Hacking a mag trainer

[barefoot]

I've had my trainer - a Blackburn Trakstand, I think it's called - for years. The vast majority of its use has been to hold a bike upright to be worked on and adjusted... very little actual use as a trainer.

But now I've got myself a Powertap wheel, and a set of Sufferfest videos... I've come up against a fundamental problem.

My trainer doesn't have enough resistance.

With my bike in top gear (50:11) and the trainer on resistance level 3 (or 1... whatever the most resistive one is), I am putting out about 250W while spinning at 90 RPM. My FTP is about 245W.

So, I could max my trainer out for about an hour at 90 RPM then collapse in a heap. But what if I want to do shorter, higher power intervals? The only way I can do that is to spin faster. But there's a limit to how fast a cadence I can spin before things get very rectangular and bouncy (eg. it looks like the trainer has ~400W of resistance (AT) 130 RPM in top gear, which is pretty damn spinny).

How to increase the resistance a bit beyond the maximum setting of the trainer?

I've got a good supply of rare earth magnets. I'm thinking that cracking open the resistance unit and doubling the magnets up might be an option. But I'm not sure exactly what the guts of one of these things looks like, and I'm certainly wary about potential balance and centrifugal issues if I were to add anything to the spinning shaft of the trainer (which must be whizzing around at quite a disturbing number of RPMs, if my wheel is doing ~600 RPM). I'm hoping I can add magnetic field strength to a stationary part of the unit instead.

Anybody ever done such a mod to a trainer?

Or do I just throw it away and get a better trainer?

tim

[Squigs]

I would just buy a new trainer if you have the money, but it's worth a try modifying the current trainer if you have all the things you need.

If you are going to buy a trainer, a kurt kinetic road machine is a very good trainer which has resistance up to 3000w.

[marty_one]

With my mag trainer, if I want more resistance I increase the pressure of the resistance unit being applied to the rear wheel (ie: screw it down harder!!!)

[earlybird]

If I want more resistance I put the Giant P-SL0 wheel with P-SL1 tyre back on

[bigfriendlyvegan]

A bigger fly wheel is a standard mag trainer hack. Interestingly, Graeme Obree says remove the flywheel altogether,

[Xplora]

Tim, get a new trainer. Or consider nice rollers. I just dumped my kk road machine because I wanted the core stability work that you just can't get on a trainer. Sportcrafters does the cycle ops rollers and they make a variable resistance option that allows 900w+ of resistance.

[barefoot]

If you are going to buy a trainer, a kurt kinetic road machine is a very good trainer which has resistance up to 3000w.

That would certainly do the trick... but my indoor cycling career certainly doesn't justify spending serious money. I mean, I've ridden this one I've got about 3 times in 10 years. It was a bit of a ridiculous extravagance putting a tens-of-dollars bid on a Sufferfest video pack at a charity auction a few weeks back (and winning... oops), so I've already overspent my indoor cycling budget for this decade. I'm really just trying to set myself up with a trainer that allows me to get some value from the videos I've got

With my mag trainer, if I want more resistance I increase the pressure of the resistance unit being applied to the rear wheel (ie: screw it down harder!!!)

The whole function of the trainer is to absorb power. That energy has to go somewhere - and without fail, it will end up as heat somewhere in the system. The magnetic thing gets hot (for comparison sake, 250W is about a quarter of a hair dryer running continuously). For the extra-pressure-on-tyre approach to work, I'd be relying on the tyre absorbing a reasonable amount of power, which would make it get hot. Not sure I want to subject my tyre to that... nor to the flexing and deforming that would cause it to absorb power.

A bigger fly wheel is a standard mag trainer hack. Interestingly, Graeme Obree says remove the flywheel altogether

The flywheel will take power to spin up to speed, but in the steady state it doesn't do anything. Well, it does work to smooth out your lumpy pedal strokes (which might be why Obree doesn't want a flywheel, to force you to pedal smoothly). But once it's up to speed, it doesn't absorb any power.

If my "add magnets" idea doesn't work, I'm wondering about adding resistance by adding aerodynamic drag. Ideally, I could add a fan to the flywheel (even better if it's aiming at me, so I get the pleasant cooling breeze and don't drip so much sweat), but any kind of non-aerodynamic addition to the flywheel will mix up some air and absorb power. It's just a matter of strength and balance. By my calculations, that shaft is spinning at ballpark 15000 RPM... so getting a fan or paddlewheel to stick on, and not vibrate itself off or self-destruct into a million pieces, will be a challenge.

tim

[bychosis]

no harm in trying to mod it. You have parts, some engineering knowledge (esp as evidenced in the disc braking thread) and what sounds like a willingness to give it a crack. Plenty of levels of outcome between success and failure.

Worst case you end up with a bike stand (no resistance) and a blown budget (new trainer).
Best case your mod actually works, you post it on here and others also complete the same mods.

[Xplora]

If you just don't want the Sufferfest videos anymore, I'll gladly buy them off you

[S-train]

Hack away, but you'll eventually kill the mag trainer, (all the below is based on a sample size of 1 with a minora trainer for hacks and sample of 1 for a kinetic)

The simplest hack is to adjust the settings of the fixed magnets, set them in closer to the steel wheel, which if it's like mine had steps in it, kindof like a kids drawing of a castle bent around a circle, the closer the magnet the more field is applied, or experinced by the steel crenulated disc. thats how the cable is increasing your resistance.

But at some point you'll hit a maximum, the magnets and or steel disc will heat up and will be less effected by the magnets and resistance will decrease, at some point the magnets will stop working.

