Bouncing rear wheel.

Hi.

I've got a problem with my rear wheel, it sorta "jumps" whenever I make a somewhat sharp turns above a certain speed.
Could this be solved with fine tuning my rear suspension or is it just something I have to live with?

Specs of rear wheel setup:
Bike: Mongoose triple black diamond
Motor: QS 3000W 205 50H V3
Rim: 21" 1.6 MotoRim
Tyre: Fortune 80/90-21
Suspension: FOX VAN RC

Regards.

Might be suspension. lot of weight compared to the original MTB wheel.

Combined with the way you ride like mad. 8)

Me? Ride like a mad, nah. It can't be that 8)

You're probably just feeling all that unsprung weight.

I've ridden some dual suspension bike with 19lb motor wheels.. and yeah, you can feel it around turns and on bumps.. but i've never ridden anything as heavy as what you've got.

It sure is a heavy rear wheel, 21kg assembled
But I guess that motorcycles and scooters also have a lot of this unsprung weight :?
There must be a golden unsprung weight to sprung weight ratio that I'm unaware of :wink:

Just out of idle curiosity ... how would hub weight do that? Well, unless it's pushing the tire out of round or something, but I guess that would be as bad or worse at slower speeds.

Rebound control is pretty limited on most bicycle shocks. Thus it is the most difficult part of the rear suspension tuning when you fit a hub motor. While some shocks only require thicker oil, some other can’t be tuned to dampen a heavy wheel without mod. It is usually enough to mod the damper valve to a smaller minimum opening, in order to slow down the rebound.

Air shocks are PITA to mod, because in fact you’d need a bigger one. Older models of coil shocks with large volume are the easiest.

Somethingg you could check is if under sideloading the swingarm is binding and releasing.

Lower tire pressure might help a lot.

Kimovitzh said:

It sure is a heavy rear wheel, 21kg assembled
But I guess that motorcycles and scooters also have a lot of this unsprung weight :?

Click to expand...

Motorcycles do what they can to _minimize_ unsprung weight in the wheels since it's so bad for handling.

You've just hit the magic formula for resonance, with the spring constant in the shock and tire, the weight of the wheel and the loading caused by the higher G forces in the turn. Changing the spring rates, increasing damping or decreasing weight will help.

MadRhino said:

Rebound control is pretty limited on most bicycle shocks. Thus it is the most difficult part of the rear suspension tuning when you fit a hub motor. While some shocks only require thicker oil, some other can’t be tuned to dampen a heavy wheel without mod. It is usually enough to mod the damper valve to a smaller minimum opening, in order to slow down the rebound.

Air shocks are PITA to mod, because in fact you’d need a bigger one. Older models of coil shocks with large volume are the easiest.

Click to expand...

My suspension has a rebound nub I'll try to dampen the rebound and see if it helps.
The suspension is the one on the picture, blue nub is compression, red is rebound speed. Tension on the spring can also be adjusted.

amberwolf said:

Somethingg you could check is if under sideloading the swingarm is binding and releasing.

Click to expand...

Just had the entire swinging mechanism disassembled, no signs of binding were to see.

fechter said:

Lower tire pressure might help a lot.

Click to expand...

I'll try that, I'm at 2,3bar right now.

billvon said:

Kimovitzh said:

It sure is a heavy rear wheel, 21kg assembled
But I guess that motorcycles and scooters also have a lot of this unsprung weight :?

Click to expand...

Motorcycles do what they can to _minimize_ unsprung weight in the wheels since it's so bad for handling.

You've just hit the magic formula for resonance, with the spring constant in the shock and tire, the weight of the wheel and the loading caused by the higher G forces in the turn. Changing the spring rates, increasing damping or decreasing weight will help.

Click to expand...

I'll try a combination of the suggested answers. I already tried some tuning but it didn't help, but it's good to hear that their might be a solution.

Thanks, I'll keep this thread updated when I try out the different combinations.

Your Van R has a good rebound control, but I don’t believe it’s enough for the weight of your wheel. Set the rebound knob at the slowest. You will notice some improvement, but to slow down enough the rebound of your heavy wheel it will need some internal tuning. It could be tricked with thicker oil, making it slower to pass the damper valve. The other tuning options on your shock are to be tuned according to your weight, terrain and riding style; they have nothing to do with rebound control, unless they are way out of tune.

Wider rim, bigger tire, lower PSI, will help too of course. That is why we suggest to those building with a hub motor to use the biggest tire that fits the swingarm width. The rim should be at least half the tire width, 2/3 is ideal, more than that is good but it will need smooth terrain to ride. The wider the rim, the lower the PSI a tire can take, because of volume increase. Mounted on a wide rim, some tires can ride very low pressure and do hrlp s lot to better tune the rebound.