load (the vertical force) shifts forward to the hitch, and thus away from the axle... regardless of whether the axle is on leading arms, trailing arms, or leaf springs.....

I think the concern expressed by some is leading arm would have more tendancy to let the wheels act as something of a fulcrum during hard braking. Trailing arm less so. The greater the down angle the greater that tendancy.

My thinking is that the hitch and TV rear tires will take pretty much the same force no matter which axle type. Hitch acts like front wheels on a car when stopping, more force goes there than stays on the trailer (rear) wheels. Which is what it sounds like you are saying happens.

My guess (and it is just a guess) that under some braking conditions the leading arm by being angle in the direction of travel may have more traction, but at under extreme braking they may go squirrely in a different way, or at a slighty different point.

E.G leading arm might crow hop if locked up and trailing arm might skid.

[RogerDat]

Originally Posted by Brian B-P

The leading axle should look exactly the same - just change the "direction of travel" arrow on the diagram (if it had one). The bracket is longer in one direction, not because the rear side should be longer, but because the bracket should be where the load is, and that's where the hub is.

Al-Ko tends to make their brackets nearly symmetric, rather than longer on the side that should be longer. I think I know the reason, but it doesn't matter - the Torflex bracket is right the way it is, and shouldn't be any different if you turn the unit around.

Look at the picture Harvey posted of the old axle, you can see it does not extend to the hub center, or anywhere close to it. Instead you can just spot the vertical welds where it attaches to the drop floor frame.

I'm guessing that being welded into the corner of the drop floor frame is what imparts the strength that would be associated with having the bracket longer in that direction and having the bracket run straight down the frame.

What your saying makes sense. What I said about having a bias toward sticking with what the original engineer came up with is what makes me willing to wait and see if Al-Ko will make the axle with that exact same bracket.

You also raise a good point on the toe-in.

[ruscal]

It seems that a trailing arm design would overcome bumps in the road better than a leading type, due to the direction of travel when compressing. You would think that when the tire strikes an obstacle in the road that it would be driven upward and backward if un-restrained. That would compress a trailing arm following fairly close to its travel arc which would go upward and backward. A leading arm would travel upward and forward during compression and the arm if set at around 23 degrees down would be nearly aiming straight at the oncoming path of the force. Since the arm doesn't telescope into itself to absorb the bump the trailer will just jump up over the obstacle instead of the suspension absorbing the hit which it is supposed to do. If the starting position of the axle was near 0 degrees (level) then the differences between leading and trailing link would I think be minimal. Just speculating about this.
Russ

[RogerDat]

Originally Posted by ruscal

It seems that a trailing arm design would overcome bumps in the road better than a leading type, due to the direction of travel when compressing. ..........
If the starting position of the axle was near 0 degrees (level) then the differences between leading and trailing link would I think be minimal. Just speculating about this.
Russ

To some extent I too think this is the case in my own speculation. If you think about it a wheel barrow acts much like I expect a leading arm would, you can push it over most bumps but if you need to go over a curb you have to pull it over.

The down angle going forward makes it so the tire "runs into" the curb. For that matter leading or trailing arm can get trashed cutting a corner too tight and hitting the curb.

I'm guessing a trailing arm can handle some unknown amount greater bumps or pot holes than a leading arm. I think tire size has a lot to do with it also, bigger tire has an easier time rolling over bumps. I'm staying with 13 inch so I can continue to use the rear spare tire bolts. Trailer is pretty much all original so I am more inclined to stay pretty stock with my changes.

I'm going to try and get under the trailer and see if I can find and read the axle model plate. Might be some info on there that would let me know what the original axle configuration was.

Of the two bump shock vs. scraping on steep inclines I'm more concerned about the scaping. But have wondered if 10 degree might be better, I would only be giving up 1" of height. 0, 10, and 22.5 are each adding about an additional inch to my current low rider axle.

[Brian B-P]

Originally Posted by RogerDat

I'm going to try and get under the trailer and see if I can find and read the axle model plate. Might be some info on there that would let me know what the original axle configuration was.

This is a Torflex, right? If so, the start angle is visible if you remove the wheel. Assuming you know what series (size range) it is, and the rubber isn't cut down to a lower capacity, everything else is visible dimensions.

[RogerDat]

Originally Posted by Brian B-P

This is a Torflex, right? If so, the start angle is visible if you remove the wheel. Assuming you know what series (size range) it is, and the rubber isn't cut down to a lower capacity, everything else is visible dimensions.

It's whatever was original equipment on scamp 13 in 1977. I think I have heard some say Al-Ko might be what they used back then. If it is there should be a model/serial plate on it.

Have not gotten under it to look for identification of axle. Had shoulder surgery a couple of months ago and it's taken awhile to heal up to the point where getting under trailer was a possibility. I get arm in wrong position or pull/push at the wrong angle and it pretty much puts a damper on my enthusiasm for getting any work done for awhile.

