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Tire "Height"
#1
Hi All - I've been into bicycling and repair since I was a kid, but I've been away from it for a while. But as they say getting back into it is "like riding a bike". hehe - sorry...

Anyway, I have and 1992 Trek 950 that has a Manitou Magnum front shock. The original tires (Matrix 26x2.0) were working well, but the rubber is old and cracked and needs to be replaced. I never had any issues with clearance on the front fork/shock.

As I am currently living in Taiwan, I had to go local - I bought a pair of Kenda Blue Groove tires which seem quite nice. They are also labeled as 26x2.0, but the sidewalls are much higher than the original Matrix tires, and as a result the outside diameter of the tire once mounted is bigger. Now the tires are rubbing up against the front fork which obviously is a deal-breaker.

My question is have tires changed so much as that they all have bigger sidewall height, and thus will present a higher profile on the rim? I have't seen any measurement or labeling that would clarify the tire's "height", so finding a replacement for the smaller height Matrix tires now becomes problematic.

Any thoughts and suggestions would be apprecitaed.

Eric
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#2
From Sheldon Brown

"Dishonesty in Sizing

Competitive pressures have often led to inaccuracy in width measurement. Here's how it works: Suppose you are in the market for a high performance 700 x 25 tire; you might reasonably investigate catalogues and advertisements to try to find the lightest 700-25 available. If the Pepsi Tire Company and the Coke Tire Company had tires of equal quality and technology, but the Pepsi 700-25 was actually a 700-24 marked as a 25, the Pepsi tire would be lighter than the accurately-marked Coke 700-25. This would put them at a competitive advantage. In self defense, Coke would retaliate by marketing an even lighter 700-23 labeled as a 700-25.

This scenario prevailed throughout the '70's and '80's. The situation got so out-of-hand that cooler heads have prevailed, and there is a strong (but not universal) trend toward accurate width measurements."

Since it's an early nineties bike, this could have happened as well. Your old tires may not have been as wide as reported to keep the weight down. This era is well documented for gram shaving, so it would make sense.

Just a guess though.
Dedicated scholar of bicycles
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#3
First time I ever heard of this problem but it would seem to make sense. I emailed Kenda to find out if there is any special instructions of installing in case it was not put on properly. I did find this link http://www.kendausa.com/bicycle/JohnTomac.html . Your model is near the bottom. It has a folding bead, which I am not too sure of but will find out more information when they email me back. To answer your question about tires changing in the bike world, yes they have different tread for different applications. Very competitive market since the '90s. If you like you can email them too and explain your situation.
Good maintenance to your Bike, can make it like the wheels are, true and smooth!
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#4
Hi JR14 - yeah, I saw that post via Google too. While it could be possible that the earlier tire was not properly labeled, I don't think the tire was so far off from 2.0 in width to cause this issue. The issue is more of sidewall height rather than tire width. But I guess the only way to know for sure the old tire's height and width is to put it back on the rim. I was hoping to avoid that ;-)

Hi Bill - thanks for your mail. Yeah I cruised Kenda's website as well. Unfortunately there is nothing there about sidewall heights on their tires - just dimension and width. But even a newbie can eyeball the new vs. old tire and see that this new tire (I don't want to put it on Kenda, this may be something that happens with all tires these days, and this is what I hoped to understand) has higher sidewalls, thus the outside diameter of the tire is much bigger that the early 90s tire.

As mentioned, the issue is one that my front fork/shock is now rubbing the tire once the shock compresses, which is of course dangerous. While I don't have the specs from the service manual, I think the stated minimum clearance on this shock is around 73mm or so. But with the new Kenda I only have 40mm when the shock us uncompressed.

I guess the simplest question would be does anyone know of any good modern all-mountain tire that's 26x2.0, but has a narrow(er) sidewall profile, thus I can keep the shock in place. As an alternative I guess I can try a modern 26x1.50 - maybe that will approximate my old early 90s 26x2.0 :-p

Thanks all!

Eric
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#5
Please allow me a moment to share with you a "moment of the blazingly obvious" - at least for you guys who are older that 40. For you young whipper snappers out there this could be a bit of a history lesson and give you insight into the roots of my problem above...

My Trek 950 is of a classic design. Produced in the early 90s, the top tube was pretty much parallel to the ground. So for anyone wishing to have a front shock in the 90s, the overall length of the shock and the amount it could compress were somewhat limited because if you make the shock longer than the original stock fork, that would mean that the front of the bike would be too high and it would be like you are driving a chopper.

As I get back into the sport, I am seeing that most bikes these days have a top tube that is angled upward toward the head tube. This allows the head tube to exist in a much higher off the ground than it did in the past, and this allows you to make shocks that have a much deeper compression (and thus more clearance too between the tire and the top of the fork).

I can't say that I am up to speed yet on why the frame geometry has changed over the years, but obviously one benefit is to allow for much more clearance and compression on the front shocks. And obviously this allows the sidewalls to be taller and for the outside diameter of the tire to be bigger too.

This is somewhat concerning as it appears that most tires being offered today are of this high sidewall design. Owners of late-model bikes don't care because their front forks/shocks have the clearance, but my early 90s fork needs a tire with a smaller sidewall height to accommodate the clearance issue.

