In Search of a Bobsled

[Carl Russell]

George, I think that the different design features have everything to do with the seasonality of the use. I want a sled I can use any time of the year, not just on snow. I think that his design gets away with inefficiencies that I don’t want anyway, but specifically during dry-ground use. The value of the single-bunk sled is, to me, too high to limit it to a few months use.

With the bunk back and log weight forward the fulcrum of extra runner length increases the horses ability to lift the weight, AND the friction of the load is reduced by shifting the dragging weight of the logs from that portion on the ground to the narrow steel shoe under two runners. These two factors contribute first by enhancing the bio-mechanics of the horses lifting power, which in and of itself reduces friction (by lifting a portion of the runner off of the ground), but having more weight on the runners than on the dragging logs, the load is easier to pull overall. This latter detail is minimized when working on snow and ice, but in my mind efficiency is efficiency, regardless of the circumstances, and with live power we can maximize our efficiency through the physical design of the equipment we use.

Also by affording increased lift to the front of the runner there is increased maneuverability by lifting the front of the runners off of the ground when turning. With the configuration of my sled, the turning occurs on the heels of the runners more than with Dwayne’s design, which again is less important on snow than on dry ground, but easier is easier….. and it increases the functional application of the equipment (more seasons that just winter).

When I say the horses shouldn’t put lateral pressure on the pole for turning, I mean that when they step gee or haw the sled should turn by being pulled in that direction from the evener, not by being levered over with the pole through the neckyoke. The pole should “float”. As true as this is, there are no doubt instances where there is some lateral pressure on the pole, and in those instances (holding back going around a corner on a steep hill) that I want the strongest pole/roll configuration I can have. I see the A-frame as the kind of insurance that when I need that kind of leverage that I can get it without compromising the integrity of the hitch and jeopardizing the safety and well being of myself and my horses.

As far as turning, even when turning at a stand still, I expect the inside horse to step forward. The power of horses is forward, not sideways. I will have them step over with an empty sled, but there is next to no pressure on the pole/roll connection in that maneuver. When turning a load, it is extremely inefficient to ask a horse to do so in a lateral movement. Time and planning should be taken to give the horses room to move forward while turning. By moving forward at the same time that they move to the side the power is translate through the evener under the load, and the load is turned with no lateral(lever) pressure on the pole.

I think that with long runners and the bunk centered, there is no advantage either for lifting nor for turning. Having the horses closer to the load in this case may seem like they should have more lift, but there is no mathematical advantage, runner length equal front and back, which amounts to a dead lift. And when you combine the fact that you are dragging more log length, it is a lose-lose situation. Then add in the longer runners that create more lateral resistance for turning, and you’ve lost my interest.

By placing the bunk back, it allows more weight to be put on the bunk(more to lift for sure), but by creating mechanical advantage (more runner in front than behind), you make up for that, and then when drawing the load friction is reduced, which in turn increases efficiency for the real power of the work, moving the load over the land. Shorter runners/less shoe combined with the front-of-the-runner-lifting makes much better turning ability increasing overall functionality…….

My sled was originally built by a man who lived near Walden. His family raised Brabants for years, and they logged and sugared with them. I didn’t buy the sled from him, I bought it from Walt Bryan. I never knew whether it was custom-built for Walt based on his own design, or whether it was a standard design that they used for their own purposes. I cannot remember the family name for the life of me….. maybe Dwayne knows who they were/are.

Carl

[Mark Cowdrey]

“By placing the bunk back, it allows more weight to be put on the bunk(more to lift for sure), but by creating mechanical advantage (more runner in front than behind), you tend to make up for that,”
My inserted italics into Carl’s quote. I’m not sure the compensation would be complete, but it is certainly in that direction.

“but there is no mathematical advantage, runner length equal front and back, which amounts to a dead lift.”
I am picturing some mechanical advantage, just not as substantial. Picture a 6′ plank. Put a rock in the center. Lift one end. It takes a certain amount of work. Now lift just the rock (dead left). It takes more work. Now move the rock away from the end of the plank you are lifting, it takes less work to lift the plank end (& the rock.

Am I right on the above comments (Carl? Andy?)?

Carl I am with you all the way on your general reasoning; safety, versatility and economy.

Great thread. I feel like this sort of thing is one of the most valuable contributions DAPNet can make to the draft animal power community.
Mark

[Carl Russell]

@Mark Cowdrey 32529 wrote:

“By placing the bunk back, it allows more weight to be put on the bunk(more to lift for sure), but by creating mechanical advantage (more runner in front than behind), you tend to make up for that,”
My inserted italics into Carl’s quote. I’m not sure the compensation would be complete, but it is certainly in that direction.

