long travel fork atomlab pimp 2 DJ frame rear 5304 79v

[pianoman]

Hi everyone,
I'm upgrading my bike almost completely, new frame, fork, wheels, etc., very few things will remain from the previous bike which was a standard xc hardtail 9c 9x7 rear, 48v 30a controller, 14ah sla, front suspension.

Now it is going to be a higher power setup:
5304 rear motor
lyen 72v 18 fet controller
a123 24s5p (about 79v) from cell_man
CA
atomlab pimp 2 frame
rockshox domain u-turn 115m-160mm fork, 20mm thru axle, 35mm steel stanchions
bmx handlebars for upright riding position
somewhat ugly, but very comfortable seat, long seatpost
atomlab pimp 26 inch rims
Maxxis hookworms, dh tubes and slime
Avid Bb7 203mm on front and bb5 160mm rear mechanical disk brakes

I’ll have a custom made long and strong seatpost to ride at normal height, batts will fit in and some over the triangle. I like to pedal at all times, that way it still feels like a bicycle while doing some light workout as well (even if it doesn’t help much to 5304, ha, ha…), so I'll put a 65tooth chainring (from http://www.raptobike.nl), rear is 7speed 11-32tooth, to be able to pedal at 60-70kmh. I'm planning to program the controller at 60 amps, that would make the top power of 4800w aprox. I decided for a hardtail because of lower cost, more room for batts, simplicity and because I ride almost only on smooth pavement. I guess this frame is much stronger then the one on previous bike that was a low price 6061 aluminium xc hardtail. Atomlab DJ frame is cromoly, but I'll make torque plates anyway for safety and they'll keep the wheel in if the nuts go loose. I bought this frame for being cromoly, strong with horizontal 5mm thick dropouts, I know it’s probably not ideal frame for ebike at high speeds because of a somewhat shorter wheelbase, also it has a smaller triangle to fit batts, I'll have to put some over the top tube too and then hide it all with custom side covers. But this was the best and strongest economically viable frame option I could find were I live.
What I’m trying to figure out is if I can run my travel adjustable fork at full 160mm, or that’s too much stress on the headtube. Headtube is reinforced, but only on the downside, frame is build for 80-100mm fork, but atomlab.com and most sellers say longer forks can be used for mtb style geometry, but doesn’t specify how much longer, on one sellers site I found that for freeride etc. 100-130mm fork is suggested for this frame, so maybe 120-130mm would be ok, but can it be more? I hasn’t been able to find any more info online about this, just in general it seems that very long forks are not recommended for hardtails (well perhaps it’s really unnecessary for non powered riding other than DH or freeride anyway, and for those FS is standard). The only reason I would like to run it long is just for slacker angle and slower steering, that I guess is better for high speeds. I’ll ride almost exclusively on smooth pavement, no potholes. If offroad, I can always lower the fork if the trail is too bumpy.

What max travel of the fork would you suggest? I see DJ frames are not that common for ebikes, well something like greyborg would have been ideal, but shipping to Chile where I live would be expensive, customs etc., and the frame itself is much more expensive, eventhough a very good quality and FS.

Has anyone tried DJ ebike with longer fork at high speeds, is it stable? My bike is still not assembled, because I don’t have all parts yet. When I’ll have the bike completed, I guess I’ll find out experimenting about the difference between running it at 120mm or 160mm, if those 40mm extra are a significant improvement in handling, or really not much or even the opposite. I just wouldn’t want the frame to fail, especially not at 72v speeds.

Here’s the link to my frame
http://www.atomlab.com/mtbframep2.html

Your opinion is highly welcome, best regards.

 

[manitu]

I would'nt worry to much. The Pimp's are cro-mo frames made for dirtjumps, skate-parks and street-style. I think they are warrantied up to 130mm for such use. The head tube is reinforced and if you plan to use it as a street-bike or mild offroad , I think you could use the full 160mm. I am usually against using longer forks than the frame was designed for , but DH and DJ bikes are the exeption. Offcourse , this depends on the use.. Dont do 6 feet drops with a 160mm fork!

