# Bike Trainer Axle and Flywheel Build (Lots of Pics!)



## macardoso (Jan 15, 2021)

My wife used to go to the local Y to do some spin classes before COVID hit and has really been missing doing that. We road bike a lot in the summer, however once it got cold we haven't gotten out. I shopped around for spin bikes and just couldn't believe how expensive they are. Turns out my parents had an older model rear wheel trainer that they haven't used in probably a decade, so they gave it to us. While it is pretty nice, there are some issues.

The trainer was designed for bikes with a bolted rear axle and my wife's bike has a quick disconnect axle leaving nothing for the trainer to grab onto on the bike. I contemplated this issue for a while and then headed down into the shop to make a new axle. I didn't get a ton of pictures of this process, but basically I started with a bar of 12L14 which was centerless ground to 5/8". I had to reduce it down to .1875 over a 7.5" length.

I did the Joe Pieczynski method (I'm sure he wasn't the first) of turning down to the final diameter in one pass and incrementally pulling out more stock from the chuck. This part didn't need to be precision, but it worked very well. A super sharp VCGT insert was used.




I preserved the center on one end so I could get some tail stock support as I worked. The finish isn't perfect, but still not bad. Dimensionally it varies by less than 10 thou over the length, although I could do better on a second part.




The part was cut to length and a 10-24 thread put on each end with a die. Additionally I made two tapered nuts from the same barstock to fit the conical inner feature of the bike trainer. I hand filed the flats and did no measurement until I was done. One nut was .503" over the flats and the other was .508". Pretty close!




She has been using the bike like this for a while now and really likes it, but I don't think it has the "feel" of real road biking. There isn't the momentum of your body moving forward, so the moment you ease up on the pedals, the wheel slows down. I am hoping that this momentum can be simulated with a heavier flywheel. The one on the bike is a 4.75" diameter, roughly 1/2" thick disc of die cast zinc which mounts to the friction brake on a 10mm shaft. It probably weighs less than 2 lbs.




I went on ebay and bought a 5.38" x 2" piece of 4140 for about $30. It came labeled as 4140 HT but it doesn't feel that hard.




I started to go to work on this with a little CCMT insert tool but it became quickly clear that it wasn't going to cut it. I have an ISCAR CNMG holder with a 3/4" shank which I milled down to 1/2" so it would fit my lathe. Good god can that thing do work. It would take any cut I could throw at it limited only by my lathe's HP. I was taking cuts 0.060 off the diameter with a 0.020"/rev feedrate at 200 rpm. This calculates to something like 2.5HP on my 1.5HP lathe. It was definitely slowing down a bit in the cut but really cut beautifully.

I have sworn to people that a 12x36 was probably too small to use negative rake tools, but now I'm a believer. Interestingly, the finish on the part was garbage until I started cutting harder - then the tool would leave a scalloped but mirror finish.




Here is the first side roughed in. The tool I was using could probably take 40HP on a real lathe, so I was having a hard time pushing it enough to break a chip. It would chip break OK on the OD when I was maxing out the lathe, but not on the face. My shop is FULL of deep blue noodles.




From here I drilled the center to 0.375 and reamed to 10H6 using a super cheap metric reamer I got in a set from 1mm to 13mm for something like $12. They leave a garbage finish, but it was the best was I knew to get a decent 10mm hole in this thing.

Here is where the trouble started. I flipped the part around to finish the back and used jaw pads to not mar the part. The jaw pads are brass sheet with a dense fiber backing. Even with the chuck crushing down on these, the part wanted to chatter like crazy when roughing. I later switched to solid brass pads which worked a lot better. I couldn't believe how much of a difference that little work holding change made.

When the part chattered, it must have moved a bit in the jaws so my attempt at finishing the OD did not line up well. Additionally when I smacked it back in the chuck, I couldn't get it to run true. I learned a few things:

My drilled and reamed hole was a bit crooked and not square to the finished face and OD
My chuck jaws are not ground square when flipped for OD part holding like they are. They are off by 2.5 thou!




Here is an interesting shot where you can see the effects of surface speed on a carbide tool. near the OD, the surface speed is relatively high and produces a good quality finish. Once the cutter started getting near the part center, the metal began tearing out a bit, giving a much worse finish.




I sat pondering how to correct the alignment errors in my part and decided move the part off of the back of the chuck so it was "floating" in the jaws. I know I need this to run true to the bore, so I decided that every other machined surface was wrong and only the bore could be correct (and the concentricity of the bore to the OD on the first side). I spent 20 minutes going back and forth between indicating the bore and the OD near the first face. When the OD was wrong I corrected by adjusting the chuck jaws, when the bore was wrong I whacked the face with a dead blow to tip the part in the jaws. Each adjustment would affect the other measurement. I kept switching back and forth until I got both reading within 3 tenths. I then machined the OD and face to match this orientation.

