Compressed Suspension = Shorter Wheel Base?

[Aluminum_Butt]

While talking with a coworker about a recent track day he did, he mentioned that he actually desires suspension compression before/during a turn because it effectively causes his wheelbase to get shorter, which improves his cornering.

 

I'm trying to wrap my mind around how the wheelbase shortens. I understand that if the whole fork assembly gets shorter (the hypotenuse, if you will), then the wheelbase should get shorter too. Still, it doesn't quite compute. On top of that, I'm wondering if it's really a desirable thing, versus maintaining ground clearance for additional lean angle, and just maintaining overall stability.

 

No matter what, it goes against everything I've ever read around here. Thoughts? Opinions?

 

If it matters, he rides a Honda CBR 1000.

[russell_bynum]

As I understand it, it isn't so much about shorter wheelbase, it's about tightening up the geometry in the front end. i.e. Less rake/trail, which makes the bike less stable. It's easier to make a bike with less rake/trail turn, and it has less desire to straighten up on its own. For extremes, think of a chopper with a raked-out front end. It's VERY stable and doesn't want to change directions. The other side of that is a modern sportbike with less rake and trail...it changes direction very readily, but isn't quite as stable.

 

That technique is certainly valid and is used by some pretty big names. If memory serves, Freddy Spencer is a big proponent of this technique.

 

Ground clearance isn't much of an issue because race bikes have loads of clearance anyway, and you get on the gas pretty much immediately after you get the bike turned, and that extends suspension and restores whatever clearance was lost.

 

BTW, this technique is totally inappropriate for street riding since it puts lots of load on the front end and requires late-braking into the corners.

[Firefight911]

+1 to Russell's comments on this.

 

Although during compression the wheelbase does shorten, its overall affect on the bike is negligible as compared to the change in rake and trail.

 

The detrimant to this "compressed suspension" theory is the fact that with this scenario in place, the suspension does not have the necessary range of motion left in it to overcome too much in the bumpy road category.

 

If, in fact, this individual is seeking this feeling or action in his suspension, he is going about it the wrong way. Suspension is there to reduce the oscillation caused by road imperfections and prevent their being transmitted to the bars/rider thereby increasing the control capability of the motorcycle. It is also essential in maintaining a tire in contact with the roadway surface.

 

To get to this set up requires a change in vehicle geometry. This is accomplished by raising the rear by various methods, effectively changing the rake and trail that way or by changing the front triple clamps geometry by way of a different triple tree geometry relationship to the frame.

 

Other areas that can affect this is by selecting a different profile tire. Again, this is a geometry thing that affects turn in, mid corner stability, etc.

 

Hope this helps to clarify things a bit. When you start digging into the depths of suspension it is very easy to get lost and head down a dangerous path. All changes in one element of suspension, be it geometry related, compression, rebound, spring rates, etc. can have a positive effect to the area you are working on and conversely, negatively affect an area that may have been fine all along.

 

Suspension set up is a fine balancing act of compromises.

[quacker944]

I don't post here very often, but in this case I have to agree and disagree. Using the front brake to load up the tire and change the rake/trail to improve cornering - trail braking through the turn to the apex is a tried and true method that every racer i can think of uses to his or her advantage, just watch AMA superbike or Moto GP. Secondly using the front brake in a similar manner on the street in conjunction with good body position can give a safety net of adjustable line that could save your arse when gravel or road debris appear unexpectedly by using less lean angle to negociate the same radius of curve and on a thicker part of the tire contact patch. Personally I never discount a way of doing things on a motorcycle and find track technique flows over to the road and vise versa. The type of front braking is controlled and slight, we are not talking about lever 'force' rather 'squeezing a baby bird'

[Firefight911]

I don't post here very often, but in this case I have to agree and disagree. Using the front brake to load up the tire and change the rake/trail to improve cornering - trail braking through the turn to the apex is a tried and true method that every racer i can think of uses to his or her advantage, just watch AMA superbike or Moto GP. Secondly using the front brake in a similar manner on the street in conjunction with good body position can give a safety net of adjustable line that could save your arse when gravel or road debris appear unexpectedly by using less lean angle to negociate the same radius of curve and on a thicker part of the tire contact patch. Personally I never discount a way of doing things on a motorcycle and find track technique flows over to the road and vise versa. The type of front braking is controlled and slight, we are not talking about lever 'force' rather 'squeezing a baby bird'

 

The OP was not about brake technique, but geometry as a result of braking.

 

I trail brake, etc. on the street as well. What I think we are viewing differently here is the degree of braking on the street as compared to the track. The lines taken on the street are not the lines taken on the street for obvious reasons.