Anyway , the handling will probably be better around 130-140mm than fully extended.

.manitu

 

[pianoman]

Thank you very much manitu, I'm not planning on doing 6 feet drops :lol: Do you know if the handling of this setup would be acceptable or too unstable for speeds around 60kmh? My previous bike was a cheap xc with entry level rockshox dart 2 100m fork, I had a rear motor, upgraded controller to 72v, had 2 14ah sla in triangle and 4 sla in custom rack at each side of the headtube.

Too fast steering though, but still driveable, I got used to that, but didn't feel very stable. Plus I was always concerned about having thin front axle and open QR dropouts, thin 28mm stanchions with losts of flex when braking, not a safe feel at high speeds, but was ok for around 40kmh at 48v I guess... That's mainly why I'm upgrading to Rockshox Domain fork with thick steel stanchions and thru axle on my new build. Thought a little about double crown too, but my intended use is more for speed on pavement, some mild offroad, not really jumps or rocky trails, and besides I guess double crown would be way too much for a hardtail dj, perhaps would even look kind of funny :lol:

On the other hand I have other bike, similar setup but with thick steel suspension fork (double crown, about 100 or 120 mm at most, very low quality and stiff suspension, in fact I don't feel the suspension at all exept when really huge bumps, but the good part is it has thick steel dropouts and huge stanchions that won't flex) and front 9c with 2 torquearms 72v, 2 slas in triangle and 4 over the rear axle, custom racks next to the axle, 2 sla each side. The fork looks funny on that frame though...


Handling is way better at 60kmh cmpared to the first bike, I don't know if it's because of some gyroscopic effect of the front motor, lower front center of gravity maybe? Headtube angle was almost the same, maybe 1 degree less than the rear wheel drive bike. I don't mind front wheel drive for flat and smooth pavement, but it's no good for offroad or higher power as there is not enough traction and too much stress on fork-headtube. So I'm curious about how the DJ frame with longer fork and 5304 will handle at high speeds. It will weight much less too, no more led, a123 will be way lighter and more range as well.

 

[Hillhater]

Why do you want such a long travel fork if ..."intended use is more for speed on pavement, some mild offroad, not really jumps or rocky trails".. ?? ( other than the "bling" factor)
Some sus' is good, but too much can give you a sloppy feel and poor road handling.
Long travel is for DH and serious off road use.

 

[pianoman]

just for slacker headtube angle and slower steering, if it gives more stability at high speeds. I just didn't really like the very fast steering feel of one of my bikes, felt like a little too fast of a move of the handlebar and the bike would go out of control. I got used to riding it anyway, but just didn't like it.
(I'm certainly not looking for the "bling factor" :lol: ).
I know that for my intended use some 100mm would be good enough. Mainly I have this fork because of the thru axle and thick stanchions, 203mm disk brake compatibility, and travel adjust is nice too, I got it slightly used and cheap.
For sure I agree that long travel is aimed for DH and offroad, my intentions were to use the increased fork lenght just to change the geometry if it would improve the high speed handling and if the frame would be ok with it, if handling doesn't improve much or gets worse, I'll just use it somewhere around 130-140mm, that's the great thing about adjustable forks. I'm glad manitou reafirmed that the frame is probably warrantied up to 130mm, so no problem with that then.

Will the slacker angle when at 160mm improve high speed handling feel or not really, or make it worse? I guess small maneuvers at low speeds would be better with shorter travel. I haven't ever tryed different geometries and my Dj frame bike is not assembled yet, so I haven't been able to try this fork yet either. It is 115-160mm adjustable anyway, so if the longer setting doesn't work well, there is no problem.

 

[manitu]

There's a lot of factors wich dictates the handling. When you put on a longer fork, the head angle decreases (good for high speed) , trail increases (often bad) , weight moves backwards , and your weight moves upwards with the pedals.

