Gear hub failure point
Hello, I've got a cargo trike (a Nihola) with a SRAM P5. There is a lot to like about the hub, but I'd like more range (especially lower) and smaller steps. I've got a 24t sprocket in back, a 38t sprocket in front and a 47-559 tire, giving gear-inches from about 26 to 65. The recommended sprocket ratio is 1.8, so I'm already running beyond the approved spec for this hub. I'm only about 155 pounds or 70kg but I stand on the pedals (or worse) in 1st from time to time.
So SRAM makes a S7 hub one of which I have on a bike, and I have two used ones spare. It has a little lower 1st gear, a higher top gear, and slightly better spacing. Trouble is they recommend an input ratio of 1.83 or so, suggesting that the hub is less strong than the P5. I would not want to use a smaller sprocket combination that 24t & 38t as I use on the P5. So, the question is, would I kill the S7?
I notice that the P5 and S7 both only use one planetary gear set at a time, so its fairly easy to visualize how they work. Is the weakness of the S7 in the gear set that gives 1st and 7th gears? On the P5 Cargo, I see they made the gear set that gives 1st and 5th less aggressive compared to the standard P5. Are they worried about the total torque coming out of the hub, or are the small gears more likely to be broken? If I break a S7, is it likely going to be first gear that dies?
On another subject, is the iMotion 9 off the market or something? Can't get any of them around here and I've possibly never seen one. Real sad situation considering the nice even steps, 1.73 recommended input ratio and the availability of a coaster brake. I love coaster brakes.
Last edited by trike_guy; 07-16-11 at 03:03 AM.
Reason: gave wrong input ratio for S7
Sachs had a Cargo Hub, version, I believe [FWIK], that was German manufactured,
the SRAM take over, had the production all shifted to Taiwan , where it is now..
More Profitable part of the plan worked, Lowered costs.
Now you are the 70 KG on the pedals . what has the Load weight & slope/distance climbed,
been over the life of the hub .
and you say you exceeded the ratio relation spec for the hub..
you may just hav e number of rebuilds ahead of you..
Not the end of the world.
Last edited by fietsbob; 07-15-11 at 11:40 PM.
Yeah the P5 Cargo, seems a lot like a normal P5 except the 1st gear is higher and the 5th gear is lower, i.e. a less aggressive planetary gear compared to the standard P5. Did they do that to limit the overall torque on some part of the hub that is not specific to the selected gear, or did they do that because the small gears needed to get the lower ratio where too weak?
Originally Posted by fietsbob
In other words, is a S7 stronger in 2nd gear than in 1st gear?
That's above my pay grade.. No MS in mechanical engineering either..
so, speculate away..
I'd ask at the Hubstripping blog, http://hubstripping.wordpress.com/
or perhaps a place where the ex Sachs engineers hang out..
people say that the NuVinci CV Hub is strong, but thats hearsay
from my perspective..
Last edited by fietsbob; 07-16-11 at 08:43 AM.