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Geometry question

G

guzzirider

Cruisin' Guzzisti
Joined
Nov 2, 2008
Messages
160
When I converted my Jackal into a cafe racer, I made the following changes to geometry:

Rear wheel diameter increased from 17 inch to 18 inch (to match 18 inch front)
Forks shortened and dropped through the yokes.
140/70 18 rear tyre (standard is 140/80 17)
110/80 18 front tyre (standard is 110/90 18)

Now first the good news- my back of a fag packet calculations have worked out in as much as the bike now steers very sweetly, and is still very stable even though she is now more poised and "pointy" just as I wanted.

And the bad news- loads of ground clearance but I am very close to running out of front tyre which is worn to the edge, but with half an inch left of rear tread unused. This is making me a little reluctant to push too hard just in case I lose the front.

My question is- do you think going back to a higher profile 110/90 front tyre will help, and allow me to get the most out of the rear tyre?
 
I think the larger front wouldn't hurt, but I'd try a 130/70 X 18 on the rear. Sometimes going down a size will improve things.
 
Tires should be sized to rim width. With a properly matched set of tires running the correct air pressure you should run out of tire at both ends at about the same time. If your rear tire is a little wide for the rim width it can pinch the tire in and cause a change in profile. That can lead to the symptoms you describe. Changing either the front or the rear tire is typically the solution, but with out knowing the width of your rims and the brand of tire you are using it is just a guess which way to go.
Too high an air pressure in the front compared to your air pressure in the rear can contribute to those symptoms as well.
 
Thanks for the replies- the tyres are the standard widths for the rims- I'm running Pirelli Sport Demons which are about the stickiest tyres you can get for narrow rims. They do have quite a raised profile which I guess speeds up steering.

Tyre pressures are 39 PSI rear and 36 front. Think its worth dropping a few PSI in each tyre and see what happens?
 
guzzirider said:
Thanks for the replies- the tyres are the standard widths for the rims- I'm running Pirelli Sport Demons which are about the stickiest tyres you can get for narrow rims. They do have quite a raised profile which I guess speeds up steering.

Tyre pressures are 39 PSI rear and 36 front. Think its worth dropping a few PSI in each tyre and see what happens?
Click to expand...
I don't run air pressure that high in any motorcycle I own, but I realize that many people do run pressures that high.
Since the issue is the rear tire not using all the tread surface if you wanted to try adjusting the air pressures I would drop the rear a one or two psi or add one or two psi to the front to see if that helps. Bias ply tires are very sensitive to air pressure.
Personally, in my Griso, V11, and Daytona, I run around 34 to maybe 36 in the back and 32 in the front. I am not telling you to do what I do, just mentioning it for your info.
And what are your rim widths?
 
Since you started with a Jackal, the recommended pressures for that bike are 33 psi front, 36 psi rear.
 
Well its an easy thing to try so i'll give it a go. Saything that I've been running those pressures for years with no problems, but I'll see what happens.

Rim sizes are 2.50 front, 3.50 rear. Tyres fitted are same width as OE.
 
Based on your rim widths, your rear tire is a little wide for the 3.5" rim. It is within allowable specs, but it is the narrowest rim that tire is recommended for. Your front rim is right in the middle of the recommended rim width range (and is probably the ideal rim width for it). A 140/80-17 and a 140/70-18 Pirelli are not made for the same width rim. The 140/70-18 is made for a wider rim. That may be contributing to the difference in profiles between your front and rear. Adjusting air pressures may help, but a 130 rear tire may be your best answer.
 
Here's the Avon rim/tire guide: http://www.avonracing.com/club_page.aspx

I have a 3.5 rear rim on my racer and use a 130/70/18. According to Avon that is perfect.

I would suggest going to a 130 rear too. On my street bike I run 100 front and 110 rear and it handles beautifully so I'm a big fan of skinny tires.

My opinion only, not trying to be a know it all. :D
 
Thank you for your replies- yes I think I may go for a 130 next time around once I have worn out the current Pirelli.

Looking at profile of the Sport Demon, it is very pointed, which I guess speeds up steering but a tyre with a slightly less radical profile may also help.
 
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