Scratch Build Questions

[nnnnnate]

I'm just about to get started on a scratch built trailer and had a few questions that I couldn't find answers to easily.

I'm trying to determine the axle width I need and am curious: how much clearance do I need from the tire to the tub of the trailer. I'm planning to run a 3500# axle with matching weight leaf springs from a local supplier.

The other question I have is about tracking. I understand most people build their trailers so that the tires will fill the same path on a trail. How important is it to have that tracking be exactly the same? I guess I'm asking how big of a deal is it to be off by an inch or two or even a couple inches each side? What issues could or would arise from this? I don't want my tires to rub but I also don't want them to stick out super far either.

I guess I'll explain a little about the conundrum I'm having. I want the tub interior dimensions to be 48"x72". I'm using 2.5" square tube for the frame so add 5" total to the exterior width and I'm at 53" tub width. My JK axle width is 64.5" (I'm running 1.25" spacers) so if I'm trying to mimic that I'd have 5.75" sticking out from the side of the tub on each side. My stock JK rims have backspacing of 6.25 so I don't have enough clearance. If having the trailer track the same is imperative I'll shave down the tub to get things to fit but if it isn't I'll consider widening out the axle.

Thanks for the help and direction.

 

[jwiereng]

If trailer is the same track width as the tow vehicle it will cut the corner so to speak unless the distance from rear axle of TV to coupler is the same as the distance as trailer coupler to axle.

 

[croix]

I've seen a few debates over track width and my personal take is that it comes down to intended use. If the most your trailer will ever see is a lot of maintained roads and some forest service roads then matching track width probably isn't too important. If you are planning very technical trails with narrow tracks through rocks and trees then you probably don't want a wider track on your trailer than your JK. There have also been discussions regarding towing in snow and/or mud. If trailer track is more or less than tow vehicle track then you will encounter additional resistance as the trailer has to break new ground. This can be overcome by some of the larger tow vehicles, but the JK isn't know for being a power house.

I'm certain that there are some effects of track width not matching TV width on handling, but in MY OPINION they are fairly insignificant; especially in the case of a wider trailer than TV. I think that suspension setup and distance from trailer axle to tow vehicle have a much greater impact on handling.

Regarding "how much clearance do I need from tire to the tub of the trailer" - that depends on use also. To know for sure you could do a bunch of crazy math involving triangles and suspension travel. You'll probably find though that you don't need as much as you think. Most trailer suspensions don't have a great deal of vertical travel so there isn't much horizontal shifting with a solid axle. Of course with a split suspension you wouldn't have to worry about that at all. My trailer has just over 1" (like 1.125") of space between the tire and the tub at static height and with 4" of upward and 3.75" of downward travel I've never had a rubbing issue. I would like to remind you though that tires stick out beyond the rim - so your 6.25" of backspacing will look more like 7-8" depending on tire bulge when the rubber is installed.

I fretted over this part of my build for a while too. I didn't want to drop several hundred dollars on a custom length axle to get it wrong. I did the crazy math over and over with a bunch of assumptions regarding tire sizes etc. and was ultimately able to get my track width within 1/8" of my JK with 48" exterior tub width. I assume you would have to go 5" wider with your plan. I would probably either build the frame and measure from there before ordering the axle, or make my best guess on the axle and not start the frame until I had the axle in hand. It's a critical decision - good luck.

-croix

 

[Umtaneum]

I will echo what Croix says above, with one addition: on deeply rutted two-tracks, or in deep snow, it's nice but not critical to have the track at least CLOSE to the tow vehicle's. Narrower is better than wider in these sorts of conditions. When your truck is down in the ruts, so is the trailer more or less. When you climb up out, the trailer will usually follow. With a large discrepancy between tracks, the trailer will constantly be up when you are down, or vice versa. My experience with dog trailers when bird hunting has been that slightly narrower trailers will follow behind very calmly, wider ones jump all over the place.