This time I opted to use a CCGT aluminum finishing insert to cut the finish passes and man did it leave a nice finish!




I need to work on this again tonight. I will flip the part back to side 1, indicate it true and finish the OD and face to be true to the bore. Thankfully I don't have any dimensions to hit on this thing and all that matters is the 10mm bore and the rest running very true.

I also need to drill and tap a hole for a set screw to hold it to the shaft. I originally planned to drill this on a steep angle from the face, however I'm questioning that logic now and might drill in from the OD. Should probably do 2 holes 180 degrees apart to maintain balance. Any thoughts?


----------



## macardoso (Jan 15, 2021)

I forgot the real kicker of this story. I'm working from home right now and look out the front window. The 4140 bar for the flywheel weighed roughly 15lbs. The eBay seller shipped it wrapped once in cardboard and then stuffed in a USPS padded envelope. 

So anyways, I'm working and watch the USPS carrier walk up to my house holding this tiny package like a shotput. She stopped short of the sidewalk going to my house and chucked it a good 4-5' onto my porch. Sounded like a gunshot. I think it even surprised her because she jogged back to the truck real quick and drove off before I could get to the door to say something. 

Thankfully I'm in a rental because it put a good dent in the wood porch


----------



## mattthemuppet2 (Jan 15, 2021)

that's a fun project! 

In my limited experience, the best way to get something like this true to the bore is to make an arbor (and some means of securing the work to the arbor), stick the work on the arbor and then mount the arbor between centers. You'd only want to do light finishing cuts, but it will let you true up both faces and the ID and have all of them true to the bore.


----------



## macardoso (Jan 15, 2021)

mattthemuppet2 said:


> that's a fun project!
> 
> In my limited experience, the best way to get something like this true to the bore is to make an arbor (and some means of securing the work to the arbor), stick the work on the arbor and then mount the arbor between centers. You'd only want to do light finishing cuts, but it will let you true up both faces and the ID and have all of them true to the bore.



I think that would be the best solution here. I don't currently own anything that would make for a good arbor or a lathe dog to drive it. Certainly could make them, but I was able to get it pretty darn true with some elbow grease and a mallet.


----------



## matthewsx (Jan 15, 2021)

Nice work....

But you need to get in touch with @erikmannie about getting a couple of fat bikes. Shouldn't let a little snow get in the way of your biking fun  


John


----------



## macardoso (Jan 15, 2021)

matthewsx said:


> Nice work....
> 
> But you need to get in touch with @erikmannie about getting a couple of fat bikes. Shouldn't let a little snow get in the way of your biking fun
> 
> ...


I've never touched a fat bike! My fair weather bike is a tri bike with 20mm slicks. It rides great, but I have fallen a few times when it is wet or icy out so I am more cautious than I used to be.


----------



## matthewsx (Jan 15, 2021)

I haven't ridden one either in winter but there are a bunch of folks up around Traverse City MI that ride all year. I do understand about being more careful but if you ride trails with good equipment you'll probably be fine.

John


----------



## macardoso (Jan 15, 2021)

matthewsx said:


> I haven't ridden one either in winter but there are a bunch of folks up around Traverse City MI that ride all year. I do understand about being more careful but if you ride trails with good equipment you'll probably be fine.
> 
> John


We are going to be moving to the Detroit area this year so that might be a new hobby I need to take up!


----------



## mattthemuppet2 (Jan 15, 2021)

studded tires would make you an ice speed demon and probably the most hardcore tri bike rider in the mid-west 

I used to winter commute on studded tires (central PA and SE WA) which was pretty wild. Several times I had enough traction to ride and stop, but not enough to put my foot down so had to time my stop signs and traffic lights to avoid stopping.


----------



## macardoso (Jan 15, 2021)

mattthemuppet2 said:


> studded tires would make you an ice speed demon and probably the most hardcore tri bike rider in the mid-west
> 
> I used to winter commute on studded tires (central PA and SE WA) which was pretty wild. Several times I had enough traction to ride and stop, but not enough to put my foot down so had to time my stop signs and traffic lights to avoid stopping.


You are a mad man. I tried summer bike commuting to work a few years ago in Cleveland, but there aren't very many bike aware drivers here. Almost got hit 3 times in a week and decided to go back to the car. Too busy, no shoulder, no sidewalk, and people just don't know how to drive with a bike in their lane.