“but there is no mathematical advantage, runner length equal front and back, which amounts to a dead lift.”
I am picturing some mechanical advantage, just not as substantial. Picture a 6′ plank. Put a rock in the center. Lift one end. It takes a certain amount of work. Now lift just the rock (dead left). It takes more work. Now move the rock away from the end of the plank you are lifting, it takes less work to lift the plank end (& the rock.

Am I right on the above comments (Carl? Andy?)?

Carl I am with you all the way on your general reasoning; safety, versatility and economy.

Great thread. I feel like this sort of thing is one of the most valuable contributions DAPNet can make to the draft animal power community.
Mark

Inadvertently working my way through that, my assessments were more comparative than comprehensive.
There is some need to have runner length behind the weight to strike the balance between the dragging the load lift the front of the sled (by tipping backward, which is the reverse of the force that allows the horses to move this load forward), and giving the horses the mechanism to use the lift in their favor. Putting the weight in the center is not as advantageous as moving it behind center.

In either case the advantage given to the horse through facilitation of its lifting power via the fulcrum in the design of a bobsled, is magnified by the reduction of sled-shoe friction. There may be some ability to lift with a centered bunk for sure, but not with the advantage of the lengthened forward runner/fulcrum. By making it easier to lift the font of the runner, you are not just lifting the load, you are reducing ground-friction (inertia) which reduces overall energy exertion. I can’t begin to approach this mathematically, nor do I feel the need personally (although I would look it over with enthusiasm if someone else did), but I can tell you from first hand experience that the design of my sled goes a long way toward bringing all of these factors to a single balance point, making it an extremely valuable piece of equipment.

Carl

[Andy Carson]

Here 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 :).

[Jay]

My sleds seem to have the bunk about 3/5 of the way back – just past half. Jay

[Gabe Ayers]

My sled has a bunk placed in a position similar to Carl’s design, and it was built from Carl’s plans. Before John and I rebuilt the sled, the bunk was centered on the runners. Having used both arrangements in succcessive winters, I can tell you that on snow and ice the new sled is much, much easier for the horses to get moving. The increased lift really does make a difference. I have not use it much on dry ground, but from what I have done I have noticed that the smaller amount of runner behind the bunk makes the sled turn more easily.
-Brad

[Carl Russell]

Looking at my sled this afternoon it dawned on me that my sled is set up just like Dwayne describes, only with a foot of the runner cut off the rear. In other words, my bunk is just as far away from the horses as in Dwayne’s model. The difference is the increased leverage under the load, and the 6″ that separates my hook position from that of Dwayne.

As far as the front of the runners coming off of the ground, it is only a miniscule amount, and it is only in those instances when the draft is at its peak, but it is that physical action of lifting forward that actually allows the weight of the load coming back down to ground to add force to propelling it forward……. Draft Buffer.

Sleep……:confused:

Carl

[Andy Carson]

Turning 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…

[mitchmaine]

Carl,
I have a set of sled irons with the skid rests in the bunk clamps, suggesting that wood was rolled up onto the sled. The wood is mostly gone but the bolts show that the bunk was centered on the bearing surface of each sled. With the rear sled a foot longer that the front sleds. Wonder if your set was the front half of a double set, cut back to use single?

[mitchmaine]

hey andy, didn’t see your reply. do you think the bunks were centered for use as a double set of sleds with cross chains? a tight turn might require the bunks centered, and lift might be let go in favor of turning? just a thought.

[Does’ Leap]

Carl, you make some convincing points. More convincing than anything is that you have logged a lot of hours behind a sled. However, there are a couple of points that I still am not getting. For arguments sake lets assume that Dwayne’s sled has just as much wood in front of the bunk as yours so that we can concentrate on bunk placement.

1. By placing the bunk back on a fixed runner you are getting greater leverage to lift the load – agreed. However, you are getting less lift with the bunk further back. If you lift one end of a 2×4 off the ground 6″ and measure which section of the 2×4 is higher – the middle or 2/3 back, the middle is higher. Placing the bunk in the middle give you greater lift but less mechanical advantage compared with it further back.
2. With the bunk back I agree that is should be easier to turn the sled, but at a cost. More weight on the back of the runners means poorer tracking with more likelihood of slipping on a side hill. It should also adversely effect the ability of the sled to float on snow. Ask any skier, leaning back means a loss of control.
3. By centering the load on the runner you are distributing that load across the whole runner. It is the runner that facilitates movement of the load through less friction. As you move the bunk back you are disproportionately loading the back of the runners and centering the weight over less of the runner. This would create tilling in certain conditions as Andy mentioned. Irregardless of the conditions, you are still creating more friction this way compared to centering the weight over the entire runners.

