IGH Max Speed
 

Hey guys, another IGH question which was lost in another thread. I know mostly people worry about the IGH's lowest gear to get up that hill, but what about when you get to the top? In high gear, what is the highest speed you've obtained?

In my crap mtb I've never managed much beyond 25mph. After that I'm just spinning air.

I think it depends on your cog size and the terrain. I have a 16t cog and can easily reach 25 mph on a slight downhill. I haven't tried much larger hills (it is winter after all) but I'm sure there'd be resistance in the pedals to go faster.

In my 3-speed Bianchi Milano I don't know for sure how fast I was going, but I didn't spin out the day I commuted on it and rode down the longest hill. I am pretty sure I could reach my usual max I do on my commuter bike a Trek MTB with a 48 -13 as the highest gear which is plenty fast for me. So in short I think 25mph is pretty attainable on the Bianchi.

Pretty much the same top speed I get on my similarly geared derailleur bike.

A Sturmey-Archer XRF8(W) IGH fitted with a 19T cog, driven by a 53T chainwheel and on a bike with 700Cx23mm tires would have a 238 gear-inch top ratio. Pedaling 90rpm on the cranks, this would give 64mph.

See starting 2:17 here.

Mine is 42/18 and I can ride 18-20mph on flat road pretty easily. I've found down hills I start spinning out around 25 mph with that gearing combination.

That's my fears squashed. Afterall, for all the talk about commuting it's really about the little boy/girl in us all really just wants to fly down hills fast! Whohoo!

Would you void the Sturmey-Archer warranty if you tried to accelerate your 23mm tires to cruising speed by standing up on level ground? Sheldon Brown's site has a very good gear table and all the major IGF hubs are represented. Elsewhere on his site are the minimum cog size (and maximum chainwheel size) that can be used safely with the hub. I'm thinking an effective gear of 238" falls outside of specs even if it is obtained using an approved cog size.

H

I thought they just specified a minimum gear ratio.

unterhausen is right. this is per Rohloff's website.

To prevent overstraining the hub, a minimum sprocket ratio of 2.35 must be used. With the available sprockets these minimum ratios are achieved by: 40/17, 38/16, 36/15 and 32/13. This resembles a derailleur transmission of 22/34. Larger chainrings can be used without exceptions.

I run 42/16 and top out in the high 20s downhill.

I've had the chopper bike in my sig (has a Sturmey XRD8) going 30+ MPH downhill, no lie. That was a steep downhill, with me pedaling furiously. I think it was a 32T ring and a 25T cog, in 8th would make it 110+ gear inches.

I took my Bianchi Milano with an Alfine on my commute once, which includes a 300'ish descent that I usually get up to 40 or 45 mph on my touring bike that has a computer. It didn't complain but I wasn't pedaling much at that speed.

And so am I. Both cog and chainrings have to fit within the parameters necessary to achieve the "minimum sprocket ratio". I don't know the MSR of a 53/19 but it seemed a little high. Either that or that is some hellacious top effective gear... 238"??!! Maybe its me but that sounds mighty high...

H

There's no maximum sprocket ratio on a gear hub. If you wanted for some bizarre reason, you could definitely have insanely high gear inches.

I don't personally know the exact minimum sprocket ratio on the SA-8 hubs, but they come with a 25 tooth cog and Sturmey Archer makes 30 tooth cranks to go with them. That's a much closer ratio than any other gear hub out there right now, but the SA-8 is designed differently too, with first gear being direct drive (and there being no reduction gears at all)

The problem with a gear hub should never be worrying how high your top gears can get, at the very least. It's more of a problem with whether or not your other gears will be appropriately spaced and low enough at the same time.

Agreed, it's all about usable gears. If it has a max speed of 35 MPH but you can't ride it on windy days, then it doesn't do much good.

gearing the hub for the low, and fitting the chainring size needed to one of these cranks ,
http://www.schlumpf.ch/hp/sd/sd_engl.htm
makes a chainring 1.6x larger via a planetary over drive gear ..
crank arms will then turn slower than the chainring..

R'off 38/16 top gear 91.93" becomes 151.68" with the Schlumpf Speed drive overdrive Crank.. can you imagine spinning that one out..

NB you still have air resistance to overcome ..

My Metropolis weighs so much, I don't have to pedal at all to go fast down hills. I get the rolling juggernaut effect. Even more fun with panniers full of groceries.

6.22643 mph is the max speed of any IGH. It's a fact of physics.

...When your measure point is the speed of the wheel relative to the centre of the axle at any arbitrary point suited to the particular measurement in question:D

That is my pet method, fully loaded and rolling I bet I am hitting the scales at close to 300*, massive amount of inertia there. Also don't have to worry about spinning out anything over ~75" either.

Aaron :)

I've had my Rohloff-equipped tandem up to 46.5 mph. I wasn't commuting at the time, though.

I've had my Sturmey Archer 8 speed hub, 16" wheel folding bike up to 45. That was on a commute.

I had my Shimano nexus-8 equipped touring bike up to 43 last Sunday. I've had it faster before, but don't remember the specifics.

In a mtn. bike with 44/11 high gear and 26"x1.9" wheels you'll be going 30mph at 100rpm. You just need to pedal faster and ride until the knobbies wear down.