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Andy Carson
ModeratorI see what you are saying, Tim, and I agree that if a certain degree of sinkage is inevitable, it is best to do this slowly and this would be best with a light nose. I am sure some degree of sinkage is always inevitable, but the amount of sinkage does not have to be constant. Lets say we are talking about a 4 inch wide runner that is 6 feet long, that’s 2 square feet of surface area, if it’s holding a ton, that’s only about 7 psi overall. 7 psi will result in some degree of sinkage, for sure, but more pressure at some parts of the runner will cause more sinkage. Rearward placement of the bunk and lifting up on front of the sled causes the increased pressure at the rear rear edge of the sled, even if the nose does not come off the ground. How much more pressure is calculable, but would vary by upward force on the front of the sled, bunk placement, soil conditons, etc. This would be the result of 1) increased pressure througout the rear skid due to rear bunk placement and 2) the equal and opposite force exerted by the rear edge alone that counteracts the backward rotational force (around the axis of the bunk) from lift at the front of the sled. So, I think that by unweighting the nose we make the angle of approach easier, but will also increase the total amount of sinkage. Is it better to have more sinkage and preconditioning, or less sinkage and no preconditioning? This probably depends on soil/ice/snow conditions and probably is a compromise between these two extremes. In my mind, this still favors a bunk placement towards the rear, but I still like the wider rear edge as this distributes the additional force over a wider area and minimizes high pressure areas that might increase total sinkage.
ModeratorI think your logic is sound and I agree with almost all of this, Carl. I also think that the bunk would be best placed to the rear of center. The only thing I am still not sure about is the statement that because the front of the sled is not lifted up very often or very far that the tillage aspect is not important or teh effect on drag is minimal. On hard packed ground or ice, this is probably true, but on soft ground I bet it’s not. At the bottom of this post I have attached a link to some work from Tim and Tillers comparing the draft of a sled vs a stoneboat over different surfaces. On hard surfaces, the draft was nearly equal between the two setups, but on soft surfaces the draft was substantially higher on the sled versus the flat bottom stoneboat. Not just a little either, the draft went up by 25% on firm soil and 31% on tilled and settled soil. To me, this demonstates that sinkage (and possibly tillage) is an important consideration on dirt. In a perfect world, perhaps the load would be evenly distributed over the runners to maximise floatation and minimize sinkage and tillage AND allow for maximal loading of log weight onto the sled and provide maximal leverage to lift the load in the front. These goals are not mutually exclusive. If one simply made the rear portion of the runner wider (maybe 50% wider) than the front portion, one could place the bunks 2/3 or the way back and still have maximum floatation (equal weight per area front and back), minimal sinkage/tilling, provide a high degree of leverage, and allow for more log loading. I am thinking something like the rear of a parabolic ski. Of course, I would still rocker the end of the runner (again like a parabolic ski) because I have watched my skids wear and know how much drag comes from that far rear edge when it’s square.
ModeratorTurning this over in my head, I can’t really understand the advantage of placing the bunk so close to the center in these designs. For the reasons Carl mentioned (placing the load on the sled instead of dragging and providing lift) I would predict that one would place the bunk very close to the rear of the runners. Perhaps tradition brings them close to the center, but I think there is probably a better more scientific reason. In other words, there have to be something “bad” that happens when you put the bunk too far back. I think this is probably excessive tillage or compression at the rear edge. It wouldn’t take much to add alot of drag. I think you a right about the lift storing power, Carl, as long as there is not tillage or compression at the rear edge. This is probably power you don’t get back. Probably not a big concern on ice, but on soft dirt I think this is probably important. So, I think the optimal bunk position would be as far back as possible so long as tillage is still prevented. To simply prevent front end edge lifting during stead-state pulling on the flat (I modelled this with 2/3 of the weight on the sled and a friction coffeicient of 0.4 for the sled and 0.6 for the dragging log end) the best bunk position would be around 3/4 back from the front. Pulling up a hill increases these draft forces, and might make the optimal position for the bunk closer to 2/3 back from the front of the runners. I my mind, the optimal bunk position is probably represents a compromise between differenr “real world” conditions. This analysis of bunk position assumes no partial compression or tillage, by the way, which I am still not sure is a good assumption, but you have to make some assumptions or you get nowhere…
ModeratorHere are my general thoughts… This is not a difficult thing to model if we assume that friction is going to be relatively constant in a sled reguardless of how the weight is distributed. I am not sure I this this is a good assumption in all cases. In an extreme situation, one could envision the front of the sled being picked up off the ground, which redestributes the rear weight onto the extreme rear edge of the runners. If they are square, they would dig into the ground substantially and increase drag quite a bit. In the real world, I doubt the front of the sled gets picked up off the ground very often (or does it?), but the inequal weight distribution would still result in more sinkage (less floation) in the rear and possibly tillage which would be important on soft ground. On ice, or some other very hard surface, I bet this wouldn’t matter much though. So I am going to split my thought into 1) Ice 2) dirt.
