New Bike Handling Question

[tnr4]

Hey folks,

So, I've been posting less since I've been Ninjette-less, but I've been experiencing an interesting issue with the new bike, and wondered if any of you wizards might have an explanation.

So, the new bike is a 2008 GSX650F (0 mile leftover, so no damage or anything). I love it -- it's freaking perfect, blah blah blah. Okay, now the interesting part: at about 2000mi in, I started leaning the bike over a bit more (still street safe, but moderately aggressive, 270* clover ramps and stuff), and I felt two things that seem weird.

First, the front end feels like it wants to 'flop'. That is, when I get the bike leaned over, and I'm pushing on the inside handlebar, it feels like the front wheel wants to flop down INTO the turn, like I was taking a slow speed turn. I've never felt this with my other bikes.

Second, the rear seems to turn slower than the front. I know that sounds weird, but the rear seems to be 'out' from my line. It's definitely not sliding or anything -- it just doesn't seem to be tracing my line.

These *could* be separate, but they feel like part of a weird handling characteristic: the front wants to flop in quickly and easily, and the rear wants to kick out.

Potentially related info: the stock tires SUCK. Old-gen, single compound, hard ST tires (Bridgestone something or others), with one model up front, a different one in the rear (that's OEM -- weird, I know). Also, the suspension is adjustable, but I don't know what I'm doing, so I haven't touched it. Front is pre-load only, rear is pre-load and rebound. Finally, the bike is heavy -- over 500lbs.

This really is a minor thing -- it's not terrible by any means, but it's strange, and I wondered if it's something to be fixed, or just the different characteristic of a different bike. I'll spoon on good rubber in the spring and take it to its first track outing, so I'd love it if I could improve feeling leaned over before then.

Thoughts???

[Sinister]

Sounds like the tires are squared off a bit.

[CC Cowboy]

Check your tire pressures.

[algs26]

These are the original 2008 tires? Though unused, aren't the tires 5+ years old?

Is the manufacture date stamped on the sidewall like car tires?

[tnr4]

Yup, these are the original tires, although I haven't noticed a manufacturing date; I'll have to check that. And I don't think they're squared off -- I've ridden a bike with squared tires, and that feeling is like there's a 'lip'; I don't get that here (plus, they don't look squared at all).

Air pressure is a good call. That was my first thought as well. CC -- with what I explained, which way would you expect them to be off? The OEM rec is 36/36 solo, and 36/40 two-up. Since I'm a bigger guy, I've messed around with everything from 36/36 to 38/40. But I think my pressure gauge sucks. Maybe it's time to invest in a decent one.

[Motofool]

Originally Posted by tnr4

..........First, the front end feels like it wants to 'flop'. That is, when I get the bike leaned over, and I'm pushing on the inside handlebar, it feels like the front wheel wants to flop down INTO the turn, like I was taking a slow speed turn. I've never felt this with my other bikes.

Second, the rear seems to turn slower than the front. I know that sounds weird, but the rear seems to be 'out' from my line. It's definitely not sliding or anything -- it just doesn't seem to be tracing my line.............

Travis,

Your bike is over-steering.

That is related to subtle changes in steering geometry and tire characteristics.

Read pages 111, 112 and 317 in the following link:

Motorcycle Dynamics
By Vittore Cossalter

"Tires have a large influence over bike handling, especially on motorcycles. Tires influence bike dynamics in two distinct ways: finite crown radius and force generation. Increase the crown radius of the front tire has been shown to decrease the size or eliminate self stability. Increasing the crown radius of the rear tire has the opposite effect, but to a lesser degree.

Tire inflation pressures have also been found to be important variables in the behavior of a motorcycle at high speeds. Because the front and rear tires can have different slip angles due to weight distribution, tire properties, etc., bikes can experience understeer or oversteer. Of the two, understeer, in which the front wheel slides more than the rear wheel, is more dangerous since front wheel steering is critical for maintaining balance. When understeering, the steering angle must be greater, and when oversteering, the steering angle must be less than it would be if the slip angles were equal to maintain a given turn radius."

The bike of a famous member had the opposite tendency (under-steering) and was discussed in this thread:
http://www.ninjette.org/forums/showt...285#post622285

[tnr4]

Hernan: I thought you may have an answer for me! Thanks much. So it sounds like the sideslip angle of my rear tire is greater than that of my front tire. It also sounds like air pressure can have some effect on sideslip (although presumably that would be limited -- different tires would have a greater effect). So then how would I decrease the slip ratio (rear/front)? Does adding air increase or decrease the sideslip angle? Since the oversteer feels fairly minor on my bike, I wonder if a few psi offset on each front and rear might help a bit, but I'm not sure which direction to go.

Thanks again!

[Alex]

Put decent tires on the bike before chasing your tail. They sucked when new. They are beyond terrible at this point.

[CycleCam303]

Originally Posted by Alex

Put decent tires on the bike before chasing your tail. They sucked when new. They are beyond terrible at this point.

Plus eleventy billion. You tune a bike or car to what's actually touching the ground. The things that make the single biggest change on everything, tires.

[Motofool]

Originally Posted by tnr4

Hernan: I thought you may have an answer for me! Thanks much.........Does adding air increase or decrease the sideslip angle? Since the oversteer feels fairly minor on my bike, I wonder if a few psi offset on each front and rear might help a bit, but I'm not sure which direction to go.

Thanks again!

You are welcome

Yes, adding air decreases the sideslip angle, as the carcase is less deformable under the lateral forces of turning.

I would try reducing a couple of psi at front and increasing another couple at the rear, as long as suspension and carcase temperature are not affected much.

[SteveL]

I had a GSX650F for three years and 23000Km the OEM Bridgstone's where not the best on the road, but they were neutral. Turning in too fast is incorrect pressure squared off, combination of the two and not enough power in the turn the GSX is a heavy bike 257Kg it needs acceleration in the turn to track well. I changed the OEM's to Bridgstone BT023's a much better tyre neutral handling very predictable especial in the wet.

Steve

[subxero]

sounds like you might need a second opinion. Just drop the bike off at my place for a week... or two... or three that should give me enough time to get a feel for what is going on and I can verify that it is the bike and not you

fill her up before you drop her off please.

[SteveL]

Just remembered I have a pdf version of the workshop manual for the GSX650F
44Mb if you are interested.

Steve

[tnr4]

Originally Posted by subxero

sounds like you might need a second opinion. Just drop the bike off at my place for a week... or two... or three that should give me enough time to get a feel for what is going on and I can verify that it is the bike and not you

fill her up before you drop her off please.

Originally Posted by SteveL

Just remembered I have a pdf version of the workshop manual for the GSX650F
44Mb if you are interested.

Steve

Ooooh, that would be great. I'd love to get a little more handy with bikes.

And as far as replacing the tires: I will definitely change them out in the spring, but I can't bear to drop a few bills on tires that I will put 6000mi on this winter, straight up and down commuting. The Bridgestones will be just fine for that, and I'll drop some sporty tires on in time for track days next year. I just wanted to learn a bit about what I was feeling.