 

[rustED]

This is some good info! I'm already 3/4 of the way through my scratch build, ive been working on it off and on for almost 4yrs. Due some changes made along the way, I ended up having to add wheel spacers and now my track width wms to wms is 65" compared to my TV of 58" and I'm trying to decide whether to go back and make changes to my frame and nearly completed tub so I can narrow up my track width, or just finish it as is and see how it performs. Sorry OP for the thread jack, I wish I'd found this forum and ask these questions before I started to build!

 

[nnnnnate]

Great stuff. Thanks for the feedback.

I stopped by the local trailer axle place on my way home from work and got a bit more info on what they need to build me an axle. I'm still figuring out some numbers but I'm leaning towards shrinking the box dimensions to get the track closer to stock JK WMS. I'm running 1.25" spacers on my JK and it appears that I'll need to run 1.5" spacers on the trailer axle to make the axle hub centric.

Croix- I just looked at your build thread and your frame is basically the same design that I'm wanting to do. I had initially planned to build the box them figure out the axle but I want to extend the rear brace out both sides to start the boxing for the side steps and tie in for the fender. I feel like I need to know my dimensions before I make that cut so its long enough. It looks like per your diagram your side extend out 14.5" and that you needed to add a bit more to cover your tire. Did you do that intentionally or do you wish you would have the metal extend out more to give full coverage?

My crude drawing. On the left picture the X'ed area is the box, the front space is open tongue area.

 

[GeoTracker90]

Very good discussion. I have a utility trailer that I built using a straight axle and I left just a touch over 1" clearance between the tires and the sidewall of the box. After many years of use (and abuse) it now has witness marks on the bed where the side walls of the tires have rubbed. Not a big deal really. If I was concerned about it I would just add some wheel spacers and be done with it.
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For my current off-road trailer build I had a few criteria that I started with.
1) Axle width needed to be the same or narrower than the tow vehicle.
2) The off-road trailer will use the same wheels and tire size as the tow vehicle.
3) Maximize the trailer main box (and frame) width.
4) Keep the tire to main box clearance greater than 1.75".
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With these goals in mind I put together a spreadsheet to help me evaluate what my choices were. As part of this I acquired a stock S-10 15x6 steel wheel and mounted a 31" tire so that I could take direct measurements. Here's the spreadsheet that I came up with:
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After all of this I settled on the 42" frame/box width and the 60" axle width. Giving me 2.25" of clearance between the box and the tires in theory. I need to get the trailing arms built and the brackets machined so that I can verify all of this still.
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Now regarding the trailer width vs. the tow vehicle widthI would just echo some of what has been stated above. A trailer axle that is the same width or wider than the tow vehicle axle width will take a path inside (smaller radius) if the tracks laid down by the tow vehicle. A trailer axle that is narrower than the tow vehicle axle will more closely follow the tow vehicle axle during a turn. Now how much this impact your decisions depends on a lot of the criteria that others have mentioned. For me, I decided that I would keep the width the same to keep the trailer tracking just right behind the tow vehicle. This will become of great importance to me if I end up in bad road conditions with lots of heavy slush. I would rather have the trailer cut corners a little rather than have to fight it with slush and bad roads. Just my preference.

Mike.

 

[nnnnnate]

Thanks for chiming in GeoTracker, I looked through your current build as well as the older one a few days ago and am super impressed with the great work you've done.

 

[Box Rocket]

Nate, who is building your axle? I'm really surprised that they are saying that you MUST run a wheel spacer. Is it because of the wheels you are using on the trailer?

 

[croix]

It looks like per your diagram your side extend out 14.5" and that you needed to add a bit more to cover your tire. Did you do that intentionally or do you wish you would have the metal extend out more to give full coverage?