I cranked my mag trainer way the hell up very very close and then added rare earth, souly so i could get high high resistance at low cadence, worked great for awhile, (several months) then one day while suffering away and feeling the resistance starting to bleed off, bang zero resistance, while standing leads to falling on top tube gentlemen berries first. You have been warned.

The kurt kinetic is very good, but hard to do a low cadence high effort, but it's very stable for power vs rear wheel speed after 5-10 minutes, if your saving dollars for EPO or what ever go the hack, adjust a little bit and keep going till it breaks

[barefoot]

I cranked my mag trainer way the hell up very very close and then added rare earth, souly so i could get high high resistance at low cadence, worked great for awhile

You, sir, are a gent and a scholar.

This is exactly what I was thinking to do, and you've confirmed that I'm on a track that might work. For a while, at least.

tim

[Xplora]

zero resistance, while standing leads to falling on top tube gentlemen berries first. You have been warned.

[barefoot]

zero resistance, while standing leads to falling on top tube gentlemen berries first. You have been warned.

Indeed, and I'll take the warning in good faith.

But I get the impression that S-train tweaked his trainer a whole lot further than I plan to tweak mine. I don't expect to be doing any high-torque low-RPM standing kind of work. I'm just looking for a bit more resistance at 80-100 RPM.

tim

[Xplora]

The level of resistance you are seeking will inevitably result in the same path strain took

[barefoot]

The level of resistance you are seeking will inevitably result in the same path strain took

Oh well, I've fathered all the children I plan to. Don't need 'em any more

[bychosis]

Perhaps part of the tweaking should also be padding for the top tube. Don't need to worry about aero, just strap a cushion on there with a belt.

[trailgumby]

If you go the air impeller method, factor several sets of earmuffs into our budget. One pair for each family member

[KGB]

You say you only want a little more resistance but you're concerned about increasing the friction on the tyre and causing too much heat?
That would be the first thing I'd try since its free and easy. Drop 20psi and put the roller harder against the tyre. If you're really concerned, do some tests and gradually increase the time interval.
Being concerned about a tyre yet willing to cook your mag trainer makes no sense to me, unless you're just keen to tinker and play around with it...

Alternatively, get another rear wheel and tie and solder some blades to the spokes like a cheapo exercise bike. It will be windy and noisy but won't create additional heat.

[barefoot]

You say you only want a little more resistance but you're concerned about increasing the friction on the tyre and causing too much heat?
That would be the first thing I'd try since its free and easy. Drop 20psi and put the roller harder against the tyre. If you're really concerned, do some tests and gradually increase the time interval.
Being concerned about a tyre yet willing to cook your mag trainer makes no sense to me, unless you're just keen to tinker and play around with it...

Trainer was about $50, at least 10 years ago.

Tyre replacement cost is somewhere about the same... plus the hassle of changing it, and potentially being stranded somewhere inconvenient with a buggered tyre (it's not a dedicated "trainer tyre" - refer back to comment about the hassle of changing it... I'm a very lazy person).

The prospect of killing a not-very-good trainer is cheaper than the prospect of killing a tyre. And if I kill a tyre, I'm still left with a trainer that isn't up to the task of providing adequate resistance against my thoroughly mediocre guns.

Alternatively, get another rear wheel and tie and solder some blades to the spokes like a cheapo exercise bike. It will be windy and noisy but won't create additional heat.

Get another Powertap wheel? Ahaha. Not going to happen.

I have contemplated some kind of clip-on paddle blades for my spokes. They don't have to be as well engineered as a mod on the trainer shaft, because the wheel is spinning about 1/25 the speed (ballpark... the roller is probably a bit over 1" diameter, the wheel is within coo-ee of 27" diameter...). That's somewhere down the list of options.

Better to actually make the trainer work the way a trainer is supposed to work.

I do wonder whether this trainer might not be somewhat defective. As I understand it, they work by putting the magnet bits close to the spinny bit, and inducing eddy currents. The closer you get the bits, the more eddies they current, and the more power they waste. Mine really doesn't have much resistance at all, so it could be that the magnets aren't as close to the disc as they're supposed to be.

It's time to crack the case open and see what's inside. See if I can turn it from a somewhat useless thing into a more useful thing.

tim

[Xplora]

I hate to be difficult, but ruining tyres is what trainers DO. It's utterly unavoidable, and one reason I am going to use rollers instead. You need to generate heat in the tyre, because you are dealing with friction. The resistance is you, pushing the tyre against the roller. You'll cook the tyre regardless if you increase the resistance, because your tyre has to resist the opposing force on the roller.

200W sounds like a defective trainer, for sure. But I think you're missing the forest for the trees on the tyre wear issue. The assumption that the powertap wheel would be a good solution with the cheap trainer was flawed. (been through that entire process a year back myself). Sucks, but at least you can move forward.

[S-train]

not getting a proper trainer tyre is false economy in my book, my current trainer tyre has been through about 150 hrs of training so far this year, it still looks brand new and i've only just started to wear the mould mark out of the centre, the thing is just about indestrutable and my training mat is'nt covered in rubber fluff. But god help me if i take that wheel out on the road and it's wet, worst tyre ever made including ones that are on my daughters tricycle and are solid plastic!

If you crank the trainer into the tyre hard enough so that it does'nt slip then the tyre /trainer interface wont get AS hot and the tyre will last longer but it will have it' life cut short, and what is more expensive a new trainer tyre or the call to the misses while out on a training ride into the middle of no where, 'honey my tyre has exploded and i'm hours from cilivlization please come pick me up and bring a muffin and a coffee'

But hack away , it does'nt take moving inboard much to improve the resistance