springer speed wobble warning/problem

[rick74304]

So, earlier in the week,the weather finally reached comfort level (50-some degrees) and my son and I head out for the 1st real ride of the season. As usual, i'm riding point on my chopper, he's riding tail on his 20' blue Macargi. As you can see in the pic, the spring has been completely removed from the springer for maximum lowering.

He rode around like that all last fall, but doesnt recall feeling anything weird as far as handling issues.
Anyway, our return trips home always involve coasting down a fairly substantial hill, I cant locate any firm numbers regarding elevation or length, but I estimate it at roughly 7 to 8 stories tall and 1/8 to 1/4 mile long.
About 1/4 the way down when I spun my hat backwards (aerodynamics :wink: ) he was right behind me.
When I got to the bottom and had to brake for a red light, I turned to speak to him, he wasn't there. I looked around, figuring he possibly had to brake to allow for pedestrians or a car, I couldnt see him anywhere.
Then I looked wayyy back up the hill at the right turn lane, and saw a black blob not moving at the side of the road.
I tell you, despite being an out of shape middle aged smoker on a single speed overweight chopper, I got back up that hill pretty frikkin fast!!
Thankfully, he is basically ok, some decent road rash on both arm and leg, and a stretched shoulder ligament.
He says about 1/2 way down, the front end went into a speed wobble, which worsened as his speed increased. When he tried to brake, the bike reacted violently, which ended up tossing the front wheel all the way to the left.(into traffic) He then felt his best option was to lock up the back brake, steer right(away from traffic) and try to skid it to a stop. Unfortunately, as sometimes happens when skidding, the tires hooked, which flipped him sideways.
He figures he skidded on his side the entire length of the turn bay.
http://maps.google.com/maps?f=q&sou...d=113968679613716985170.00000111deb536ad558ec
{for reference, the point marked "A" on the map is where the hill actually starts, pretty gentle drop at first, its when you get to the 2nd bus stop icon that the drop really comes on}
I've gone over the bike, the forks still seem square, the rim and tire seem nicely straight and well balanced, handlebars and stem are lined up with the front tire, the works. I figure I was rolling about 25-30 mph, so his speed should have been somewhat less, maybe 20-25mph. I've been down that same hill on my orange krate (with spring) and dont recall any signifigant issues.
What would cause such a speed wobble to occurr? Anyone else with lowrider/springer experience got a handle on this?
Rick

 

[icyuod2]

with big apes and low stretched out forks, i'd chalk it up to driver error. the bike wanted to zig, and your boy wanted to zag. alittle over compensation, and its all down hill from there. (ya said ya checked all the bolts)

he probably tried to man out the speed wobble only to apply his brakes alittle too late. if his front wheel wasn't straight when he hit em, we'll by the sounds of it he managed to over compensate his steering one last time before he hit the ground.

with the front wheel that far infront of the head tube and the apes straight up like that, those bikes are not the quickest responding. basicly he jack knifed his bike, just like an 18 wheeler. as the distance between the front axle and head tube increases, so will the risk of this happening.he had all his weight wanting to go forward, and his front wheels trying to stop/turn left.

glad to hear he's ok!

 

[Anonymous]

First of all,I HOPE the boy is OK.

Next,go over ALL the bolts in that springer.Front wheel bearings.Head tube too.

I've never had a speed woble problem but heres a suggestion to try...

"Speed Balance" your front wheel until it doese'nt stop spinning in the same spot all the time...a spoke reflector or simalar weight opposite of the valve stem is a good start IF the valve stem stops at the bottom all the time.

A tire could have a "heavy" spot in it too...ya just got to deflate it and spin its orientation on the rim to balance it out.

 

[Dorian]

The reason is that lowrider forks are designed for looks and not for steering geometry. The farther you get away from the mechanical design of a stock bike the worse your steering is going to be. It gets worse the faster you go with a bent lowrider fork. It doesn't help that a lot of repop spring forks are cheaply made either. Theres a thing called rake and trail which is a little complicated but is at the root of your steering problem. I believe that lowriders have way too much negative trail, making a lowriders wheels behave like the ones on a shopping cart, fine at low speeds but terrible when you go faster. If you put a stock fork on your bike it it will steer great. Lowriders steer poorly, I wouldnt want to go fast on one.

Dorian

 

[Dorian]

Here is a motorycle article that talks about rake and trail. Its for motorbikes but the principal is still the same http://64.172.168.34/neatstuff/rakeandtrail.htm

The best posible trail for a bicycle is between 4 and 0 inches of positive trail so I hear, because they dont go as fast as motorcycles. You can't always get these numbers if your running a real long fork. Lowriders are the worst for steering geometry as you can see from the one illustration :shock:

Dorian

 

[Anonymous]

I am FAR from an expert...but look at the caster on a 300 mph.Top Fueler.

The more caster,the more stable they are at high speeds...but I know for a fact and first hand experiance that a ton of positive caster on a bike is a HANDFULL to ride at slow speeds!

THIS will give ya an Upper Body Workout! (it wants to fall over at slow speed)

the Wild Indian viewtopic.php?f=10&t=5490

 

[Dorian]

A long chopper bicycle with a straight fork and a raked neck will steer poorly at low speeds but will steer better when you get going faster, if it has tons of positive trail. DRag motorycles have lots of positive trail which is ok becuase they go over 200 miles an hour, they need it.

A lowrider bicycle will steer at low speeds (sort of ) but the steering will get tricky and dangerous if your going really fast, because it has tons of negative trail.

Good steering on the street is all in the rake and trail numbers.for motorbikes or bicycles

http://64.172.168.34/neatstuff/rakeandtrail.htm

Dorian

 

[user 135]

Springer forks are not made for speed. I have a friend that took his repo Schwinn on our Bogus Bomb Run (16 miles, 2600 foot elavation drop). His fork is the nicer reproduction with the pivot behind the steer tube. He had speed wobbles all the down. He had no loose bolts just a fork not designed for speed.

 

[stretch]

i have a springer on my favorite rider and it occasionally gets loose at the steer tube, where the arms bolt on. that long bolt that goes thru the whole thing gets loose and wobbles. so maybe thats what happened?

 

[Dorian]

Springer forks are not made for speed. I have a friend that took his repo Schwinn on our Bogus Bomb Run (16 miles, 2600 foot elavation drop). His fork is the nicer reproduction with the pivot behind the steer tube. He had speed wobbles all the down. He had no loose bolts just a fork not designed for speed.

It may have been more a case that you went beyond the intended use parameters designed into the stock set up. The rake and trail numbers on a basically stock bike are meant for use within a certain speed range, go beyond it and you find increasing problems. More problems if the spring was weak too.

Dorian