Welllll... I wouldn't say that it was intentional I did a lot of math with backspacing and tire bulge etc. to make sure the inside of the tire wouldn't hit the tub, but didn't really think about the outside of the rim/tire. I also did consider what would happen if I went to a bigger/wider tire in the future and ultimately decided that 35" tires are as big as I will go (although a skinny 37 would probably fit... hmmmm..)
Edit - keep in mind that my tub is wider than the rails of my frame. I actually only have about 13.5" steps vs. 14.5"
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To answer the second part of your question, I don't wish I had extended the metal. The width of my frame is almost identical to the width of my jeep body. I have spacers on my jeep rims and on my trailer rims. This allowed me to set WMS-WMS the same and if I change to aftermarket rims later with proper backspacing I "should" be able to remove spacers all the way around. BTW - I'm not looking forward to buying 7 rims - lol. So the spacers on the jeep or rims with less backspacing cause the tires on the jeep to stick out a bit. Having the tires on the trailer stick out a bit and adding the rubber flares kinda makes it blend more with the jeep. It's aesthetic more than anything.
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One more piece of advice as you design your frame - keep future improvements in mind. I spaced my cross beams very specifically to fit 10 gallon water tanks later on. I found the tanks I want and used those dimensions to determine cross bar location. I wish I had considered a few more things (like propex heater) because I would have liked to mount that underneath as well, but now I don't think it will work out. My side steps were sized with gas cans and propane bottles in mind. My tongue was designed for a box, but I didn't account for the lid hitting the tub so now I wish my tongue was about 6" longer. All part of the fun right???

-croix

 

[dstock]

Nate, who is building your axle? I'm really surprised that they are saying that you MUST run a wheel spacer. Is it because of the wheels you are using on the trailer?

If he is running hubcentric JK type wheels, the opening in the wheel will not fit over the hub of the axle. In my research, 1.75 inch spacer can just barely make it with some mods to the dust cap. I went with 2" to avoid the issue.

 

[nnnnnate]

Here is another thread I found that talks about the JK rims on trailer hub problem. Its the rims fault, not the axle maker. I'm going to have it done locally at a place called Henderson.

I have been thinking about what future mods or add ons I'll want and I guess I'm torn. This isn't an expo trailer exclusively and will be used for home projects more than camping trips. I'd like to add some water storage but I have Scepter cans that work fine for that. I'm going to mount can holders on the tongue and get a pump (either manual or otherwise) fitted on a lid that I can exchange between water cans but I don't see myself ever doing hot showers via my trailer. Something I have spitballed in the past is building a width wide removable rack for the rear for firewood hauling. I figure I'd add three hitch receivers on the bumper and build it off that. I know this could throw off the weight distribution but its something I'm going to look into a bit further.

This is a good point though, thinking ahead and trying to see how or what might change in the future.

Its a balmy 66* today in SLC today, heck of a winter we're having, so I'm planning on cracking out some progress this afternoon and tomorrow.

 

[nnnnnate]

I'll just add that my inspiration for this trailer comes from what Adam (Box Rocket) built. I've poured over his build thread many times and have asked more questions about it than I ought to and he keeps answering them. Adam is a pretty awesome dude.

 

[nnnnnate]

I decided to keep overall width as well as wheel path in line with the JK I needed to shrink it a bit. I had wanted to have an interior dimension of 48" wide but that seemed to screw everything else up. Going with an OD of 48" and ID of 43" I can get the numbers to work.

I made a stop at my local axle place and got some good info. They can't do anything about shrinking the center of the hub which I expected. The guy said it will only take 4 or 5 days to build after I get back to them with measurements which I thought would take longer. Springs are included with all these prices.

No Brakes $187.45
Brakes $288.66
Brakes with Parking Brake $325.66


That pretty much settles it for me going no brakes on this trailer. If it was close (i.e. $50 or so) I would probably add them but don't think I will for an extra $100.

Enough of the numbers crap. I got some metal cut this afternoon and did a quick mock up on the ground.

Does anything think that there will be a strength issue by having the front angle then flatten out like I it sketched above? Would it be stronger to have the angle pieces but directly to the receiver tube? I like the esthetics better with the front platform flattening out but if there could be strength issues I'll do it the other way.

 

[GeoTracker90]

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One-Swell-Trailer