If your bike has short wheelbase and a high bottom bracket , the handling will easily be destroyed by too long a fork.
On the other side , if your bike is front heavy and have a really low bottom bracket , it's a perfect candidate for a longer fork.

My freeride bike , a Santa Cruz nomad has the high-end version of your fork a, Rocksox lyric. At 115mm , the bottom bracket is really low , and the pedals hit the ground all the time. At 160mm , It's one superb ride. This frame has a short top tube , wich encourages you to put weight on the front.

My old bike , a Mongoose teocali in large came with a 95-130mm Fox fork. It was a great bike on fire roads and such , But it never gave me the confidence to bomb downhill. I have now put on a 110-150mm marzocchi fork, wich is longer than the Rockshox lyric/domainat 160.
This transformed the bike, but not without drawbacks. The handling in steep downhills got much better because of the head angle, and it feels more stable at high speds. However, banking turns and loose gravel performance became worse. The bottom bracket , wich was high before became silly high, and it feels like you have to hang over the handlebars to get weight on the front wheel in turns. I ended up using it at 130mm, same travel as the fox but about 20mm longer axle/crown. I only use full travel on that bike at really steep downhills.

.manitu

 

[pianoman]

Thanks manitu, I saw from your profile that you are an ebike mechanic, have experimented with different bike setups and so really have a knowledge about this matter. Those are some great bikes you have!
Headtube of the frame and high speed stability were my main concerns and you have helped me with this, I guess I’ll just have to try and find out which travel will be a good compromise in my case once the bike is completed. The handling and stability of an ebike is equally important as its power I think, otherwise it’s really not that enjoyable.
Ideal lower cost than FS solution perhaps would have been to have a rather large cromoly xc type frame (or at least something in between DJ and XC) with reinforced dropouts and headtube, and slack headtube angle so geometry wouldn’t need to be changed. But such a frame simply doesn’t exist or cannot be easily found, at least not here. I bought a DJ frame thinking that it must be made strong as intended for jumps and tricks, which put a lot of stress on it, so it would be a good candidate for a medium cost ebike project. I’ll see if it handles good enough.
My rear 9c bike at first had 4 sla right next to rear axle, even with low center of gravity it had a very funny handling, would pop wheelies easily and felt really uncomfortable and front shaky. Then I placed 2 of the sla batts in triangle and it got much better, but still not great. When I changed to 72v, I left 2 sla in triangle and placed 4 sla in racks next to top tube as seen in pic, handling and stability somewhat improved, the other benefit was that the rear wheel didn’t go out of true so easily as before, which was a problem, that was one of the main reasons why I built those front racks. And no more wheelies. Front shaky feeling remained, but was less disturbing. But the bike was really heavy because of the 6 14ah sla, and 4 sla mounted so high didn’t feel right, but still it was better than having 4 sla, motor plus myself (and sometimes even my daughter) over the rear axle. I’m still curious though why my other front wheel motor bike feels much more stable, because the geometry apparently isn’t very different. I wonder if the weight and rotation of the front hub motor has something to do with it. The feeling is that even if I steer a little abruptly, the bike doesn’t go easily out of control and is not front shaky, feels rather solid, maybe it is the hub motor weight inertia because of which the wheel doesn’t turn sideways so easily as does a light wheel with no motor, and that results in more controlled steering at high speeds?
BB height on my new atomlab pimp 2 frame is 325mm, I guess that’s specified with 100mm travel, so at full 160mm it would raise to slightly more than 350, higher than normal, but is it silly high? I’ll see, maybe it’ll not be that bad. Head angle is 69, I guess with 140mm fork setting it would be about 67 and with 160mm 66, not DH slack but slacker then my previous setup. All batteries will be in triangle and over it, so there will be some weight to the front wheel, but high handlebars, upright riding position which I prefer for more relaxed longer commute riding with no stress on wrists, and rather short chainstays will sure lead to some rear heaviness I suppose, and because of that probably steering on gravel will not be at its best, but perhaps will feel all right on pavement. Anyway for sure it'll be way better than a rear motor bike with all sla weight on back axle as I had it at first setup.