 

My post dealt with what I viewed was the meat of the subject, and that was how a change in motorcycle geometry had an affect on motorcycle action and how the OP's co worker was going about this.

 

I will add to this discussion that there are quite a few excellent written references out ther to help wade through the myriad of options, actions, and reactions of suspension, suspension gemotetry, and motorcycle geometry.

 

I highly recommend one study up on suspension. It is one of the greatest determinant factors to an enjoyable motorcycling experience, whether on the street or track, IMHO.

[russell_bynum]

If, in fact, this individual is seeking this feeling or action in his suspension, he is going about it the wrong way. Suspension is there to reduce the oscillation caused by road imperfections and prevent their being transmitted to the bars/rider thereby increasing the control capability of the motorcycle.

 

My personal opinion is that the change in rake/trail under braking is a side effect (as opposed to something you specifically work towards), but you can put it to good use.

 

The tough part is timing everything and modulating all of the forces. Ideally, you'd compress the suspension under braking. Then you'd ease the brake off as you tip the bike in....with the goal being to add cornering force to the front end at the same rate that you're removing braking force. If you do it right, the suspension compresses once (when you start braking), and stays compressed until you start opening the throttle instead of compressing under braking, extending back out as you release the brakes, compressing again as you tip it into the corner, and extends again as you get on the gas.

 

If you watch the Rossi's of the world in slow motion, they're doing this.

 

For a putz like me...the best I've been able to achieve is a small extension before it re-compresses under cornering load...and I only got to that point after working at it on the same corner 100 times in a day...and I only got it right a handful of times.

 

Again...on the street, this is a bad idea. Get your braking done with, get the brakes off and the suspension re-settled prior to starting the turn so that you can maximize the suspension's effectiveness.

[Paul Mihalka]

This is a very interesting discussion. Just let's not forget that it applies very little to BMW's Telelever front suspension, as it changes very little steering angle, trail and wheel base, even at full compression.

[Firefight911]

This is a very interesting discussion. Just let's not forget that it applies very little to BMW's Telelever front suspension, as it changes very little steering angle, trail and wheel base, even at full compression.

 

Excellent point that I did consider bringing up, however, since suspension action/control can be so knot tying on the brain, I felt better to leave it out.

 

As I recall, BMW actually had to ADD inherent suspension "dive" to their bikes due to riders feeling so out of place without suspension dive. All done with geometry.

[Joe Frickin' Friday]

This is a very interesting discussion. Just let's not forget that it applies very little to BMW's Telelever front suspension, as it changes very little steering angle, trail and wheel base, even at full compression.

 

...and doesn't compress very much even under heavy braking. I suspect most of the (small) change in steering geometry on a Telelever bike comes from the unloading of the back end (which you can control with some rear brake).

[russell_bynum]

This is a very interesting discussion. Just let's not forget that it applies very little to BMW's Telelever front suspension, as it changes very little steering angle, trail and wheel base, even at full compression.

 

Correct. The whole purpose of Telelever in the first place was to avoid those geometry changes, and to counter the bike's natural tendency to dive under braking.

[bakers_rt]

I've been taught that a touch of the brakes will enhance turn in, especially in really fast corners like over 100 mph where it is harder to overcome the gyro effect of the motor and wheels this can be very effective.

[RFW]

While talking with a coworker about a recent track day he did, he mentioned that he actually desires suspension compression before/during a turn because it effectively causes his wheelbase to get shorter, which improves his cornering.

Well, one of the interesting things about the Hossack front suspension (on the K1200S/R/GT) is that it does not materially reduce wheelbase under compression.

[ShovelStrokeEd]

Neither does the regular BMW suspension. In fact, the front wheel moves slightly forward as the suspension compresses as compared to normal telescopic forks. It is built into the geometry of the suspension.

 

Kinda invalidates the technique for a modern BMW. No substantial rake or trail change (trail might actually increase slightly) and a longer, not shorter, wheelbase. Somebody ask Nate Kern what he does.

[russell_bynum]

Neither does the regular BMW suspension. In fact, the front wheel moves slightly forward as the suspension compresses as compared to normal telescopic forks. It is built into the geometry of the suspension.

 

Kinda invalidates the technique for a modern BMW. No substantial rake or trail change (trail might actually increase slightly) and a longer, not shorter, wheelbase. Somebody ask Nate Kern what he does.

 

Just to clarify...when you say "Regular BMW suspension", you're talking about the Telelever, right?

 

The BMW's with a traditional front end behave like any other bike with a traditional front end.

[ShovelStrokeEd]

Yup, should have been clearer. I meant the telelever. Ride my '78 some time. It drops to its knees under hard braking. One day I really have to go over those forks. The thing is like a 50's Buick compared to a bike with modern suspenders.