Reduce Drag Stark

[imchrisf]

I have been riding dirtbikes for many years, but not really repairing or working on them. I have been riding Mountain Bikes for just as long and work on them all the time. I like it when my wheels spin freely front and back so I have no drag if at all possible. My Stark is a different story. The rear wheel does not spin freely. I am not an idiot, I know the chain and all the heavy stuff will never let it be like a Mountain Bike, but wouldn't it be worth it to remove every bit of drag we can so our precious battery can last longer? Is it even possible? Mine does not even go one rotation on its own if I push it by hand. Is that normal? What can I do to make it as smooth as possible? Sorry, you all probably know this.

 

[FYR]

I have been riding dirtbikes for many years, but not really repairing or working on them. I have been riding Mountain Bikes for just as long and work on them all the time. I like it when my wheels spin freely front and back so I have no drag if at all possible. My Stark is a different story. The rear wheel does not spin freely. I am not an idiot, I know the chain and all the heavy stuff will never let it be like a Mountain Bike, but wouldn't it be worth it to remove every bit of drag we can so our precious battery can last longer? Is it even possible? Mine does not even go one rotation on its own if I push it by hand. Is that normal? What can I do to make it as smooth as possible? Sorry, you all probably know this.

I too experienced this and shared the same concern when my Varg was new. My assumption was/is that most of it is due to the direct drive to the electric motor as well as the mechanical advantage of the gear reduction. There is no separation of the drive from the driven wheel like there is when pulling the clutch or shifting into neutral on a traditional motorbike. I will say though, that after several hours and a decent amount of miles ridden, that it has freed up a bunch and is nowhere near what it was like when brand new.

 

[Philip]

I am confused what you guys are talking about. Drag with the chain installed or not? You cannot make the motor spin freely, but with the chain removed it shouldn't be too bad.

Make sure the brake caliper is centered and the pads don't drag. Polish the wheel spacers and lube the seals. Install a better chain (from a bicycle, lol)!

 

[FYR]

Hmmm... I was assuming the OP was referring to the bike being as-ridden. Yes, my rear wheel spins like a top without a chain attached!

 

[imchrisf]

I am refering to the spin when on the stand. I have not tried spining it without the chain yet. I will try that. BTW, there is no master link on this chain. Should I add one? I am used to being able to take off the chain easily on my MTB's. I had not expected that the motor would have any resistance when powered off. So if it is still dragging without chain i can look at my brake setup. My Varg is under 10 hours if that helps. I also have 2 wheels one is an 18 with a larger softer tire. I bought a second rotor from Stark and mounted it as directed. I have not changed the brake mount, since it seems like that does not depend on the wheel size, only the rotor size. I will let you know how it goes. I have been riding eMTB for a while, and I can tell you that every bit of drag removed can make a difference in battery longevity. Thats why I am so interested in it. Plus, I am 260 LB and want as long of a ride as I can get.

 

[Theo]

I had not expected that the motor would have any resistance when powered off.

Here, @ 14:29, there is an explanation about the spin loss that we experience in the permanent magnets motors like the ones in our Vargs:

BTW, there is no master link on this chain. Should I add one?

If you want to try, make sure that the chain guide is wide enough for it; some people have written that it's not if I recall correctly.

Anyhow, super interesting topic!

I like learning about technical stuff like power usage, torque, drag, etc. and I do it trying to use the most reliable sources of information that I can find. Here is my opinion but remember that I don't even have a degree and that I am not a professional in this field.
Surely trying to increase efficiency is important on an electric dirtbike, but I think there is little that we can do.
• On asphalt, the greatest amount of drag is the aerodynamic one and this holds true for motorcycles, cars, bycicles...; the force of this kind of drag is estimated to be directly proportional to the square of the speed and the power consumption is directly proportional to the cube of its speed. Typically, at top speed something like 95% of the power is used to overcome this drag. For example, a 30 HP 125cc 2T sportsbike can have a top speed of 100 mph because at that speed it takes almost 30 HP just to move through the air, but in order to double the speed in theory you need 2³=8 times that power: 240 HP for 200 mph; as you can see, it's decently realistic. Now a dirtbike usually stays below 50 mph, but its Cx is aweful: the shape is very far from a drop and the front wheel has knobs and has a huge gap between them and the fender; surely the turbulence is awful. Electric cars like the Mercedes EQXX have very refined shapes with covered wheels and if there are mirrors instead of cameras probably that's only because of regulations.
I don't know whether hand guards can imporve the Cx.
• The drag generated by the knobs with the ground has to be accepted since I think that nobody would trade grip for better efficiency and personally I wouldn't know what tyres to use for that anyways.
• Of course at Stark they have already done their best to save weight; the lighter tubes should especially help saving energy because they draw energy just to be spun and, while my front one failed maybe at the 4th trackday, the rear one is still good after 52 hours and Wass says that they have improved them, so maybe you could give them a try.
• Interestingly, Stark didn't choose an oringless chain which would save some energy.
• Maybe it would be worth to invest in some lower friction wheel bearings and grease seals if they are available.

 

[imchrisf]

Thanks, that is all very helpful. I also enjoy geeking out on this stuff. Some is a bit above my level as I am an IT Pro and not much real world engineering training. (Other than watching lots of Mythbusters and "How its Made" and the like.) I would agree that if most of my riding was at 20 - 50 Mph the aero factors would far outweigh any small improvements in drag on the rear wheel. When I am messing around on the road, I can really feel the turbulence and pushback on any speed higher than 35 or more. However, since I am primarily riding trails at 5-15 mph, I think the equations change. When climbing up a hill, or rolling down the other side, the limiters are 2 IMO. First is overall weight. Bike + Me. I am 260 and even losing 20-30 LBS will noticably help range. Second is any resistance from moving parts, Wheel, Chain, tires, brakes drive motor. At the speeds I run this stuff could make a difference I think. Especially since any energy lost to unnecesarry resistance can't be put to propultion, or regen and is just left on the side of the road. (Metaphorically) I have seen blocks of wood with polished axels beat the most aerodynamic pinewood derby cars when they dont have their wheels and axels set up right. I would guess spending a bunch of money on expensive chains, bearings, seals or etc could be a waste, but polishing, straightening and replacing worn stuff could be on the right track. I just don't have enough knowlege of how these motorized thiings work yet. Thanks for all the help, I have already learned a lot.