Ok, have at it:)…. Tim H, you have done a lot of work in this area. Any thoughts?

George

[Carl Russell]

So as we have discussed, the runner doesn’t actually spend much time in the air, and therefore the plowing aspect is really a minor factor. I can assure you we are talking about taking a huge amount of friction off of the ground and onto the narrow shoes, which far outstrips the small amount of friction that may be added momentarily by the tillage at the rear of the runner.

Also in the lifting, the height of lift is not as important as the mechanical advantage given to the lift, which affects the limited available power from the horses to actually move the load forward.

The tracking issue is possibly important, but I just locate trails with that in mind, and place slue logs to hold the sled in track, etc. As far as floating on snow, that just doesn’t happen. The first load or two, that may play important, but if there is enough snow to float that sled with a few thousand pounds on it, then it will be so deep that it will pile up under the load and compound the drag. After that the snow will be packed down, and the floating issue is moot.

The component of additional friction, or distribution of ground pressure, is not nearly as significant as the overall reduction in friction created by putting the load on top of the sled, especially as compared to the functional advantage given to turning. The other advantage that the longer front section of the runner gives is in turning. The 2:1 approximate ratio provides mechanical advantage in overcoming the lateral resistance of the runner. Centered-bunk sleds would have 1:1 ratio which doesn’t prevent turning, but doesn’t facilitate it like 2:1.

Putting the bunk too far back will cause the sled to pitch backward which will lift the point of draft, which will decrease the horse’s lifting capacity. There is significant advantage in any sled set-up by providing a low fixed point of draft in relation to a mass that is held off of the ground at a point higher than that hitch point. By having a bearing surface located on top of a runner, the mass is redirected onto the plane where the power is attached. Of course mathematically, the load is more evenly distributed in a centered model, but that even distribution contributes to other factors of resistance that affect functional application.

Based on a long period of experimental application I can say that I see where the design of the sled I use successfully addresses all of these factors to make an extremely effective piece of equipment. Having no experience with another style, my opinion is anecdotal specific to that design, but based on my own mathematical/physical knowledge, I would still prefer the design of the sled I use.

Carl

[Andy Carson]

I 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.

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&ved=0CDcQFjAA&url=http%3A%2F%2Fwww.tillersinternational.org%2Ffarming%2Fresources_techguides%2FEstimatingSledandStoneboatDraftTechGuide.pdf&ei=p7M7T5qQHvSk2gWB7aXzBg&usg=AFQjCNH8RXpSARJY6owvencT05WZ5wGKhw&sig2=tEgV_MIV1Oot254qoo1PYg

[Tim Harrigan]

I agree that turning is probably the key advantage of that bunk placement and also with the lift and leverage observations. There has been some discussion of even load placement over the runners and I am not sure if I agree with that. If you think about what happens at the leading edge of the runner it is important in pre-conditioning the track. If the snow or soil is soft the curved front of the runner begins to firm it up so that the runner that follows actually has very different conditions, much better conditions actually, firm and smooth with less motion resistance. Think of pulling a wagon across tilled ground. It is tough, and on the return trip you are probably going to ride in the tracks that you already made. Now think of the micro-topography of the trail under the runners. If you have bumps, surface roots, loose soil, snow etc you want to compress those if possible, and if they are not compressible you want to ride, actually ramp yourself, over the top. You do not want to be nose-heavy. Think of a large diameter wheel compared to a small diameter wheel rolling over a rock or out of a rut. A nose heavy runner is like a small wheel or a short ramp/lever. A light nose is more like a larger wheel or ramp/lever.

If you pre-condition the tracks with a lighter leading edge you minimize soil or snow disturbance. It is a differential process that occurs from the front end of the runners to the back, so in most conditions trail under the back half of the runners is in the best condition to carry the load with the least runner/ground resistance because it is firm and smooth. It might look like there is some tillage going on, but that probably happened at the front. If you are nose-heavy it will be worse.

George, you mentioned skiing and that made me think of one of the top skiers from probably the 68 Olympics, Jean-Claude Killey. Pretty sure he was a gold medal winner. I remember him because he had an odd style. As he approached the finish line he would sit way back on his skis and concentrate his weight on the back half of the skis with the fronts carrying almost none of the load. Everyone wondered what the hell he was doing. Well, he was pre-conditioning his track with the bowed fronts and concentrating the load on the back half of his skis where there was the best track and the least motion resistance. So he was in a controlled, out-of-control run for just a second or two, but maybe just enough to gain those few hundredths of a second that he needed.

So this bunk placement seems a nice compromise between maneuverability and carrying the load on an optimal track. Farmstead engineering rocks!

[Andy Carson]

I 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.

[Author]

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