Tire Size for '77 750

[TheNose]

Help! I'm confused. I need new tires for my '77 GS750. The tires on the bike are 3.25/85H19 in front and 4.00 18 in back. The specs I found for this bike on another site are the same, except they have 4.00H18, 120/18 for the rear. This doesn't compute - 4.00 x 25 = 100, not 120?

The JCWhitney catalog says even stranger things. My question is, what size tires should I buy? I'd like to stay close to stock, but I really don't have any idea what will work.

Thanks,

Terry

[robinjo]

According to the original sticker on my '78 750 the correct sizes are 3.25" front and 4.00" on the rear, I have a 110/90/18 on the rear that seems to work fine.

[Anonymous]

i had a 4.50 on back on the 78 750 it fit just fine but was rotted so i went with a the stock 4.00.

[Anonymous]

I found it hard to find standard measures, so I went 100/90r19 front, and 110/90r18 rear. it's about the same as before, although I didn't have stock rubber I was replacing.

[Anonymous]

I was going to post the same question when I found yours. I have 325H19 in the front of my 81 GS750E, and it's the tire I really want to change, but this size doesn't seem to compute much except for a small handful of tire types. Is there are metric equivalent to this size or is this dangerous? And is replacing a tire yourself similar to bicycle tires, just bigger, or do you all take them to a shop and then get the wheel balanced too? Plus, what's the difference between the H version and the V version of tire?

Sorry Nose for adding to your post, but it seemed the place.

Roger Moore

[Anonymous]

Right, here goes:

1. The metric and imperial sizes do not directly cross over, i.e. a 4.00 is not equivalent to a 100.

2. The correct metric sizes for a GS750 are 100/90-19H front and a 120/80-18H rear. A 110 is too small.

3. The H and V are speed ratings, H up to 130mph, V above 130mph. This is the capabiltiy of your vehicle, not the speeds that you actually travel at!

4. The 80 and 90 are aspect ratios, i.e. the 100/90 is 90% high as it is wide.

5. We went metric with tyre sizes over here about 20 years ago, its now very difficult to get 4.00 and 4.25 tyres in a good quality tyre (but who'd want to?) You'll get a better tyre in a metric size. Doesn't matter for most of you guys, but we get lots of rain!!

6. Unless you're very good, I'd take your wheel to a tyre specialist and get them to balance the wheel as well as fit the tyre.

7. No its not dangerous to use a metric size, in fact quite the reverse. The US is about the only place in the world still using the old sizes. Hey, even we dumped the imperial system, and we invented it!!

[Anonymous]

Awesome! This helps me a lot... and hopefully Terry too. So many more choices available now.

RAIN -- heck, I live in Seattle, the wettest spot in all the country this year. That's why I want to change that front tire. It's old and weather checked from UV, besides being an odd, "old fasioned" tread design. Looks like the tread pattern on my car instead of a bike pattern (flatter instead of rounded. Doesn't corner nice going slow). It's a Metzeler tire though.

Roger Moore

[TheNose]

Thanks brit7.11, that's exactly what I was looking for. It doesn't make sense that the tire sizes don't cross over directly. That's what was confusing me. I do agree with you that we Yanks should change to the metric system. It would make things so much easier. Too many Americans think they would have to convert everything to metric, instead of just forgetting about the Imperial system and learning metric.

Now, all we have to do is to get you Brits to learn to speak and spell English words correctly. (schedule,tyre).

Roger, I was in San Antonio last month when they got 19 inches (475 mm) in a 24 hour period. I'll bet it beat Seattle for being the wettest spot in the country on that day.

[robinjo]

If the correct size front itre is 100(mm) then why is my 3.25"front tire 89mm wide??? wouldn't a 100m tire become to sqeezed together at the rim and make it turn unstable?[/quote]

[Anonymous]

19" is a lot, even for here. Being a Texan 40 years removed, but still with roots there, it's been pretty crazy for them I've heard. I doubt any tires can shed this much water. :?

[Anonymous]

If the correct size front itre is 100(mm) then why is my 3.25"front tire 89mm wide??? wouldn't a 100m tire become to sqeezed together at the rim and make it turn unstable?

Not all 3.25 tyres are 89mm wide, this would vary from maker to maker. Metric 100mm wide tyres area different profile and are designed to be wider, its not just like fitting a wider tyre (or tire!! ) The imperial sizes were an older profile. I.E. same size rim, imperial size tyre is narrower and taller than metric.
You are right though to say that fitting an excessively wide tire to a rim distorts the profile and could lead to handling issues. A metric 100/90 tyre is designed for the 2.15" rim.

[Anonymous]

OK, Now I'M confused. I was told a 100/90 - 19H tire would be correct to replace my 3.25 x19H tire. Is this still true, or is it true if I only get a particular brand of tire?

Roger Moore

[Anonymous]

Hey Roger Moore,

If you want a personal recommendation, go with the Dunlop 491s. I have used them for year around riding and the stability & traction is great under severe weather conditions. I've even ridden with them in black ice and while things were squirrely (to say the least) I was able to maintain enough control to get me safely to my destination.

[Anonymous]

I was able to maintain enough control to get me safely to my destination

Well, I think that's what we're all shootin' for

I have a good (metric) Metzeler on the back, so I was hoping to go with the matched tire of its type (ME 33 I think), or something similar. But (giving myself away) I'm not one who has to ride on the bike in those conditions, so I don't.

Two of my co-workers commute every day on ferries here in Seattle, so they always ride their bikes to defray the cost of the commute. But personally, I think they're nuts! :P I've got enough vehicles that I could drive each once a week and never use the same one twice -- if I include my bicycle, which I do commute on if I need to.

I like the motorcycle when it's nice out, but with crazy Seattle drivers (they are the worst :twisted: ) -- black-ice is the least of my concern when it starts getting crappy around here.

Cheers,

Roger Moore

[Anonymous]

Yeah, I've heard that Seatlle traffic is nothing short of horrible, even in the best of conditions. I lived in Kirkland 20+ years ago and even then I took the bus everywhere because the traffic, even then, was just too much for the streets to handle.