Topic: Getting a 17" rim, should I go 120/60 or 120/65 or 120/70 on the front tire?

[Foyil100]

Hi All

Just as the title says, I'm swapping out both wheels on my 1978 CB750 SS with 17" rims and am getting a 160/60 rear tire and was just curious if any of you had any experience going between 120/60, 120/65 or 120/70 for the front wheel or had any suggestions or opinions on the matter. I understand the 60/65/70 denotes the height of the side wall so being 6'3 my first instinct would be to get the taller one to avoid dropping the height of the bike too much but after reading a bit more I'm not too sure what to go with... The rims should be here next week so I'd like to go ahead and put the order in soon.

Thanks in advance!

[flybox1]

I prefer the 120/90-17
i'm 6'5"

[Foyil100]

I prefer the 120/90-17
i'm 6'5"

That's a beast of a wheel isn't it? I was looking at sport wheels (Michelin power pures)

[calj737]

I'd say the answer lies in what frint suspension changes you've made. Tire aspect ratios should ideally match the steering geometry design and the rim design.

Many modern sport bikes use 120/70 to provide better all around handling. A taller profile ('90) would diminish handling and stability some, but might better match the rear aspect.

Make any sense?

[Foyil100]

I'd say the answer lies in what frint suspension changes you've made. Tire aspect ratios should ideally match the steering geometry design and the rim design.

Many modern sport bikes use 120/70 to provide better all around handling. A taller profile ('90) would diminish handling and stability some, but might better match the rear aspect.

Make any sense?

Hey Calj,
As far as suspension is concerned, I've added a front end from a 2007 GSXR 750 so I'm thinking somewhere between the 120/60 and 120/70 would work best (not the 120/90) since more sport bikes use /60 or /70. The rear is also going to have the 160/60 so would a 120/60 match best? I've heard the 120/60 makes for a rougher ride than the 120/70 so maybe a 120/65 would be somewhere between the two (obviously)?

[calj737]

Using Devin's hub? I'd stick with the 70. It's a perfect match to the rear tire on many sport bikes. True, the 60 front has less sidewall and thus a slightly stiffer ride, but neglible. The 70 is widely available in great tire brands, so it would be my choice.

[Foyil100]

Yup, using Devin's hub in the front and a 76' cb750 hub laced to the Sun rim in the rear. Thanks for the input  sounds like 70 is the way to go

[MCRider]

you haven't said, or I missed it, what width rims you are using. I tried to put a 2.50-17 on my K2. It interfered with install and remove of brake caliper bolts. Had to go to a 2.15-18.

Rim width will determine range of tire widths.

[calj737]

If he's running a GSXR front end with Cognito hub, I'd suspect a 3.25x17-

[Foyil100]

If he's running a GSXR front end with Cognito hub, I'd suspect a 3.25x17-

That's correct, 17x3.25 in the front and 17x4.25 in the rear

[calj737]

Don't think you'll be happy with a 160 on a 4.25. I'd want a 5.50 or put a 140, max 150 on there. With as short a sidewall, the 160 is probably too wide. Best double check that tire/rim for fitment before ordering.

[754]

Oh..oh... I had a 150 on a stock 18 inch rim.

[Foyil100]

My apologies..My front is actually a 3.5x17 not 3.25.  Also, I spoke with Devin and he recommended putting a 160 on the rear wheel, he said I will most likely need offset sprockets and on his bike he uses a 160 and it just kisses the chain but not enough to cause any issues. I'll upload some photos when I get the wheel in and get it all mounted up.

FYI - a lot of supermoto bikes run 160 tires on 4.25 rims

Edit - also just found out that Husqvarna uses 160/60/17 tires on a 4.25 stock. Seems like there is a lot of back and forth on other forums as to if this will work on not..I guess I'll find out!

[MCRider]

http://www.mtsac.edu/~cliff/storage/gs/Motorcycle_Rim_Width_Tire_Size_Chart.pdf

Don't know about your SS. Lots of things start scraping at 140+ on a K.

[Foyil100]

http://www.mtsac.edu/~cliff/storage/gs/Motorcycle_Rim_Width_Tire_Size_Chart.pdf

Don't know about your SS. Lots of things start scraping at 140+ on a K.

That table confirms it will work and from my understanding the SS had a wider swingarm than the K. Can anyone confirm? Additionally, not sure why a 140 would scrape.

[MCRider]

http://www.mtsac.edu/~cliff/storage/gs/Motorcycle_Rim_Width_Tire_Size_Chart.pdf

Don't know about your SS. Lots of things start scraping at 140+ on a K.

That table confirms it will work and from my understanding the SS had a wider swingarm than the K. Can anyone confirm? Additionally, not sure why a 140 would scrape.

On a K, I know from first hand. My 140 does not scrape, but a 150 would scrape the brake stay. I moved my stay out a tad. Next (or before?) would be the chain case, then chain. I'm using a custom chain case. The chain would stop it at 160 I think. Need a lot of HP to justify more width.

[Retro Rocket]

If you are using a GSXR triple clamp you have added trail which will slow down steering response, then you are adding a 17 inch rim which speeds up steering response, then you have the frame with 27 degree rake, these combined rake, trail and wheel numbers are not ideal, the Honda was designed to run well with a 19 inch front wheel and 50mm offset onthe clamps, therefore a higher profile side wall {70} on the front tire would be more benificial than a lower {60} as it gets the rolling radius closer to the original 19 inch, although its still not ideal....  A 160 on a 4.25 is quite acceptable these days as the tires are designed to fit the 4.25 inch rim, most tire manufacturers have this on their fitment charts.....

[Foyil100]

If you are using a GSXR triple clamp you have added trail which will slow down steering response, then you are adding a 17 inch rim which speeds up steering response, then you have the frame with 27 degree rake, these combined rake, trail and wheel numbers are not ideal, the Honda was designed to run well with a 19 inch front wheel and 50mm offset onthe clamps, therefore a higher profile side wall {70} on the front tire would be more benificial than a lower {60} as it gets the rolling radius closer to the original 19 inch, although its still not ideal....  A 160 on a 4.25 is quite acceptable these days as the tires are designed to fit the 4.25 inch rim, most tire manufacturers have this on their fitment charts.....

Thanks retro for the feedback, I went ahead and got the 70 sidewall so hopefully that helps. I totally understand the whole 'not ideal' and couldn't agree more. That said, I have a trackbike that I use for trackdays and racing in the CVMA circuit here in SoCal so when it comes to specifics/geometry/setup thats my bike; this cb750 is something I bought to putt around town with my girl or go on short day trips down to San Diego, no plans to tear up any twisties on this old boy so I think 'not-so-ideal' just makes it into my 'i'm not worried about it' category 

Oh and yea I agree, after hearing this feedback and doing a bit more research I believe the rear 160 will do just fine. Pictures to follow in the coming month.