Ducati Sporting Club

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 15-Nov-2006, 04:06 PM
phil_h's Avatar
phil_h phil_h is offline
Registered Forum User
Ducati Meccanica
 
Posts: 2,527
Join Date: Sep 2004
Mood: 900,900,851,750,750,750, 600,600,450,350,350,250
Tire Size vs. Rim Width

I've just noticed that the fancy rear mag brembo on the beast is actually a 6in rim.
The 748 has a 5.5, and the steering feels pretty similar on them both.
I have thought in the past that I have no idea what the 851/888 guys are talking about when they say that the steering is a bit flighty ... and that would certainly slow down the steering action.
Anyone else running that size rim ?
Reply With Quote
  #2  
Old 15-Nov-2006, 04:18 PM
keefer's Avatar
keefer keefer is offline
Registered Forum User
Ducati Meccanica
 
Posts: 2,519
Join Date: Jul 2004
Mood: Found a CR250. Let the bone braking begin
I have a 6in rim
and run it with a 190/55
works fine for me
Reply With Quote
  #3  
Old 15-Nov-2006, 05:20 PM
Shazaam!'s Avatar
DSC Member Shazaam! Shazaam! is offline
DSC Club Member
Big Twin
 
Posts: 1,167
Join Date: Nov 2001
The size of rim to select is best answered after you decide what tire size you need. In other words you should use the rim width specified by the tire manufacturer to get the proper contact patch. The issue of tire size versus handling has been discussed extensively on this forum, so I'll try to sum it up.

Tire Size vs. Rim Width

Motorcycle tire sizes get larger from year-to-year so it's easy to think that bigger is better. However, chassis design development is the principal reason larger tires can be used to go fast, not the tire size by itself. So it's best to stay with Ducati's recommended tire and rim size, especially on a stock bike.

The size of the tire is selected by each bike manufacturer during development testing of a new design. The manufacturer develops their chassis and suspension system components in combination with the tires in order to balance wear, traction, tread design, stability and handling design objectives. When you bought your bike, you paid for a lot of development and testing costs, so think carefully about what advice to take when making changes. It often will come down to individual riding styles and rider preference so be critically honest about your own riding capabilities and needs compared to others.

When you modify your bike you often need to reconsider your tire needs. More horsepower generally means a larger tire is needed. If you decide to put on a larger tire, then check the rim width specified by the manufacturer of the tire. You should consider the manufacturers recommendation for rim size as the starting point in evaluating what's best for your bike's handling and your riding style. Put too wide a tire on too narrow a rim and obviously you changing the manufacturer's intended profile so the tire becomes more - or less - round, perhaps making it wear out faster in the center, roll harder, or make it more difficult to get over in the corners. But that aside, you can safely fit several tire sizes to the same size rim. Another related issue to consider is front-to-rear mismatching of profiles and changes to the ride height. Be prepared for some design development of your own since changes to suspension settings will also be beneficial when used with larger tires.

Some tire profiles are more sensitive to rim width than others. Dunlop, for example, says that their 180 slick works fine with up to a 6.25 inch wide rim. In the past, both Michelin and Dunlop have stated that a 5.5 inch wheel is suitable for 160 through 180 tire widths. Different tire manufacturers specify different rim widths for their tires.

So the question might be would you rather have more cornering grip along with a heavier turn-in and slightly lower top speed or have a quicker turn-in, less apex speed, but more speed on the straight? Getting the power and braking to the ground through the larger contact patch of a larger tire needs to be traded-off against the handling degradation experienced with a larger tire.

Ducati 74X/9XX's come with 5.5 inch rims standard, although some 916's did come with 6.0 inch rims from the factory. In order to get the correct tire profile, the recommended street tire size for 5.5 inch rims specified by the Pirelli and Michelin is 180/55 and for 6.0 inch rims it's 190/50.

The outside diameter of both size tires is the same so a switch won't require a rear ride height adjustment. The important difference is that the 180 is a 55 section meaning that it's height is 55% of the width cross-section. The 190 is 50% of it's width. This means that the 55-section tire has a steeper profile, it's taller.

When you mount a 190 tire onto a 5.5 inch rim it's profile becomes slightly incorrect. The too-narrow rim forces the tire's outer edges inward into a tighter curve so that you can't use this part of the tire effectively. A correct tire profile creates a correctly-shaped road contact patch essential to optimum handling, better sidewall stability with less tire flex and, and better overall tire wear.

When developing the suspension for the 916, Ducati had World Superbike racing in mind so when they sold models for the street they decided to mount 190/50 tires to 5.5 inch rims, a good combination for stable handling. It's been pointed out that WSB Ducati's then used 19/67 race tires, roughly equivalent to a 190/60 road tire. So, we got the wide tire look without the quicker turn-in handling characteristics of the 60 section race tire.