Have also read here that you can figure out original articulation by looking at the square in center of arm and figuring out what the arm would have been at if the shaft was in a certain postion. Got the gist of it but not the details.

[Roy in TO]

FYI I did the leading - trailing swap on my boler American.
http://www.fiberglassrv.com/forums/f...arm-37248.html

Excellent price on the axle, once you get it done and are satisfied you will have to post the location. Now that Canada has raised it's duty free limits to $800 after 48 hours it is definately worth the trip to MI.

[RogerDat]

Originally Posted by Roy in TO

FYI I did the leading - trailing swap on my boler American.
http://www.fiberglassrv.com/forums/f...arm-37248.html

Excellent price on the axle, once you get it done and are satisfied you will have to post the location. Now that Canada has raised it's duty free limits to $800 after 48 hours it is definately worth the trip to MI.

I did read that post and have been following it. Axle price is a little better but most of the difference in labor is I expect due to the extra work involved in switching from leading to trailing. There was a fair amount of fabrication and welding in doing that job.

Still waiting to find out a price from Al-ko on axle with direct copy of the original bracket based on shops measurements.

Not too worried about leading axle with brakes since dexter and al-ko both make them but would have been nice to have heard if any of the older 16 ft Scamps came that way.

[RogerDat]

Axle is ordered !!!
Not sure when it will be available, mostly depends on when Al-Ko gets it done. Trailer place makes runs down to Elkhart fairly often, if it's done they will pick it up.

Interesting side note on leading axle and brakes. The guy at Al-Ko said they would NOT do brakes on a low angle leading arm such as a 0 degree. No information on the why, just that they would not, since I was getting 22.5 degree down angle brakes were OK. Don't know about 10 degree.

Also was told there is no toe-in on these axles. Camber yes. Enough positive camber so that under rated load the axle flexes out to zero camber, over loaded yields a negative camber and inside tread wear.

[Brian B-P]

Originally Posted by RogerDat

Interesting side note on leading axle and brakes. The guy at Al-Ko said they would NOT do brakes on a low angle leading arm such as a 0 degree. No information on the why, just that they would not, since I was getting 22.5 degree down angle brakes were OK. Don't know about 10 degree.

The horizontal or up (I would have to think more about which) arm angle maximizes the lift reaction to braking. Remember the trailer axle is like a car's rear axle: you want braking to pull down to counteract braking dive, not push up to make it worse. The steady-state load tranfer is the same either way - this is about transient reactions and body movement.

I wouldn't expect the angle to matter that much within the range we're talking about, but they're the ones building the axles...

[RogerDat]

Originally Posted by Brian B-P

.... The steady-state load tranfer is the same either way - this is about transient reactions and body movement.

I wouldn't expect the angle to matter that much within the range we're talking about, but they're the ones building the axles...

I'm thinking that what your saying makes sense to my limited understanding, it's about how it moves from it's initial location to the steady state. And how that transition impacts handling, or braking.

[RogerDat]

I'm starting to look at brake controllers it looks like the P2 is the best bet, unless I want to spend $20 more for the diagnostics of the P3. I could go down to the primus and save $30 from the price of the P2 and it still seems like a good product.

Anyone have experiences with any of these they would like to share? Or an alternative brand?

[Thomas G.]

Originally Posted by RogerDat

.............
Anyone have experiences with any of these they would like to share? Or an alternative brand?

I have been using the Primus for about 18 months and think that it works great on my little 13 footer. It has adjustable, proportional, accelerometer based braking, which is the key attribute I want in a controller. Maybe if I had a P2 or P3 I'd be even happier, but whatever advantages they offer, I have not missed them.

When I think of the spectrum of trailers with electric brakes, on one end I see triple axle equipment trailers hauling bulldozers and on the other extreme end, I see 13 foot FG campers. So, for me, basic need, basic controller.

[RogerDat]

Originally Posted by Thomas G.

....
When I think of the spectrum of trailers with electric brakes, on one end I see triple axle equipment trailers hauling bulldozers and on the other extreme end, I see 13 foot FG campers. So, for me, basic need, basic controller.

Thanks for the feedback. One of the things that is sort of frustrating is trying to separate the really important features from the marketing hype so as to avoid the tendancy to purchase way more "features" than one needs due to lack of experience.

[Thomas G.]

Originally Posted by RogerDat

Thanks for the feedback. One of the things that is sort of frustrating is trying to separate the really important features from the marketing hype so as to avoid the tendency to purchase way more "features" than one needs due to lack of experience.

Yea, I'd be interested to hear what I'm missing.

I know that my camera and smart phone have about a thousand features that I don't use, but somehow I muddle on.