Ice: The friction coefficient is incredibly low between ice and steel, approaching that of pneumatic wheels on pavement. This is so low that if the load is challenging on the flat, it is truly an immense load. With loads this immense, it is impractical to expect that the horses are going to carry enough of the load (by the verticle vector of thier pull) to make any real difference int eh drag. This means that on ice the placement of the load front to back probably doesn’t matter other than to ensure tracking and to enable turning. I would guess the best would be long straight flat runners would be the most efficient here.
Dirt: On dirt, the friction coefficient is high enough that carrying some load from the verticle vector of the pull makes a difference. How much of a difference it makes depends on sled design, soil type, and moisture, etc. In a nutshell, different soils differ in thier ability to be tilled or compressed and also differ in how much drag is generated by this tillage or compaction. Adding some rocker to the sled runner design or a “boat tail” to the rear would minimize increased compaction or tillage that might occure at the rear of the runner. If I was making a sled runner for dirt, I would definately do this. Also, making wide runners would also minimize tillage. Once the possibility of tillage or excessive compaction is eliminated, I feel more comfortable assuming that friction is constant reguardless of weight distribution, and the optimal attachment point could be calculated. I need a little more info, though. 1) What is the weight of load that this should be designed for? (a big load, but not “one of the biggest”). 2) What is the weight of the bobsled? 3) How long are the log(s) and how are they placed on the bobsled (IE how much hangs over the bunk on average)?
I suppose if I was making a sled that would be used on ice and dirt, I would lean towards a design that works well on dirt. The friction on ice is just so low that dealing with hills is going to dominate loading, and no bobsled designs are going to power logs up a hill for you :).
Moderatorthanks for the good wishes all, I will certainly keep everyone updated
ModeratorThey are a little over 5 feet tall at the shoulder. Not as big as some of the oxen I have seen, but big enough for what I want to do (I think). They are from south central michigan, between Lansing and Jackson. The guy I bought them from said they have some growing left to do. This is interesting to me as they certainly look young to me, and I would have never guessed they were almost 5 years old. I guess shorthorns grow slowly. The guy I bought them from had said he had had them at Tillers, and spent time down there with other teams as well. I suspect that Tillers exerts a local influence in encouraging young people to try oxen. They were very sad to see them go and very happy to find someone who wanted to work them rather than eat them. They said the whole time they had advertized them (at least 2 months) they only got one other call about them, but the person never showed up. It is sad that there doesn’t seem to be a strong market for an older trainer team, even though I benefit from this.
Moderator@Tim Harrigan 32170 wrote:
What do you have with that much tongue weight?
Good question. When I really think about the length of the pole, not much. I was going to adapt a two wheeled trailer and that might be the only thing. Just have to make sure this is generally balanced, I suppose, same as with horses.
Moderatorsorry for your loss, Sean.
ModeratorCongrats! I hope you had fun and remember to keep us all updated. It’s fun to hear of others progress over time. 🙂
ModeratorThe loads are impressive. I looked up the friction coefficient of steel on ice and got 0.03 static and 0.015 dynamic (from http://www.tribology-abc.com/abc/cof.htm). This is in the range of car tires on pavement! On the flat, this would mean it would take 60 lbs draft per ton to start a load and only 30 lbs per ton to keep it going. Horses exerting 600 lbs force could move 20 tons. Mild slopes would have a huge effect at these weights and the roadway would have to be very well planned to be nearly level or downhill (with hay or snublines) throughout. I think this is an intellectually accomplishment for the people who figured this system out, hard work too, but there was alot of thought, planning, and technology as well.