In the 1995 916 owner's manual, Ducati specified the 180/55 as an "alternative" to the 190/50 and the bike's under-seat specification sticker also listed both sizes as recommended.

It wasn't too long before buyers figured out that switching from the 190/50 to the 180/55 gave a very noticeable change in cornering feel. The 180's, mainly because of their taller, steeper profile, turn-in much quicker and easier. So eventually the word spread, and everyone who has changed to the 180's has praised its positive effects on handling. The lower horsepower and slightly lighter 748's ship from the factory with 180/55's.

Keep in mind that, as any street tire wears, the center section wears down more rapidly than the sides, so a 55 section tire will drop to an equivalent 50 section over the life of the tire. Consequently, the turn-in handling gets sluggish as the tire looses profile. This partially explains the rejuvenating effect that a new set of tires will have on a bike's handling, and will give you an idea as to the magnitude of the effect of switching to a taller section tire.

Most of the perceived handling difference people are attributing to size effects is due to the fact that they have installed a brand new 180/55 tire in place of a worn 190/50 on a 5.5 inch rim.

A 180 tire is also slightly lighter. This will account for part of the subjective handling improvement experienced when moving from a 190 section tire. The weight difference between brands is greater, especially for the front tire. For example, 120/70 front Pirelli Supercorsa's (8 lbs. 6 oz.) Dunlop D207RR (10 lbs. 7 oz.) A 2 pound lighter tire will reduce rotational inertia by the same order of magnitude that you get when switching from an aluminum to a magnesium wheel.

The 190 size is somewhat stiffer because of the shorter profile. This results in increased grip and reduces the tire carcass flex (better feedback), making accelerating hard out of turns less scary. Also, if you reduce tire size, with the same horsepower you're going to stress the tire carcass more. This however hasn't been a problem, even with the most powerful street bike models.

The bottom line here is that if you compare the handling of a NEW 180/55 mounted on a 5.5 inch rim back-to-back with a NEW 190/50 on a 6.0 inch rim you'll be hard-pressed to tell the difference on a stock bike ridden on the street.
Reply With Quote
  #4  
Old 15-Nov-2006, 05:48 PM
BDG's Avatar
BDG BDG is offline
Registered Forum User
Ducati Corse
 
Posts: 3,897
Join Date: Sep 2003
Mood: Daft as MartinH after too much Smirnoff Ice
Shazaam, could you explain that properly and elaborate a bit more please.

Larry, i hope you're both keeping well and hope we all get a chance to meet up again, that was a very pleasant night, all the best, Shaun.
Reply With Quote
  #5  
Old 16-Nov-2006, 09:56 AM
rcgbob44's Avatar
rcgbob44 rcgbob44 is offline
Registered Forum User
BSB Star
 
Posts: 5,984
Join Date: May 2003
Mood: My Jota goes rumpety rump! & I have my wife exactly where she wants me!
Shaun

I have to agree with you, his articles are to to short and also non descriptive to the point of being bland.

Still for one from the collonies he does try!

Welcome back Larry.
Reply With Quote
  #6  
Old 16-Nov-2006, 10:24 AM
phil_h's Avatar
phil_h phil_h is offline
Registered Forum User
Ducati Meccanica
 
Posts: 2,527
Join Date: Sep 2004
Mood: 900,900,851,750,750,750, 600,600,450,350,350,250
Thanks Larry (I think)
I think the pair of us'd bore the pants of people if we ever got together for a tach-jaw-session

So ... some more info ...
- there I am running a 180/55 on a 6in on the beast, and a 180/55 on a 5.5 on the 748, snotting them both to the edge at trackdays (well there is about 3mm of chicked strip ) pirelli on one and bridgestone on the other, and I think they both feel pretty similar (once theyre off the centre).
- and on the race bikes I have identical wheels and tyres on the ss and monster and they couldnt feel more different if you tried

Actually, the what I hadnt thought of was the stiffening of the carcass on a wider rim, which must easily make up for the loss of height when heeled over, but make it more prone to slides cos of the reduced contact patch which is what I was thinking about - but the 748 was the one that slid easiest
Reply With Quote
Reply
  
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes
Postbit Selector
Switch to Vertical postbit Use Vertical Postbit

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Recent Posts - Contact Us - DSC Home - Archive - Top
Powered by vBulletin 3.5.4 - Copyright © 2000 - 2017, Jelsoft Enterprises Ltd. - © Ducati Sporting Club - All times are GMT +1. The time now is 02:32 AM.