Front forks advice please

[jonrev]

I'm just getting the bits sorted for my delta build and one of the MTBs has a suspension front fork. The bike is actually aluminium and I'd imagined the forks were also but the unit seemed really heavy when I took it off. Tested with a magnet and it is in fact all steel.....BUT. The saddle thingy which has the steer tube and fork tops fixed to it is ally? Don't know if this is usual and such a fork is thought of as an ally fork or this is a steel fork with an ally top??.

The thing is a pretty substantial looking casting but obviously both steer tube and fork legs must be press fitted into it I assume. This part must take the largest bending force of the whole front unit I would imagine and since I am likely to be putting my front hub motor (500w) wheel on it at some stage will have even more stress applied.

Will this unit be safe to use or should I forget it and stick with one of the all steel unsprung units? If I change my plans a bit and use a motor drive to a rear wheel instead would it then be considered strong enough?

Thanks
John

 

[Hugh]

I'm by no means an expert but everything I've read about using a front hub motor says to be very cautious installing on a suspension fork. Changing your plans and using a rear motor would probably be a safer choice or using an all steel unsprung set of forks. However it is your decision to make. I turned my delta into 2wd by using a jackshaft to route the pedal power to the left rear wheel and mounted the hub drive on the right rear wheel. A rear hub capable of having gears was used but a front hub would have been fine as well.

 

[jonrev]

Thank you Hugh. The front suspension fork has been shelved for now as it has a different type of bearing setup ( I don't know what it's called) and I don't have a steel head tube of the same type. I was very unsure about using these forks with a hub motor wheel anyway so not a problem.

I'm going to use the same system as you with a jackshaft to route drive to the inside wheel. I'm building a seperate rear frame for the wheels so they are supported on both ends of their axles and makiing it suspended at each end roughly in line with the jackshaft axis and with a coilover suspension unit. Having the front wheel suspension too would have been nice but since the rear will take a higher proportion of the vehicle weight I'll be happy with just the one end of the trike sprung.

I will start with just pedal power but if all is well I'll swap the offside wheel to the hub motor unit. It's a front drive wheel so the dropout spacing should be able to take either wheel.

All is a bit frustrating right now as my gammy leg is slowing me down and my old house is in need of some work this summer too. Not enough hours in the day as usual.

Thanks again
John

 

[Popshot]

That top casting in ally is very common on even cheap suspension forks. Used as intended it's perfectly strong enough. Putting undesigned for side loadings into it such as from a motor would likely fatigue it.

 

[jonrev]

Thanks popshot. Must admit it looks quite a beefy unit and weighs a ton. Problem is it has a non threaded steer tube which is bigger diameter than my other sets and the top tube on the MTB it came from is ally. I haven't got any tube in my stock that's the right size or even any I could bore out a bit to fit. No probs it can go into stock and maybe I'll find another bike with a steel top tube at some time or maybe turn one up if I get really bored one day!. I suspect I'll probably be making more machines once I get going.

John