Front end "chirp" | GTAMotorcycle.com

Front end "chirp"

YellowDuck

YellowDuck

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Track technique question...

Yesterday at TMP I was, as always, trying to go a bit faster. Track was dry in the afternoon and leaned over in 3 (not dragging knee - I'm not that good - just hanging well off and turned in) the front tire gave about two audible chirps and stepped out a bit. I'd say I was outside the normal line due to coming in a little hot (for me) and also being something of a spaz. But, maintenance throttle when it happened, not trail braking or anything.

I have felt front end pushes before on new or cold tires and they have always been more of a greasy slide. Is it very unusual to have more of a bouncing loss of traction in the front when turned in? It sort of freaked me out and I didn't trust the front after that and so came in for the session. Wondering what could cause this (machine-wise or rider-wise).

I am on a Ducati Sport Classic with AK-20 cartrides in the front, running Pirelli Superbike Pro trackday slicks ("redstripes").

Input appreciated.
 
Your front tire is losing contact with the track surface obviously, in an uncontrolled fashion. Too much spring, not enough spring, not enough damping, too much damping, crappy suspension, incorrect setup for tires, bad riding, poor frame/suspension geometry..any of these things could cause the problem. I think you need some on-site help with settings..and possibly a suspension upgrade to tune the bike to your traction/suspension use.
 
Well, that is quite a laundry list!

One thing I will investigate is the spring rate. When I did the forks the springs that Traxxion recommended were based on my intention at the time to use the bike exclusively in street riding. The sag seems to be in the ballpark but there is a bit of a mystery about how much of the total travel I am using. I have the zip tie on the forks but I don't know for a fact if the travel is externally limited in the normal fashion, or internally limited because of the aftermarket internals. I remember when I put in the cartidges the Traxxion guys said the forks would be extended about 10 mm more than stock, so I had to raise them in the triples by that amount - that's what makes me think that bottom is not what it appears to be externally and maybe I am using more of the available travel than the zip tie data acquisition system is indicating.

I want to change the fork oil anyway so when I have them off the bike I will figure out what the actual total available travel is. The bike is lightish (maybe 410 lbs with half a tank) but I weigh 195 nekkid (sorry for the image) - maybe 235 in gear? Current spring rate is 0.925 (0.90 left 0.95 right). Does that seem on the light side for (lower) intermediate level track use at my weight?

I doubt the issue is damping. I set the rebound *just* slow enough to prevent the forks from falling into a second stroke when they are hot and you bounce them hard - i.e., they just rise and stop. Very unlikely they are packing down because of excessive rebound damping, especially in T3 where there are no bumps to speak of.

Possibly related to this, on occasion I have had bad front end chatter when braking really hard. Oddly (to me), the fix for this both times has been to simply add air to the front tire - it's now up to 32 cold / 35 hot, which seems high for these tires but any less and I get the chatter issue.
 
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YellowDuck said:
Possibly related to this, on occasion I have had bad front end chatter when braking really hard. Oddly (to me), the fix for this both times has been to simply add air to the front tire - it's now up to 32 cold / 35 hot, which seems high for these tires but any less and I get the chatter issue.
Click to expand...

This sounds odd. I've had trouble with too much air in the tire causing less of a contact patch on the road causing chatter. Going to less psi, shouldn't have this same affect.

As I understood it, too much air pressure causes you to have a very small contact patch. This small contact patch then holds you back from warming up the tires to operating temperatures. (keeping them colder)
Too little air pressure makes a larger contact patch but will also put more heat into your tire. Overheating the tire will give it a slick and greasy feel.
 
I think it is actually more the increased amount of carcass flex that gets the tire hotter, but yeah, normally less air = hotter tire I think. Like I said, it seemed odd to me too. It was Mark Van ...(sorry, for got his whole name) the Pirelli rep who suggested it. The first time, he said add more air and reduce rebound, which I did (reducing rebound makes sense to me - reduces the chances of packing down), and it worked. The second time it happened (different day) he just added more air, and that alone worked. Maybe the "bounce" in the tire carcass itself can cause the chatter, and by increasing the pressure you change the frequency of the bounce?

Like you, me not get. Again, not even sure this is related to my chirpies under cornering.
 
I was there as well (we spoke) our Ducs are very similar. I can tell you that when we first started racing it the front end was the worst most undersprung POS that I have ever used. It took a big bag of cash and some fancy machining of GSXR kit stuff to make it good.
My guess is that you were loading the front as that corner does and basically ran out of travel transferring all the dampening to the tire. It was still cold and a small front end skip is not out of the realm of possibility.
Get someone to look at the front, made a HUGE difference on mine. Bring lots of cash though. It was just under 1800 to sort mine. Night and day though.
 
kneedragger88 said:
I was there as well (we spoke) our Ducs are very similar. I can tell you that when we first started racing it the front end was the worst most undersprung POS that I have ever used. It took a big bag of cash and some fancy machining of GSXR kit stuff to make it good.
My guess is that you were loading the front as that corner does and basically ran out of travel transferring all the dampening to the tire. It was still cold and a small front end skip is not out of the realm of possibility.
Get someone to look at the front, made a HUGE difference on mine. Bring lots of cash though. It was just under 1800 to sort mine. Night and day though.
Click to expand...

