Darkside Question - Running a smaller than normal front tire

[RealAG]

Hi All:

I currently own a 1997 Valkyrie (GL1500C). I'm planning to take a trip to the "Darkside", but have a unique situation. Based on a normal situation, the front MC tire and rear CT should be as follows:

                                         Overall Diameter
MC Front Tire: 150/80-17 =    26.45"
Car Rear Tire:   205/60-16 =   25.70"

However, my Valkyrie has a smaller than normal front tire due to a low-profile fender. In other words, I needed to mount a smaller than OEM recommended front tire because the fender sits lower than normal...therefore, my bike tires are currently as follows:

                                            Overall Diameter
MC Front Tire: 120/70-17  =    23.61"
MC Rear Tire:  180/70-16  =    26.14"

As you can probably tell, this causes quite the problem when driving in a straight-line. I did some research (hence why I included the "Overall Diameter" numbers listed above) and found out that the NORMAL overall diameter differential between a front MC tire and a rear CT is 0.75" (26.45'-25.7'). In other words, the rear car tire should sit 0.75" lower than the front MC tire. Therefore, I concluded that I will need a rear car tire with a 22.86" overall diameter (23.61 - 0.75)....so.....I went to TireRack.com and started doing some trial and error searches for the correct car tire that yielded an overall diameter as close to 22.86" as possible. I found the following sizes:

175/50-16; Overall Diameter = 22.9":
http://www.tirerack.com/tires/TireSearchResults.jsp?width=175/&ratio=50&diameter=16&rearWidth=255/&rearRatio=40&rearDiameter=17&zip-code=

195/45-16; Overall Diameter = 22.9":
http://www.tirerack.com/tires/TireSearchResults.jsp?width=195/&ratio=45&diameter=16&rearWidth=255/&rearRatio=40&rearDiameter=17&zip-code=

205-45-16; Overall Diameter = 23.3":
http://www.tirerack.com/tires/TireSearchResults.jsp?width=205/&ratio=45&diameter=16&rearWidth=255/&rearRatio=40&rearDiameter=17&zip-code=

Questions for the forum: Does anyone have advice on which size to pick for my situation? Is it even possible to fit these smaller size car tires on the rear wheel? Or perhaps someone can recommend a better size CT??? Let me know if anyone needs more information on the matter....

Thanks!
-AG 

[Gryphon Rider]

Why not put on shorter back shocks to lower the back end by one inch?  I am assuming the straight line handling problem is lack of stability, i.e. it doesn't take much to bump it off of a straight line.  The cause of this instability is a reduced trail measurement, which in turn is caused by the bike being lower in the front or higher in the rear than normal.  Shorter shocks is a better solution than a smaller, lower profile tire for a couple of reasons:  1. A smaller rear tire means higher engine RPM.  Most Valkyrie riders would like lower RPMs at highway speeds, not higher.  2. A 45 or 50 series tire is more likely to have a very square profile, which is undesirable on a motorcycle, causing it to be more affected by ruts in the pavement tossing the bike about, and is more likely to exhibit bump steer in the corners.

I'm sure you're aware your smaller diameter front tire will cause your speedometer to read faster than stock readings, which are already a fair bit higher than actual speeds.

[Jess from VA]

I have run a 130 Avon Roadrider Bias rear on front, reversed on the rim.  It was slippery for 200+ and just OK.  I did not get great miles from a rear depth tread on front, and couldn't wait to change it.

I have run two 130 (65 instead of 70, about a half inch shorter than stock) Michelin Commander II Radial fronts.  They are great tires, sticking good in all weather from new with no release agent I could discover.  Still, a 130 just does not have the strength of a top quality 150 carcass, so I am going back to ME880 radial fronts, even though I can only get 8K from them.

I think going to a 120 on front may be too small a carcass.  Have you checked the load ratings for your 120s (for the heavy valk), and also how does it match up with valk rims?

No advice on the rear. 

[98valk]

changing the front or rear heights in relation to each by tire ODs or fork and/or rear shocks, what u are doing is changing the rake and trail of the front wheel. having the wrong R/T will affect handling at different speeds.

http://sportbike.natkd.com/motorcycle_geometry_101.htm

http://www.rbracing-rsr.com/rakeandtrail.html

[RealAG]

Thanks all for the quick replies. But maybe my post was a little confusing....I already have the 120/70 tire on the front wheel. The bike used to have a rear CT and rode like a dream, even with the smaller front tire. I took it to Honda and had them put a rear MC tire on, but they couldn't match the front OEM tire with the rear OEM tire due to a low profile fender that I have on my front wheel. So they just replaced the front tire with a 120/70 since it fits with the fender, but still put the OEM rear tire on the rear wheel. They never explained the theory of "rake and trail" to me so now I'm miserable running the small front with the OEM rear. Im constantly counter-balancing to keep it as straight as possible. So all I'm looking to do is put the rear car tire back on that rode like a dream.  Unfortunately, I have no idea if the old rear CT was a 205/60 (like it seems most people run on the rear wheel) or if it needed to be smaller to match the 120/70 in the front. Additionally, after changing the rear CT to a MC tire, I replaced my defective 11.5" rear shocks with aftermarket progressive 12" shocks. Perhaps the extra .5" could be a problem, I'm not sure if a 1/2" really matters.

In summary, I plan on leaving the front tire as is (120/70). It ran well in the past with a rear CT, so I'm really just wondering if a 195/45-16 rear CT will fit the OEM rear wheel. It seems like the balancing would be virtually identical to running a normal 150/80-17 front MC tire with a 205/60-16 rear CT...here's kinda a breakdown of my reasoning:

Normal people run a dark side Valkyrie as follows:
Front MC tire: 150/80 = 120mm overall diameter
Rear Car Tire: 205/60 = 123mm overall diameter

My bike is as follows:
Front MC tire: 120/70 = 84mm
Rear MC tire (proposed): 195/45 = 87.5mm

Now, keeping in mind that I'm using 12" rear shock absorbers, is there any doubt in thinking the the front and rear tires as described in "my bike" above would have significant issues cruising in a straight line?

Thanks everyone!
-AG

[RDKLL]

I have a 205/55-16 car tire and a 170/60-17 on the front...does that mean I am going to die a fiery death?

[Bighead]

I would put the correct fender back on and run the OEM size

[mustang071965]

195/45/16 will it fit, Yes . your RPMs will be higher but it fits

[Kunkies]

   Agree with BigHead