8541Hawk

Time to talk bikes again.....

The place kind of feels like a swap meet lately.....

So why not share the latest set up stuff and unexpected results of it so far.

When I still had the stock suspension I was running the forks up 10mm, a .250" or 6.35mm ride height spacer and 16\43 gearing.

This worked pretty good. The major draw back is that it is rather easy to touch down the header in right hand turns. Also, IMHO, the ride always felt a little harsh with this set up.

So when I did the front end swap, one of the first goals was to get more ground clearance.

Buy my calculations, the SP2 forks with the cut top out springs end up to be around 3-5mm shorter then the stock forks (or equal to the stock forks pulled up 3-5mm)

I then went to a 10mm ride height spacer in the rear to get the rake and trail numbers around where I wanted them.

I also changed gearing,which lengthened the wheel base, which moves the CG forward.

Now the first step would be to look at what each set of changes does to the stock geometry.

I did cheat a little here as I had worked out all the numbers while setting up a friends Duc but as it had a rake of 24.3 with a trail of 97mm and the SH is 24.5 with 97mm of trail, I figured close enough.

Keep in mind that lowering the front ride height, or raising the rear ride height, are not equivalent adjustments. Lowering the front serves to lower the bike's center of gravity. With a higher front, raising the rear, raises the C.G.

So this is what you get:

for every 1mm increase in rear ride height:
Trail decreases 0.4mm
Wheelbase decreases 0.2mm
Height of the bike’s center of gravity increases 0.8mm
Percent of the bike’s weight on the front wheel increases 0.03 percent

The adjustment can be felt most in trailbraking - The bike stands up less on the brakes, which can be a benefit on backroads where you never quite know what might be coming up around the next bend. Also, some feel that the steering is more neutral at large lean angles.



Every 1mm that you raise the forks in the triple clamps (lowering the front end):

Trail decreases 0.2mm
Wheelbase decreases 0.5mm
Height of the bike’s center of gravity decreases 0.4mm
Percent of the bike’s weight on the front wheel increases 0.06 percent

So with this I was able to see that with the stock suspension I was running a set up that gave me a:

Trail decrease of 4.54mm
Wheel Base decrease of 6.27mm
The CG raised 1.08mm
and a .7905% increase in weight on the front end.

While setting up the new stuff, along with added ground clearance, I also wanted to raise the CG of the bike as I felt this might improve the ride quality and also add stability in turns.

The old (late '90s) Ducati factory race bike setup manual recommended raising the front 10mm to increase "flickability" in turns. Yes, I said raise, not lower. Raising the front end raises C.G., and a higher C.G. makes the bike go to the tire edge quicker.

Familiar with the Mille SP? It has the capability to raise the engine in the frame to increase C.G. to improve flickability. Same effect. Even the Mille R has the engine mounted higher in the frame to do the same thing.

Once in a turn, a higher C.G. biases the weight more to the inside of the corner which helps the bike turn.

So with this in mind, I left the forks as high as possible (I used -3mm for my calculations) and then raised the rear 10mm, like I stated earlier.

Which gave me these numbers:

Trail decrease 4.6mm
Wheel Base decrease 3.5mm
CG raised 6.8mm
and a .48% increase in weight on the front end.

So what do these numbers mean while riding?
With similar trail numbers the steering is still quick and light but with the higher CG the bike transitions from side to side quicker.

Add to this the longer wheel base, which increases stability and you have a bike that is very neutral when leaned over. In other words the bike holds it line until you tell it to do something else.

Now the difference in weight on the front wheel does look rather small but it was much bigger that I expected. In fact it took 5 turns of preload on the Ohlins shock and I had to lower the oil in the forks 30mm to get the sag numbers and bottoming correct.

The first thing I noticed was yes the front end will come up quicker but also most of the vibrations I had through the bars was gone.

The other unexpected thing I have seen is longer rear tire life.
This is opposite of what I expected with more weight on it. About the only thing I have come up with so far is that the add traction, due to the increased weight, causes less tire slip at speed.

Well that is enough for now.....

JamieDaugherty

Personally, I've found that trail numbers less than 100mm will inevitably cause you some grief. For the street it's ok. It sort of gives you a false impression of good handling because it reacts so quickly. The problem is that when you push hard, or during change of direction, it gets twitchy real easy.

You didn't mention if you factored in the RC51 triples. They have a different offset than your stock parts, which has a big effect on trail. Going with a set of 30mm offset triples really helps the VTR's handling. There is some balancing to be done, but I've found 102-105mm of trail is a good place to be. The challenge with the VTR is to get that without having ground clearance issues. I've found it's necessary to raise the rear just a bit and also the front as well. Keep in mind this is with 30mm offset triples, not stock. The stock geometry is kind of goofed so going +6mm at the shock and leaving the forks alone is about as good as it's going to get.

That's just my experiences. Of course there are so many other variables from tire brand, pressure, weather conditions, road/track conditions, etc. That's what makes it so fun!

RCVTR

+1 on the trail numbers. One of the best hings about going to RC51 or 929 triple clamps is that they have 5 mm less offset, which gets the trail back that you lose by lifting the rear and decreasing the rake angle.

Raising the rear has another very important effect on the chassis. It increases the angle of the swingarm, which causes a higher anti-squat force. this allows the rear end to remain unloaded - the anti-squat forces opposes the weight transfer. This in turn allows the rear end to remain active and supple when you are hard on the gas coming off corners.

8541Hawk

Thanks guys, I knew I had forgotten something when I started rambling yesterday.....

for every 1mm decrease in fork offset:

Trail increases 1.1mm
Wheelbase decreases 0.9mm
Height of the bike’s center of gravity increases 0.2mm
Percent of the bike’s weight on the front wheel increases 0.05 percent

So add this forgotten bit to the mix and I end up with these numbers:

Trail- increase by .9mm for a total of 97.9mm

Wheel base- decrease of 8mm for a total of 1422mm

CG raised 7.8mm

% of weight transfer to the front wheel -.73%


So with all these numbers I can see why I do like this new set up.

I also agree with the other 2 posts ( see I can change my mind even though I have been told that I never do....lol) in that you will get the best handling from leaving the forks at stock height, to keep the trail numbers and to also keep the CG as high as you can (in this case) and then get the chassis numbers you are looking for by raising the rear.

I have found that the suspension just works better (tracks better and also gives you a smother overall ride) and the bike is also more "neutral" when leaned over.