Yellow Duck.
We met at the track while I was taking photos of you (dressed).
Your photos are posted in the "Gallery" of Trackday Hub

I think you are correct about the length differential and are now too steep in the rake and when you
are at the apex and between braking and acceleration the damping lets go and you get the chirps
until you get up on the tire.
Chirping under hard braking indicates you are using up all your internal travel with too soft a spring. Adding air to the tire also lifts the bike closer to the recommended rake and trail as a move in the right direction.


I have AK20s in my 2001 Gixxer 1k and have .925 springs with a personal weight limit of 185lbs on installation
for that rate. Start by dropping the air pressure to 30 cold and lift the forks up thru the triple clamps by a measurement of 4mm only.
The sag should be spot on (not ballpark) with the right springs. Take out all the compression and rebound to get
the sag right. If necessary change springs to match (maybe .950-.975)

I spent three days with Traxxion a few years back and can vouch for the fork internals.

Are you doing more trackdays this season and where?
Can work with you if you are in our neighbourhood.

Hope this helps
Best regards Gareth
Owner of TrackdayHub.com
 
kneedragger88 said:
Get someone to look at the front, made a HUGE difference on mine. Bring lots of cash though. It was just under 1800 to sort mine. Night and day though.
Click to expand...

Well, I already have $1500 in mine, so maybe if I just stuff another $150 in each fork tube they will be perfect!

Seriously, thanks very much for the constructive input - much appreciated!
 
Supertaff said:
Yellow Duck.
We met at the track while I was taking photos of you (dressed).
Your photos are posted in the "Gallery" of Trackday Hub

I think you are correct about the length differential and are now too steep in the rake and when you
are at the apex and between braking and acceleration the damping lets go and you get the chirps
until you get up on the tire.
Chirping under hard braking indicates you are using up all your internal travel with too soft a spring. Adding air to the tire also lifts the bike closer to the recommended rake and trail as a move in the right direction.


I have AK20s in my 2001 Gixxer 1k and have .925 springs with a personal weight limit of 185lbs on installation
for that rate. Start by dropping the air pressure to 30 cold and lift the forks up thru the triple clamps by a measurement of 4mm only.
The sag should be spot on (not ballpark) with the right springs. Take out all the compression and rebound to get
the sag right. If necessary change springs to match (maybe .950-.975)

I spent three days with Traxxion a few years back and can vouch for the fork internals.

Are you doing more trackdays this season and where?
Can work with you if you are in our neighbourhood.

Hope this helps
Best regards Gareth
Owner of TrackdayHub.com
Click to expand...

Thanks so much Gareth - first for the huge number of awesome photos, so many I almost got tired of looking at myself...almost... - and second for the great setup advice. The idea about the extra air in the tire changing the rake a bit had occured to me as well, but I didn't know if it was realistic, quantitatively.

I will defintely find out what the actual full travel is and how much I am using. As for changing the geometry, is there any important reason I should be raising the front with the fork position rather than lowering the back a bit? (the Penske has ride height adjustment independent of preload). It's a lot easier to make multiple changes and experiment on the rear than the front, obviously, especially on my bike where the shock is right out there on the left side, and ground clearance is not an issue at this point. Getting the roll centre lower might also make it easier to turn in, no?

I would LOVE to make one more track day this year, but I can't seem to make the schedule work out.
 
Oh, and gentlemen, what front sag should I be targeting? I think I am currently at 35 mm total sag (bike + rider) from fully extended, with room to go either way via preload adjustment.
 
YellowDuck said:
Well, I already have $1500 in mine, so maybe if I just stuff another $150 in each fork tube they will be perfect!

Seriously, thanks very much for the constructive input - much appreciated!
Click to expand...

..........Kneedragger88 - "It was just under 1800 to sort mine. Night and day though" $1800, wow........ 30mm Ohlins kit, springs, oil, installed for less ..... just say'n. BG
 
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YellowDuck said:
Thanks so much Gareth - first for the huge number of awesome photos, so many I almost got tired of looking at myself...almost... - and second for the great setup advice. The idea about the extra air in the tire changing the rake a bit had occurred to me as well, but I didn't know if it was realistic, quantitatively.

I will definitely find out what the actual full travel is and how much I am using. As for changing the geometry, is there any important reason I should be raising the front with the fork position rather than lowering the back a bit? (the Penske has ride height adjustment independent of preload). It's a lot easier to make multiple changes and experiment on the rear than the front, obviously, especially on my bike where the shock is right out there on the left side, and ground clearance is not an issue at this point. Getting the roll centre lower might also make it easier to turn in, no?

