wheelbase length: suspension and handling effects?

[bclapham]

although it is not a "suspension" question perse, i think it best belongs in this forum!

When putting a new chain and sprocket on my bike, the chain was too long. After measuring things up, i found that i could run the rear wheel further to the front (short wheelbase) or further back in the swing arm.

What effect will the postion of the rear wheel (ie wheel base) have on turning, handling and how the suspension works etc?

Ive got an 01 YZ250 and it handles pretty good, but if anyone has any suggestions on which way i should go, please let me know!

Let the debate begin

thanks

Bruce

 

[bigred455]

Running the wheel base on the first notch compared to the 3rd or 4th notch on the chain adjuster,the bike will turn quicker..,But lose a little stability.My opinion I would run it about the 3rd notch.How bout this one,Lets say you have a race sag at 95mm and your chain is adjusted on the first notch.Your chain stretched you are on the 3rd notch now,I wonder if this would alter your race sag+-.Being your shock now, has a different leverage point compared to the rear wheel.That is my deep thought for today. ;)

 

[JTT]

Yes

 

[Moto Squid]

This local racer I know would change all sorts of things suspension wise, including changing the distance of the rear wheel. All the way out for sand tracks and all the way front for tight SX style tracks. I've messed with it some and didn't notice much difference

 

[bclapham]

i was thinking the same thing as Bigred.....further forward, better steering but more twitchy and vica versa. since work = force X distance i expect wheel position will have quite some effect on the rear sag.

i also remember MXA (i know, i know) saying one of the fixes for the CRF turning issue was moving the wheel further forward as possible.

I think i will cut the chain long first and see how it measures up and try to find the most neutral position. Hopefully, since i have an X-ring chain it wont stretch out too much.

thanks.

but i would still like to hear from the higher powers though!:)

 

[endosports]

Ok I am here for you on this one Bruce...;)

The rear wheel location will affect your handling but not as much as you think. Just check your race sag to see if it has changed at all. You can also move your forks up or down in you clamps to change your wheel base to compensate for any changes in race sag.

Did you replace your chain slider? I just saw in DR mag there is a company who sells the Ty Davis or Terry cable chain slider and they offer a 1-3 year warranty on parts for wear. Try that instead of your stock slider. I think mine lasted about 3 weeks.

Don't rack your brain to hard!! We need you working on those vaccines while your boss is away. Yeah right its moto time!!!

 

[bclapham]

you a right brian, sometimes i do think too much...but it is interesting to understand what is going on.....i will opt for the most neutral position.

no i didnt do the slider, the one on there appears to be in quite good order, but i will keep a good eye on it. Maybe it is just a problem with the 4-strokes? or i just dont ride hard enough!:)

 

[marcusgunby]

Not all bikes react the same-i remember the 97 KX250 kawasaki found it benefitted from a longer wheelbase-it actually steered better-it was 1" longer than the 96 or maybe it was the 96 vs the 95.I find if you go to the extremes of the adjuster it makes a difference.

It all depends on how much weight is transfered to the front/rear wheel by the adjustment and if that model requires a change.

 

[bclapham]

Originally posted by marcusgunby
Not all bikes react the same

i wondered where you were, we can always rely on you to answer a question with more questions! :confused:

It all depends on how much weight is transfered to the front/rear wheel by the adjustment and if that model requires a change.

i do hope by that comment about weight, you wern't trying to say that i am fat? :scream:

It's quite an interesting point, from what i can gather they havent changed much on the YZ's for quite some time, and since they have a reasonble repuation handling wise i suppose there isnt that much data out there. Thus, i would assume the most near to the mid point would be best for me. One of the reasons i threw the post out was to understand this effect and also find out if there were any specific traits of my machine. The whole suspension and handling thing facinates me, i couldnt beleive how even a minor adjustment would have such a marked effect on turning for example!

thanks for the discussion gentlemen :thumb:

PS Marcus: did you hear that James Dobb is in So Cal and is looking for 2 guys to join his British team on sunday. for a laugh i sent the organisers an email offering my services, but for some strange reason i havent heard back yet! :p

 

[dirt bike dave]

I had a dual sported '91 KDX250 and would alter the wheelbase & chain length for different rides.

For the trail, I ran a shorter chain and different gearing, which required me to pull the rear wheel ALL the way forward - as far as it would go.

On dual sport runs, I had a longer chain and ran the wheel nearly all the way back.

The bike was clearly more nimble with the wheel all the way forward. The shorter wheelbase also resulted in less sag, which kept the back end higher and helped the front dive to improve tight turning.

Even with some mods, the KDX250 is a slow steering bike, so it was still plenty stable with the wheel forward (also had a steering damper) I probably would have kept it forward for dual sport runs but the shorter chain would not work with a larger front sprocket needed for street rides.

One other disadvantage of the wheel forward is the tire would eat the plastic flap protecting the shock.

 

[marcusgunby]

Bruce im still around-problems with my ISP again-im going to Broadband.
Dont you just love people who never give straight answers;) i have found, from my work i always think of the whole problem-
eg are all bikes the same-no
do some prefer longer wheelbase-yes it was the 96 now my memorys working.
do some prefer short wheel base-yes the 92KX125 was a dog in tight turns-running the wheel forwards helped.
so why is that that these bikes vary?
well some are well engineered and turn well and are stable-the Cr and the older YZs.
the newer KXs are slow turning and the RMs skittish so these are more likely to benefit from weight changes etc.Try to remember nearly all changes have a downside-im trying to get through to some of the owners of suspension i have done, that most work is a compromise.
They dont like the jarring/kicking skittish nature of mordern sx derived std suspension but they complain a plusher set up will not take the monster landings-cant have it all ways i tell them.Factory suspension has less of a compromise but costs more than most modified bikes.

 

[Jeremy Wilkey]

Originally posted by Moto Squid
This local racer I know would change all sorts of things suspension wise, including changing the distance of the rear wheel. All the way out for sand tracks and all the way front for tight SX style tracks. I've messed with it some and didn't notice much difference

One intresting thing that I've begun to grapple with more concretely is that wheel base and swingarm length are important and do offer a tuning variable, however consider that when the wheel is forced to move, the longer the wheel base, actually the less the actual corrective force acting on it is. Of course the overall coreective force is stronger. Yet the percentage decreases.. So its no wonder the less senstive riders notice..

BR,
Jer