----------



## mattthemuppet2 (Jan 15, 2021)

I dunno, it's just what I do. In some ways I have it worse here in TX - although the weather isn't as cold (though the thunderstorms are nuts) I was commuting 50 miles a day until covid hit. About 24,000 miles in 3 years. Funnily enough I feel safer on my bike than I do in a car here.


----------



## macardoso (Jan 20, 2021)

Quick Update. In order to mount the flywheel to the bike I needed a 1/4-20 setscrew 0.36" back from the face of the little nub in the face of the flywheel. This would have required drilling in from the OD and tapping. My drill was barely long enough and my taps were not even close. A nut tap would have cost me $60 at least. I opted to tackle this feature with an angled thread from the side of the part. I set up the part on the mill using a drill press vise and an angle block to tip the part back at 60 degrees to the table.




Some quick trig found the distance along the X axis that I would need to offset in order to have the setscrew breakthrough the bore in the correct location to line up with the flat on the shaft.




Unfortunately while I was locating the part, I jogged the MPG in Y when I meant to move in X and crashed my Tormach TTS edgefinder. They don't make these anymore and there is not a great replacement option. If anyone has one to sell, please let me know. I was able to tap it back to running true within 4 tenths, but I still am not happy at all.




The corner of the nub was visually located with a chamfer tool and the 0.1851" offset in X was applied.




And here is the hole, counterbored and tapped. The location of the hole ended up perfect.




At this point I tried it out on the bike and it sounded like a helicopter was taking off on the roof of the house. Things were falling off shelves! Did the math and if you were to bike at a speed equivalent to 20mph outside, the flywheel would be turning at 5500rpm. At that speed, 1 gram of unbalanced weight (equivalent to a paper clip taped to the OD) exerts 5 pounds of force radially! So even though all features were indicated true within .0002", the unbalanced weight was too much.

I revisited the machining by drilling a second setscrew hole 180 degrees opposite to the original and mounting the flywheel on a 10mm arbor I turned from 12L14 steel.




From here, all the surfaces were skimmed to clean them up. The weak workholding (only 2 setscrews and a tight sliding fit to the arbor) made for awful rigidity and the cut just wanted to chatter like crazy. I had to finish all the surfaces at 150 rpm at 0.004"/rev feed and the smallest depth of cut possible. Even with that, the finish was not nearly as nice as it was before.

I remounted the flywheel to the bike and it was way better than before. The house did not shake, but it was just loud. Too annoying to use. At this point, I need to consider dynamic balancing the flywheel for these speeds, and probably upgrading the cheap bearings on the shaft of the trainer. It might be that the flywheel is perfectly balanced but the bearings and housing have enough slop that the shaft is able to wiggle radially and throw the flywheel out of true.

For now, the flywheel sits on the shelf of shame and will beg me to revisit it. I do have to say that even though it was too loud, it felt so much closer to riding outside. I really hope I can get this to work someday.

I have a lot more respect for the precision that is required in making those exercise spin bikes.


----------



## mattthemuppet2 (Jan 20, 2021)

you can statically balance flywheels pretty well with 4 roller skate bearings and wood. Make up a cradle that has 2 pairs of bearings to support a rod between them. Mount the flywheel to the rod (rod must be straight ). Spin it and mark the low point when it stops. Spin it again and mark it. Do that a few times. Wherever you have alot of marks, drill a 1/2" divot. Clean off the marks and repeat the process, drilling new divots or deepening existing ones. Takes a while but you can get things nicely balanced if you're meticulous. I've done it a bunch on treadmill motor flywheels.


----------



## macardoso (Jan 20, 2021)

mattthemuppet2 said:


> you can statically balance flywheels pretty well with 4 roller skate bearings and wood. Make up a cradle that has 2 pairs of bearings to support a rod between them. Mount the flywheel to the rod (rod must be straight ). Spin it and mark the low point when it stops. Spin it again and mark it. Do that a few times. Wherever you have alot of marks, drill a 1/2" divot. Clean off the marks and repeat the process, drilling new divots or deepening existing ones. Takes a while but you can get things nicely balanced if you're meticulous. I've done it a bunch on treadmill motor flywheels.



Ohhh, that sounds good. I think I'll need to beef up the bearing block on the lathe first but will follow up with what you just described.

We have some spare machinery vibration monitoring equipment at work I can mess with, so I might be able to build up a quick dynamic balancer as well.

I have enough projects at the moment so I think I will shelve this and come back to it later.


----------



## mattthemuppet2 (Jan 20, 2021)

dynamic balancing would be better, but for simple stuff like this a basic static balance should get you most of the way. Mine is cobbled together out of scrap wood but does the job.


----------

