Suzuki SV650 Riders Forum banner
Cancel
Create thread New Forums

Fork height vs rear height, need info if possible....

1 reading
Jump to Latest
61 - 80 of 157 Posts
#61 ·
I agree, the faster you go the harder it is to change directions. Still, if you cite an authority it doesn't look good when you argue against them.

It is harder to force the front wheel out of direction the faster it's spinning. That extra force is what you're feeling the wheel do to the chassis, which is lean it over harder.
 
#62 ·
I don't know if you guys are aware of this or not, but here is a forum dedicated to motorcycle physics:

http://www.msgroup.org/forums/mtt/forum.asp?FORUM_ID=198

The main guy, Jim Davis, is kind of a dick, but he's usually right.

oh look - http://www.msgroup.org/forums/mtt/topic.asp?TOPIC_ID=9348

In another thread they talked about braking distances and IIRC the best stopping bikes were "power cruisers" i.e. motorcycles that typically have good brakes, but low CoG's. So that sorta throws out Andy's reasoning for a high CoG.

Just some fuel for the fire. ;D
 
#63 ·
High and low are not absolute terms, but relative terms. Cruisers typically have more weight and can load the front tire more heavily without flipping forward (relatively low COG). What about moving the gas tank under the engine to lower the COG? That was funny to me. Gas weighs a bit less than 7 pounds per gallon. Suppose you had a five gallon tank. You're relocating, say, 45 pounds down lower (including the tank) and raising the heavier engine to do it. Curious.

So while I ate lunch I dug a bit more into this basic question: Do you drive the contact patch line out from under the bike or does the bike fall relative to the contact patch line? What got me thinking about this were the two videos I linked earlier (countersteer and the no BS bike). I could see two different things happening. The closest thing to concensus I could find is that a bit of both occurs. It has to do with the rate of transition.

It is possible to countersteer more to force the contact patch line farther out faster. This is a very quick transition from vertical to lean. On slower transitions the initial countersteer starts the bike falling and the bike can be allowed to continue falling as the rider more slowly steers into the turn to the balance point. The faster the transition from vertical, the more the contact patch line deviates from under the bike. The slower the transition from vertical, the less the contact patch line deviates from under the bike.

The other thing I'm finding is that there are lots of conflicting theories out there. A couple dozen in the last 25 years and more coming all the time. The general behavior is well understood, but there are many differences of opinion as to what exactly causes some of those behaviors.
 
#65 ·
andyauger said:
High and low are not absolute terms, but relative terms. Cruisers typically have more weight and can load the front tire more heavily without flipping forward (relatively low COG). What about moving the gas tank under the engine to lower the COG? That was funny to me. Gas weighs a bit less than 7 pounds per gallon. Suppose you had a five gallon tank. You're relocating, say, 45 pounds down lower (including the tank) and raising the heavier engine to do it. Curious.
Click to expand...
Ok, so you said earlier that a higher COG helps with weight transfer and braking, now you're agreeing that it does not. So if a high COG is not desirable for braking, or accelerating (we haven't argued about this per say, but it's true) then why do sport bikes have a higher COG? ground clearance? Is that the only reason? Certainly manufacturers could figure out a way to lower COG without sacrificing available lean angle.
 
#67 ·
It's a compromise. The lower the COG the faster the bike can transition from vertical to lean, and vice versa (lower polar moment of inertia). This can also make the bike feel a bit twitchy if carried to extremes.

The lower the COG the more the bike depends on front tire coefficient of friction to generate high braking forces (lower COG = less weight transfer to the front end under braking). It has the same effect under hard acceleration. Less weight transfer to the rear means you are dependent more on rear tire coefficient of friction to prevent wheelspin.

The higher the COG the more weight transfer under braking and acceleration, making the tire's coefficient of friction less critical. But higher COG also makes the bike more prone to stoppies and wheelies.

It's relative, and it's a compromise, like so many things in engineering design. It's not like there's one answer.

Cruisers have high braking forces mostly because they are generally heavier than sport bikes. Sorry if I didn't make that clear enough.

Lean angle is part of the issue, but it's true that modern designs can provide plenty of lean angle with a lower COG. Sport bikes, especially those with higher rear wheel torque and more powerful front brakes, raise the COG (relative to a cruiser) for the reasons above. The sacrifice in vertical transitional handling is relatively small compared to the gains in braking and acceleration potential.
 
#69 ·
andyauger said:
It's a compromise. The lower the COG the faster the bike can transition from vertical to lean, and vice versa (lower polar moment of inertia). This can also make the bike feel a bit twitchy if carried to extremes.
Click to expand...
so, lower CoG makes it faster to lean and can be twitchy but also more stable?
 
#75 ·
It's not that simple. Heavier objects take more force to get stopped. How is that braking force generated?

Suppose you have two bikes, 50//50 weight distribution, both with good front brakes. One weighs 1,000 pounds with rider and the other weighs 550 pounds with rider. Right away one has almost twice the normal force on the front tire. Suppose the heavier bike has a relatively low COG and the lighter bike has a relatively high COG. Typical to cruisers, suppose the heavier bike has a longer wheelbase than the lighter bike. Then the heavier bike can transfer more weight (increasing normal force) before lifting the rear tire than the lighter bike. Heavier bikes typically have more contact patch to work with on the front, so the coefficient of friction (given similar tires and road surfaces) can be much higher.

So the heavier bike has more normal force, less tendency to lift the rear tire, potentially higher coefficient of friction. Take all that into consideration and that's why, under real world testing, some very heavy bikes can stop in less distance than some much lighter bikes. They can generate more braking force before lifting the rear tire or skidding the front tire. I've seen similar braking tests in several locations on the web. Go look.
 
#76 ·
andyauger said:
Cruisers typically have more weight and can load the front tire more heavily without flipping forward (relatively low COG).
Click to expand...
[/QUOTE] The lower the COG the more the bike depends on front tire coefficient of friction to generate high braking forces (lower COG = less weight transfer to the front end under braking). It has the same effect under hard acceleration. Less weight transfer to the rear means you are dependent more on rear tire coefficient of friction to prevent wheelspin. [/QUOTE]

which bike is pushing tire in ground more, the one with low cog or higher cog?
 
#79 ·
well what you guys are talking about has a lot of variables in it. a bike with a higher cog in theory could put more downward force on the front wheel as it pulls weight off of the back wheel and transfers it to the front, but with a lower cog and almost twice the weight, the force may be the same. that topic of conversation all depends on the weight of the bike, where that cog is relative to the front axle, and whether the brakes are equally as strong with the weight difference in the bikes. you both need specifics for either of you to win that argument
 
61 - 80 of 157 Posts
About this Discussion
156 Replies
13 Participants
Last post:
RichDesmond
Suzuki SV650 Riders Forum
posts
2.5M
members
80K
Since
2002
A forum community dedicated to Suzuki SV650 owners and enthusiasts. Come join the discussion about performance, modifications, racing, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
Motorcycle Talk Off Topic Riders Lounge SV Modifications and General Maintenance SV650 Gen2 (03-15) Tech SV650 Gen1 (99-02) Tech