rubber Donuts for a Bike Trainer

I'm modifying my stationary bike trainer and I need to find some rubber donuts to use as an elastomer flex point.

Background: I have a cheap stationary bike trainer that is basically a steel a-fram that mounts to the rear axle of my road bike and has a magnetic resistance unit that contacts the tire. It allows me to set up the bike and ride indoors to get a workout when the weather's crap or I don't feel like riding in the dark.

Problem: The bike is held absolutely stationary, which makes it more uncomfortable to sit on it and pedal instead of swaying and moving a little like it does on the road. It also doesn't exercise some of the core muscles used in balance, etc.

Solution: One company has developed a trainer that uses elastomer to mount the 'bike mount' part to the 'floor stand' part, allowing for some rocking and bobbing but still keeping you basically upright. Unfortunately, these things are $500 - $800.

They look like this -

GRM Solution: I finally got a close look at one and it shouldn't be hard to make myself using some scrap steel and my existing trainer.

GRM Problem: I'm basically going to copy the arrangement of steel plates with rubber elastomers for flex, but I need to find something to use for the elastomers

Reqmts;

must be about 3"-4" diameter - the plates will be 4-5" across - and somewhere between 1/2" - 3/4" at full compression

fairly solid material ( I'm about 240lbs)

absorb many, many compression cycles without splitting or crushing

I'm considering poly suspension bushings, but what type? Maybe even chunks of old tire? Ideas, suggestions?

I bet the Kinetic elastomers are sold separately.
Make sure the footprint is really wide.

In reply to EastCoastMojo:

Son of a..... never thought to check. 'Scuse me a minit.......

and, yeah, I'm eyeballing a 36" wide square frame design.

Okay, replacement elastomers don't seem to be available on the open market. You have to call and file a warranty claim to discuss replacements.

the elastomers also look more like 2-3" diameter and appear to be polyurethane, so some kind of suspension bushing seems like a good bet.

Off the top of my head....

Solution 1 would be to mold some yourself. That way you can custom tailor the size and hardness to your liking.

Solution 2 is to get some rubber semi truck mudflaps and cut and stack them to get the thickness you need. But i'm not really sure if that solution would be too soft or not.

go to mcmaster.com, type "rubber mounts" into the search box and press enter. Spend an hour comparing

Hockey pucks with a hole drilled in the middle?

Casting polyurethane - I looke dinto that and as cool as it is, I can't justify the cost when there are readymade solutions out there. The mudflap idea may work, and at least it's easy to try.

McMaster-Carr - holy crikey, how have I not seen this before??!! They do have some options, but I'd prefer not to have to ship something.

Hockey Pucks - Hey, I live in the south! Seriously, how hard is a puck?

bump stops.. some trucks have some big soft ones.. especially the jacked up 4x4s.. as an extra, you can get them in fun colours

Hockey pucks are falling from the trees in my part of the country. They're actually pretty hard, but from the photo you posted it appears there is basically a single pivot point inline with the rear wheel, so I'd think it's going to need to be stiff.

Another thought would be those rubber donuts used for exhaust hangers, although they may not be hard enough.

Pick up one of those truck re-tread pieces on the side of the highway? Cut your pieces from it. Or even an old car tire, although I don't know what you'd have to do to get through the belts. Or how to make the edges of those belts safe for children/animals to be around.

How about an engine mount?

mad_machine wrote: How about an engine mount?

Beat me to it !

neato... but if you are going for the core workout and such why not get rollers? if you want more resistance push a higher gear or use the "towel" resistance method

you'll end up riding a lot smoother... and if you're real smart you can do some vids of early on when you fall off :)

EastCoastMojo wrote: I bet the Kinetic elastomers are sold separately. Make sure the footprint is really wide.

On a related note, ECM, do you have an opinion on the Kurt Kinetic trainer? I'm looking to invest in one and don't know which one to buy.

80mm 78A durometer longboard (skateboard) wheels cut to proper width???

Should run around $35 for a set of 4, so you can experiment with different widths.

I agree--a dead-solid trainer gets VERY uncomfortable after the 1-hour mark.

Like this?

http://polybushings.com/pages/bushingsbysize.html

donalson - I was wondering when someone would throw that out..... Mainly because this is cheaper since I already have all the scrap steel, and I get to weld something. I'd love to get some rollers eventually though.

gamby - hey...that's an idea...I'll look into that one

Turbo - I looked at the exact same item, and it's perfect if I can't find something local. I'd rather be able to eyeball it in person, but if nothing looks like a solution, that will be my go-to

dyi - I'm not ECM, but I'll say the Kurt trainers are about the nicest I've looked at and their resistance units have a long (lifetime?) warranty. If you have the money for one, I don't think you'll be disappointed. The one in my pic is one of 3 used by a professional fit mechanic in Atlanta that charges $250 and UP for pro level bike fit sessions. If they're good enough for him to build a business on, it says a lot.

ultraclyde wrote: The one in my pic is one of 3 used by a professional fit mechanic in Atlanta that charges $250 and UP for pro level bike fit sessions. If they're good enough for him to build a business on, it says a lot.

It's hilarious to me that "fit" is such a buzzword these days. Back in the day, you'd buy a pricey bike and maybe 15 minutes and a "this seems to work for you" was all it took.

Now (in the internet tradition of nothing ever being good enough), comes the $250+ fit session. "ZOMG your bike is worthless without a professional fitting done!!!"

I had a fitting done and I still ended up tweaking the seat height. Eventually, we all figure out anyway...

Sorry about that rant, clyde. Not directed to you...

Several of my friends swear by it. I'm not so sure.... at least I haven't invested in it yet. This guy can do all kinds of video motion capture and skeletal measurement...... I suppose if you're a pro roady that spends 8 hours a day on a bike, maybe those tweaks would matter. And I can see more value in it for a road bike where your position is more static than on mountain......

But I still haven't coughed up the dough to do it on any of my bikes.

Not to tangent, but if you have a shop you like, I figure it's not a bad idea to have someone who knows what they're doing just have a peek at you on the bike. I expect any wrench who races will instantly be able to tell if you've got some element of fit badly wrong. EDIT: "you" in this case is the generic "you" who's a person contemplating bicycle fit, since nobody here seemed to be teetering on the brink of a professional fit session.

Back on topic: Elastomer donuts that perform a similar task can be found on skateboard trucks. They'd be too small to do it in the format you've outlined, just slid over a pivot bolt. But if the bolt's down the middle and you have these guys arrayed on the left and right, I think they'd work quite well. They'd also give you the ability to tweak firmness by moving them inboard or outboard.

hmm an old set of trucks could do a ghettoriffic trial system at least

flip the trucks so the T section mounts to the trainer and then get a 2x4 or something and bolt the truck to that... could even adjust the tension (which may well be more torque then the trucks want to deal with... but could try it out

was just thinking about it.. might want to put some hard limits on the movement. While the rear wheel is still a gyroscope, you do not have forward momentium to help you stay up

a couple of bump stops out on the end of the truck would take care of that... set it up somewhat like on the bumppers on the hood so you can screw them in/out and they'll sit higher or lower

Volvo 240 exhaust hanger donuts.

also, have you looked into a roller instead?