Last night I did my first ride on my home made rocking trainer. The goal behind the rocking trainer is to let the bike tilt left and right, like it does when riding out on the road.
The two obstacles (well three?) to a rocking trainer are:
1. Does it rock realistically?
2. Does it allow you to ride it without tilting inadvertently?
3. Does it provide enough resistance?
The first is the main goal of the trainer, to be able to rock the bike realistically. What a lot of people don't realize is that when you rock your bike side to side while out of the saddle your bike actually tilts around the bottom bracket area, relative to your path. Meaning, yes, your bike tilts left-right with the tires as the axis, but when taking into account the slight wiggle you get when rocking the bike back and forth, the bike is really going in a straight line from about the bottom bracket area.
This means the tilt axis needs to be higher than the tires, closer to the bottom bracket. Your body is mostly stationary during out of saddle efforts (if your body moves then that's not the best), you're basically standing on the pedals, and the two pedals share one thing in common - they're anchored at the bottom bracket axle. This makes the bottom bracket the ideal pivot point.
If the axis is too low then you get the "my tires are stuck in a groove and I'm going to fall over" feeling", which is what it would be like if you actually pivoted around the tire/road junction.
Problem is if you have the axis too high then it gets a bit weird. Imagine if your head was the pivot point - the tires would slide back and forth a few feet as the whole bike/trainer assembly pivots around your head.
Can You Stay Upright?
The second is sort of crucial. Bikes stay upright because you can steer the bike while you move forward. Basically you catch yourself falling by steering into the fall, like if you're tilting right you steer right and now you're not falling, you're just turning right.
With a trainer there's no forward motion, meaning your bike isn't moving forward and therefore you can't really steer into a turn because you're not turning per se. Therefore it's a bit trickier to keep the bike from tilting.
The thing that helps to tilt the bike is to be able to steer the front wheel. The head tube angle and fork rake encourage the bike to lean if you turn the bar while the bike is stationary (by lowering the front of the bike). That allows the bike to tilt a bit where it wouldn't otherwise tilt.
My homemade rocking trainer.
The brown thing under the wood is a spacer to raise the left side just a touch.
Eventually I think I'll use wedges between the two pieces of wood.
Due to the forward hang of the trainer the base has to extend forward a bit. I made my base out of wood because I could, and I don't have welding skills.
Note that the folding arms and their mounts are removed from the front of the trailer (normally under where the trainer clamps the wheel). This was an all-or-nothing experiment.
I had this trainer frame after warrantying a blown up electronic trainer through CycleOps. Although I requested they send only the resistance unit (a Fluid2 since they discontinued the electronic unit at the time) they sent me a complete trainer. I suppose it's probably easier logistically to do that instead of stock a bunch of resistance units. Anyway end result was that I had an extra trainer frame. They don't wear out so I decided to use the spare to see what I could do with it.
View from above.
I've been thinking about the trainer and how to make it better. I think one thing would be to make the pivot point (where the plates are, see pictures below) below the bike, not behind it. This would support the bike better. Right now the bike dips a bit when I pedal, more when I pedal hard. It's to the point that I expect the tire to hit the floor/mats at some point.
Different view of the rocking area.
Although I asked my car friend to cut down the plates a bit, he ended up welding the plates in a different order. I wanted the top plate to be in the middle, so it could clear the other two plates while it rocked. It's okay, nothing hits so far.
(I gave my car friend a piece of plate steel in exchange for doing this work - the plate steel was probably 2'x3' and had to weigh 50-80 pounds... it was really heavy but it's stuff he could use for his car restoration hobby.)
Plates and bushings.
The plate angle was designed to allow the bike to pivot around 1/3 up from floor relative to BB.
The guy who welded everything painted it also, without me requesting the paint. Very nice.
You can see here that the plate welded to the trainer frame (middle one) is wider than the top plate. This was an error. The guy helping me out inadvertently swapped the two plates. If the narrower top plate was welded to the frame there'd be more room for it to rock.
