Bicycle Mechanics - Main Kentfield FS forksfitted with electric motor
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11-10-10, 11:16 AM
Can anyone tell me if my Marin - Spinner Odesa LX suspension forks are made of steel or Chronoley as opposed to Alloy as I want to fit an electric motor to the front wheel and understand that alloy ones would be unsafe. The bike is a 2007 Kentfield FS gents with suspension forks and saddle.
11-10-10, 12:20 PM
Hold a magnet to them. If the magnet sticks, they are steel.
11-10-10, 01:49 PM
Ummm,,,,,, aluminum is not magnetic, and steel is (-try sticking a magnet to the fork lowers-).
And cromoly (http://en.wikipedia.org/wiki/41xx_steel) is a type of steel.
-Unless the fork lowers are made of stainless steel, but I'd bet a big pile of money that they're not. (stainless can be magnetic or not, depending on the exact alloy)
11-10-10, 07:36 PM
Virtually all suspension forks have steel stanchion tubes and aluminhum sliders. I do not think that I have seen a front wheel motor assembly used with a suspension fork. As noted a magnet should tell you which parts of the fork are steel.
11-11-10, 02:18 AM
After having worked in an electric bicycle specialty shop and dealt with this problem plenty enough times, I can tell you that the situation is more complicated than just what your forks are made of.
It also depends on the power of the motor, whether the controller has a soft start or pedal first start, and if it does not then how brutally hard is the rider going to accelerate?
In situations where people are using low - mid power motors designed for power assist and applying the throttle reasonably, most forks are quite fine. It's the high powered / overvolted motors and the riders who gun it straight off the line that have troubles with fork damage / failure. The most stress to the fork by far is generated when you gun the motor from the starting line.
Additionally, suspension forks are not optimal matches for electric motors either, generally. They are prone to damage during repeated heavy acceleration at the stanchions, developing play and becoming dangerously loose - potentially eventually failing. Cheap suspension forks already tend to do this a lot even for normal bicycles, electric ones with front hub driven motors just amplify the problem that much more. Again though, a decent quality suspension fork and a reasonable usage level from the rider given their particular motor setup can make it work alright. Some electric bicycle manufacturers such as eZee use a front motor mounted to a suspension fork, but it is a moderate power motor with a soft start controller.
A Chromoly fork IS preferrable, but others can work decently if you're not going to be building some kind of scooteresque super powered abomination.
If you have any doubt, go for a rear wheel drive system instead. The bicycle frame itself is capable of handling the forces generated by the motor to a greater degree than the fork is, and there are benefits in increased drive traction and keeping weight off the front wheel, which can make some bikes handle a bit wonky. The rear wheel motor kits aren't much harder to install or deal with.
11-11-10, 06:29 AM
Thank you for a comprehensive reply.
Firstly the bike forks have steel sliders at the top (proved by a magnet). The bottom of the forks (down to the drop-outs) is NOT magnetic. I am however now wondering if they are in fact Cromoly but I have no idea if Cromoly steel will respond to the magnet test or not! Maybe someone could shed soe light on that one please?
For information my wife's Marin Kentfield has rigid forks and are marked "CS Series Cromoly rigid forrks" and do respond to the magnet so are presumably steel.
Mine however are marked "CS Series - Spinner Odesa LX"
I think from the above it is likely my bottom part of the forks are alloy?????
I was proposing to fit a UK legal 250 volt front wheel motor together with a Pedalec function using a 36 volt 12 AmpH battery, so I do not think that this is what could be a considered a particularly powerful set-up. Unfortunately I can not get a rear wheel option at the same sort of cost and in any event it would be way too powerful to keep within the UK law!
If you think it would be safe to fit that set-up, do you think I should install a torque arm and if so have you any ideas where I should go for such an extra please?
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