I would LOVE to make one more track day this year, but I can't seem to make the schedule work out.
Click to expand...

Remember it is a 5 to1 ratio when making changes. 10mm on the front equals 2mm on the rear which would lower the roll centre. 35mm is good if measured without any compression and rebound otherwise it is too soft. Give it a try.
Your photos of the zip tie and Traxxion telling you the kit is 10mm longer internally plus your description when heavy braking shows that you are packing the suspension before the end of the stroke.
Lowering your rear suspension might reduce your swinging arm angle and lose your sweet spot for traction under acceleration.

Can you test these options anywhere? (airport....industrial complex?)


Did Traxxion build your forks?
 
Supertaff said:
Remember it is a 5 to1 ratio when making changes. 10mm on the front equals 2mm on the rear which would lower the roll centre. 35mm is good if measured without any compression and rebound otherwise it is too soft. Give it a try.
Your photos of the zip tie and Traxxion telling you the kit is 10mm longer internally plus your description when heavy braking shows that you are packing the suspension before the end of the stroke.
Lowering your rear suspension might reduce your swinging arm angle and lose your sweet spot for traction under acceleration.

Can you test these options anywhere? (airport....industrial complex?)


Did Traxxion build your forks?
Click to expand...

Wow, another great post! More questions...

1. For clarity...the 5:1 rule...do you mean the effect on front *rake* is 5:1, or something else? That's new to me and seems very important so I want to make sure I understand.

2. Why does damping affect static sag? In *theory* the forks should return to wherever the spring tension and weight on the springs is in balance, no? Damping should just affect how long they take to get there. Is the issue that with higher damping, as the forks extend, eventually the spring tension isn't enough to push the oil through the valves, so the forks get stuck lower than they normally would? I will try it with the rebound wound right out and see what I get. I have no compression damping adjustment.

Thanks for the reminder about rear ride height and swingarm angle. Forgot about that. I will mess with fork height first!

Traxxion built the cartridges and supplied the springs and other internal bits, but I assembled the forks myself.
 
YellowDuck said:
Wow, another great post! More questions...

1. For clarity...the 5:1 rule...do you mean the effect on front *rake* is 5:1, or something else? That's new to me and seems very important so I want to make sure I understand.

2. Why does damping affect static sag? In *theory* the forks should return to wherever the spring tension and weight on the springs is in balance, no? Damping should just affect how long they take to get there. Is the issue that with higher damping, as the forks extend, eventually the spring tension isn't enough to push the oil through the valves, so the forks get stuck lower than they normally would? I will try it with the rebound wound right out and see what I get. I have no compression damping adjustment.

Thanks for the reminder about rear ride height and swingarm angle. Forgot about that. I will mess with fork height first!

Traxxion built the cartridges and supplied the springs and other internal bits, but I assembled the forks myself.
Click to expand...

1} Yes it is a ratio regarding effect.

2} Yes. You answered your own question. It is minimal with no compression adjustment although your internals are already giving you 10mm preload.

The combined lowering effect of both front and rear maintains the status quo of having a lower street bike but also reduces the trail on the front end and shortens the wheelbase giving it the sensation of a quicker turning bike but decreasing the stability.

Did Traxxion recommend pushing the forks 10mm thru the triple clamp?

What is the eye to eye measurement of the rear shock compared to the stock one?
 
Ben G said:
..........Kneedragger88 - "It was just under 1800 to sort mine. Night and day though" $1800, wow........ 30mm Ohlins kit, springs, oil, installed for less ..... just say'n. BG
Click to expand...

I hear ya Ben. Steve was all over it after the cartwheel at Road America before he passed. He brought the bike back from Sharrards with a ton of machining and a collection of Peris GSXR guts along with some Zx10 Kit springs that were chopped up. Along with a big bill. Gotta say it works well but that was what I said at the time as well.
 
Supertaff said:
Did Traxxion recommend pushing the forks 10mm thru the triple clamp?

What is the eye to eye measurement of the rear shock compared to the stock one?
Click to expand...

Yes, after I bought the cartridges and was installing them, the fellow from Traxxion told me he had "just realized" that the setup would cause the forks to be extended 10 mm from stock, and so I should raise them an extra 10 mm in the triples to compensate - which I did.

90% sure I adjusted the Penske to the same fully-extended length as the OEM Sachs before installation. You've given me a lot to think about and work on but I won't know the results until I can get it back on the track. In the meantime I'll verify the true maximum fork travel, measure front sag with minimum damping, and then based on the results think about upping the spring rate. Will let you know how I get along with it.
 
You sure the oil level(Fork collapsed, spring out.(as per traxxion))and viscosity is correct?

$1500 and no compression adjustment :(

If the fork isn't moving there is no damping.
 
suz1k3 said:
You sure the oil level(Fork collapsed, spring out.(as per traxxion))and viscosity is correct?
Click to expand...

Yes, absolutely. Set oil height as they recommended, and used the oil they sent with the kit.
 
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