ModeratorGood idea on using the Gothic arch! It’s substantially stronger than the circular arch, and now that i’ve seen I am wondering why so many people use the semi-circle, other than that is easy to make. Supposedly, the inverted catenary arch is the strongest free standing arch, but they might be hard to bend. Again, good idea to think about the basic shape of the hoop. More food for thought in the link below.
ModeratorInteresting… looking at these, esp the ones with the long bolts, convinces me that the Woodstock photo is a d-ring too. I agree with your interpretation of this being a rural invention. It is very interesting that the horse community in general recognized the need for an invention of this type (as evidenced by the similar patents). Despite this need, the many people did not adopt this design at all for maybe 50 years. Even now the adoption is pretty regional. It’s amazing to be that such a potentially important innovation could have been kept secret (or nearly secret) through the golden age of horse usage… I might have thought people might not be open to new hitching ideas, but the photos and harness catalog dispute this idea. I keep trying to think of some sort of disadvantage that might have slowed it’s spread, but haven’t been able to come up with anything other than that it might be harder on the leather and that it might have been outcompeted by mass produced harness in many areas of the country. I am not sure if this is enough…
ModeratorCarl,
Good idea to post the D-ring examples. I agree that the West Failee (1907) example had this, but I can’t be sure about the Woodstock (1890) photo. I went to the vt historical site and zoomed in. The front of the ring is definately round, and the attachment to the rear tugs is strait, but my view of the britchen attachment is blocked by the rear tug. It looks like it’s not type 2 or 3 (from your photos). The steep angle that the britchen strap assumes, and the fact that it is twisted, makes me think it’s not a type 1 arrangement either. With a type 1 ring, everything is sewn in solid and who would sew in a half twist? I am not sure what to make of the woodstock photo. Also, it looks like a bolt might run through the rear tug attachment. Hard to interpret…ModeratorThe photos are very interesting. From the force analysis I did earlier, I think that a d-ring in the modern arrangement (as in the west failee photo) would experience more than twice the stress of a ring that is arranged like the square d. Supporting the pole with straps from the jockey yoke to the collar cuts the stress in half, and you certainly see alot of this (as in woodstock, Berlin, and Barnet photos). The other modification that eliminates the extra stress would be to eliminate the saddle, which is also seen in all these photos but the west failee photo.
Take a look at the photo of your (Carl’s) horses in link below. You can tell that it is critical that the rear tugs and the britchen attachment have to lay one on top of the other when attached to the “d”. As the two straps are arranged on on top of the other, I think this requires a more complex shape than a simple d. Perhaps a simple d would be sufficient for a wagon, as in the Woodstock photo, but if the horses really had to exert (like with the Berlin photo) i think it would not be a workable arrangement.
http://www.draftanimalpower.com/showthread.php?3905-D-Ring-Harness-Parts
ModeratorJulianna,
You don’t have to use a leverage bit to drive. Personally, I have never felt the need for additional leverage. I am sure some horses or situation need them, but I don’t think this is a “one size fit’s all” situation. Are your horses responsive to the bit when you ride? What about when you ground drive? It is was me, if they ground drive well in a snaffle, I would keep them in the snaffle. If I felt like I really had to exert alot of force, I try a harsher bit and see how they go. That said, the horse would have to convince me that it needed it and I would likley try to move back to a snaffle later if I could. You will certainly hear different opinions from others, but many people drive in snaffles. I am pretty young and male and can pull pretty hard on the reins if I have to (even though I try to never do this), also I only drive a single horse, and I don’t take my horse in public. All of these things make using a snaffle an easier option, but it s a real option. I think there is an immediacy and intimacy that I feel with a snaffle, that I personally don’t feel with leverage bits. That said, I am not used to a leverage bit enough to give it a fair chance… The choice is ultimately yours, but a snaffle can be a valid choice.PS. Your “hands” transfer well, so trust you instincts with them and be confident. I also rode before I drove, but I have no idea how “good” or a rider I was… 🙂 The main thing I think about with a beginner is safety, so just make sure your think about what could happen if there is a spook or a runaway, and spend some time ground driving. I bet you’ll learn alot pretty fast.
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