I have to increase the height of the bushings as the trainer doesn't rock enough so I don't think I'll run into interference problems between the middle plate and the bottom one. If I do I'll have to try and cut down the middle plate. It's very tough to cut - the guy who welded it actually asked someone else with a plasma cutter to cut out the pieces for him. I don't have a plasma cutter. I may visit a machine shop nearby and ask them to cut the piece down.
A stock Kinetic Road Machine resistance unit.
The tire-roller junction is the weak point of the whole set up. I haven't done an all out sprint yet, just a few rolling jumps to make sure nothing would break, maybe 600-800w each. Nothing like 1000-1200w, and definitely no 10% jumps. However the tire slips if I do any kind of a super hard jump.
I got the KK Road Machine unit from a fellow racer. Note the spring - it's not curved around the corner. When I got the trainer the spring was curved around the corner. I didn't realize it but the various pieces of metal holding the old trainer were slightly bent. This meant I couldn't get as much clamp power to hold the roller to the tire. Now it's much better.
Note: This is a Kurt Kinetic resistance unit. The frame is a CycleOps frame that used to have a Fluid2 resistance unit on it. The frames and resistance units are interchangeable based on what I have experienced. Apparently KK has some deal with CycleOps where the frames are the same. Different mechanisms for adjusting tire tension (I prefer the CycleOps lever system although I haven't tried the two companies' newest systems) but obviously the resistance units fit one another's frames.
For the front a heavy duty lazy Susan with the front wheel holder.
This allows the front wheel to turn easily; I've turned it a bit to illustrate that.
I need to mount something to the metal lazy Susan, probably a thin piece of wood with the tire block mounted to it. The lazy Susan is an industrial strength one rated to a few hundred pounds, not something for the coffee mugs in the kitchen cabinet.
If I do that I'll need to raise the rear a similar amount, to keep the bike level. This isn't a bad thing since it would increase tire clearance to the floor.
How Does It Work?
Right now it's not working really well.
1. The rocking motion is too limited side to side. I feel like I want to move the bars another 4-6 inches to each side. That might be an exaggeration because I didn't use a tape measure or anything, but I don't think so. For sure the bike still feels too rigidly upright.
2. The bike doesn't tilt easily enough. I have to force the bike down to the side. It doesn't drop down naturally. I'm not sure what the solution is - maybe some bushings to the side of the main axis, soft ones, with the larger central bushings cut down a bit? I don't know. I'll have to think about this for a bit.
3. The rear tire gets very close to the floor.
4. The skewer has to twist in the trainer. Since the trainer frame is tilting forward on each downstroke, however slightly, the skewer twists inside the cups. Ultimately I'd like to have some kind of bushing type material or a separate cup for the skewer ends that rotates on its own.
5. Lack of peak power. The tire-roller interface is still the weak point of the whole set up because it slips at about 1100-1200w. This limits any peak power kind of efforts.
My ultimate trainer set up would be a reduction gear trainer like the one here, but that's a pipe dream for now. The challenge here is to mimic the inertia you have to overcome when accelerating without drivetrain/tire slip as well as rocking the bike like you do outside.
With the Kinetic Road Machine resistance unit you get some of that - the inertia and the exponential resistance. However there are two limitations.
First and foremost there is a definite element of drivetrain slip, aka tire slip. If I do a max effort downstroke the tire slips every time, so I've learned to tone down my initial jump by 300-400w. That's not a good habit to get into and it also prevents me from exploring max effort jumps indoors. I'm pretty sure this is part of the reason why my peak power is a bit lower nowadays - I've trained myself not to make big jumps nowadays. In 2008-2010 I was training outside a lot, I was doing many massive jumps, and I regularly hit significantly higher peak power numbers, 1400w whenever, 1550w on a great jump. Nowadays I think it's a big deal if I hit 1250w and many of my jumps are more like 1100w.
The other is the incorrect rocking motion. Without the bike tilting as much it just doesn't feel the same as it does on the road.
I can't do much on the drive train slip limitation. I can, though, work on the rocking bit, so that's my homework going forward. I'll also finish up the front tire mount as well.