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Whats the biggest rear tire I can put on a 78 xs 400?


78XS400
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And will a 650 rim/tire fit? I have a 90 on the back right now....

tyre wise I've seen a 120 x 80 x 18but it is very close to the chain guard

I run 110 x 90 x 18 BT45 on the rear.

don;t know about the xs650 as it comes in all sort of guises

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A 90 size tire in the back is not safe at 65 MPH

not sure what you mean. the speed rating is what you need for safety at speed.

90 is the profile of the tyre.

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not sure what you mean. the speed rating is what you need for safety at speed.

90 is the profile of the tyre.

agreed!! the wide tire comment is just my opinion, but thankfully that fad seems to be on the decline.

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not sure what you mean. the speed rating is what you need for safety at speed.

90 is the profile of the tyre.

What I should have said is that at speeds above 70ish I start to get steering shimmy.

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What I should have said is that at speeds above 70ish I start to get steering shimmy.

what tyres are you running?

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Size or brand? 90 in the frt and back

size, 90 is just one figure.

i run 90 x 90 x 18 front and 110 x 90 x 18 rear both BT45

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Both my front and rear are 90/90/18

hahaha, there's your problem! spec for your bike's rear is closest to 110/90-18, If my memory serves me. That is roughly a 2" difference in overall diameter. Effectively, you've changed the final drive ratio of your bike. At 70mph, what do your rpm's look like? Seems like it could be an alignment or balancing issue as well. I'd also suggest using a very small amount of rear brake to help you get out of that wobble zone safely.

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shouldn't be any wobble. there's a mismatch of tyres there as mdsims says.

also check steering head bearings for notchiness and slackness.

what are your tyre pressures?

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x2 on checking the bearings.

Smaller tires alone won't cause a wobble but they do have an effect on performance, acceleration, rpm ranges (power band), top speed, etc. A 90/90-18 is even small as a front tire. 100/90-18 is where the front should be per spec. Running the smaller front tire is also going to cause your speedometer to be inaccurate.

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Thoughts:

-I run a ribbed Cheng Shin 3.00-18 on front and a Cheng Shin K70 3.50-18 copy on back, and the Green Machine tracked like a bullet all the way to Jacksonville and back (@1200 miles on the road).

-If you're running a tire designed for the *front* on the back, that will make handling SQUIRELLY to be charitable. The bike will tip in to corners quicker, and won't be as stable on straightaways. Some tires are rounder than others; the K70 is almost square in profile and thus is interchangeable. I know for a fact that Bridgestone Spitfires aren't interchangeable, for one.

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hahaha, there's your problem! spec for your bike's rear is closest to 110/90-18, If my memory serves me. That is roughly a 2" difference in overall diameter. Effectively, you've changed the final drive ratio of your bike. At 70mph, what do your rpm's look like? Seems like it could be an alignment or balancing issue as well. I'd also suggest using a very small amount of rear brake to help you get out of that wobble zone safely.

Now it all makes sense. My RPMS at 70 MPH are at 6700.

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shouldn't be any wobble. there's a mismatch of tyres there as mdsims says.

also check steering head bearings for notchiness and slackness.

what are your tyre pressures?

Pressures are 28 frt and 32 rear.

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Guys, I am about to order the correct size tires and need to know if I will need a new inner tube to go from a 90 to a 110 tire?? I dont even know if I have tubes or not, the actual tires say "tubeless" on them so I am assuming that I dont have/need tubes. My Haynes manual says my front AND rear tires should be 3.50 X 18 so the closest to that is the 110/90/18 tire. I just ordered a new one off ebay for the back for $60 shipped. I think I am going to keep the 90 on the front as some websites that list the "specs" for these bikes has the front as a 3.00 inch which is a "90" size tire for the front.

Drewpy what size tire do you have on the front?

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Guys, I am about to order the correct size tires and need to know if I will need a new inner tube to go from a 90 to a 110 tire?? I dont even know if I have tubes or not, the actual tires say "tubeless" on them so I am assuming that I dont have/need tubes. My Haynes manual says my front AND rear tires should be 3.50 X 18 so the closest to that is the 110/90/18 tire. I just ordered a new one off ebay for the back for $60 shipped. I think I am going to keep the 90 on the front as some websites that list the "specs" for these bikes has the front as a 3.00 inch which is a "90" size tire for the front.

Drewpy what size tire do you have on the front?

see one of drewps previous posts, he runs the same as you, 90/90-18. That size tire will be fine for the front. I'd suggest getting new tubes every time you change tires. They are too cheap and too important not to change ($6.00/each). As for not needing to run tubes, yes you SHOULD run tubes. The tire may say tubeless but unless the wheel itself says tubeless you need to run them. Tubeless rims are made with different lip shapes to help a tire bead hold on the wheel better.

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