Anyway, don't want to go on this point given that the topic probably belongs in a different section of the forum, but as you can see the deeper I look into this issue the more problems I may have. I can't easily find lower profile tires, and replacing the shock seems out of the question, as it won't fit my horizontal top tube geometry :-p

Eric
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#6
The top tube slope is not really an indicator of geometry, in my opinion. What should always be measured is the horizontal length (and if you look at the bike specs of most brands, that's what they do), they call it something like "virtual length", Dan Empfield calls this measuring "centre to air". What is important (for sizing / fitting) is the "reach" (horizontal length) and the "stack" (how high the head tube end is above the bottom bracket) of the frame. The fact that the head tube seems "higher" to you is due to two reasons: the length of the head tube has decreased and integrated head sets don't add as much height (to the top of the steerer tube). So in all the lower end of the head tube has moved upwards quite a bit.

I'd say concerning the sidewall height: The cross section of an inflated tyre is roughly circular, except for the "studs", the tread pattern reaching "up" but not as much "sideways" (the sidewalls themselves have (hardly) no tread pattern) and the part where it contacts the rim. So I'd not expect two tyres with the same real width to have (much) different sidewall heights.
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#7
Hi Joe_W - on the point of geometry, you might be right with regard to measuring for sizing/fitting, but my point was regarding how this change in top tube angle has brought the front tube higher on most modern bikes (higher being height off the ground). This additional height seems to allow greater length/compression/clearance on today's front fork/shocks vs. that available to the shocks of the mid 90s. This causes complications due the tire sizing (change in measurements/cross section) that is available today.

The implication of these changes means that either 1) I need to change my front fork/shock, but I can't because newer models may be too long and cause my front end to be too high, or 2) I need to find a smaller outside diameter tire, thus giving me back the clearance on my existing shock. But since the tire sizings have seemed to have changed I may be SOL here too.

To illustrate my point, I found a pic of a Trek 950 (not mine) on the net.

http://img123.imageshack.us/i/p1000444sdl8.jpg/

You can compare this to a reasonably similar Trek 4300 on their website.

http://www.trekbikes.com/us/en/bikes/mountain_hardtail/4_series/4300disc/

You can see by comparing the two images how much clearance there is in the newer bike, yet the rims are both 26" and the tires are supposedly 2.0" wide (and labeled as such on the sidewall). Now given that the only thing that changed on my bike was I took my vintage 90s tires off and put new Kendas on, only a tire with a larger outside diameter can make the shock bottom out on the knobs. Call it what you will, but if we assume for a sec that the actual width of both tires is reasonably close to 2.0", then the only thing that could account for a larger outside diameter would be taller sidewalls.

Note: I just received the service guide from Manitou. It states the minimum clearance is just 54mm. As mentioned previously my current clearance with the new tires is 40mm, so I need at least another inch of clearance to be safe :-(

Eric
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#8
Different tires do have different profiles and some may be genuinely taller than others. However, I'm a little concerned by your measurements. You state that you need nearly a full inch more clearance than the new tire gives. Most knobby tires I see are about 2" tall. Therefore, your old tire would be about 1" tall? That seems very short for any common knobby tire. Are you sure that your fork is not bottoming lower than it is supposed too? Maybe some of the rubber elastomers inside have split (I think the old Manitous used rubber springs).

I know tires vary, I'm just having a hard time visualizing that big a difference between tires rated the same size. And I've never noticed any general "older tires have lower profile" trend.
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#9
Hi DaveM - thanks for your note. I decided to get empirical and put the old tire back on the rim so I can quell any doubt as to sizing. This is what I found (mounted on my 26" Matrix rims):

New Tire (Kenda Blue Groove): Width 51mm/2.1", Height (from rim to top of knob) 50mm/2.0"; this gives me an unweighted clearance of 36mm/1.43" between the top of the knobs and the bottom of the fork's crown.

Old Tires (Matrix circa '92): Width 48mm/1.87" (advertised as 2.0), Height (from rim to top of knob) 44m/1.75"; gives a clearance of 43mm/1.75" between the top of the knobs and the bottom of the fork's crown.

The service manual spec on the Answer Manitou Magnum fork requires a 54mm clearance.

With the Kendas on the rim it was very easy to bottom out the fork. I just need to push down with my 220lb frame and it will rub. With the old Matrix tires it doesn't bottom out, at least not easily. I jumped up and down on it but it didn't rub. It's possible it might with a significant drop, but I'd rather not test this one right now ;-)

So even though the old Matrix tires produced a less-than-spec clearance, it seems for normal street and trail riding it's ok. I never had a problem with it. There is a difference in 7-8mm clearance in the two tires, and I am guessing the bottoming out point for that fork is somewhere in that zone.

I'm guessing at this point the best thing for me to do is search around for tires that have an confirmed measurement of 26x1.8 or so. It doesn't sound like much, but it's possible the lower profile will just allow me squeak in under the limit. I am still a bit concerned that the spec'ed clearance isn't being met, so I guess I'll just relegate this bike's riding to street and easy trail.

Eric
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#10
So your old tyre is not as it was specced. As I said: tyres are almost circular, that is, as wide as they are high. You have to get a tyre that matches the old one.

Concerning the geometry: If the stack is the same, the headtube is at the same point above the bottom bracket. The slope of the top tube has nothing to do with that. Your pictures can serve as a good argument there: The saddle is about the same height above the handle bars on both pictures. So you could just run the top tube horizontally on the modern bike, without running into problems with adjustability. I first thought "well, maybe the bottom bracket hight is different", but judging from the pictures it isn't (too much).
What has changed is the maximum length of the front shocks. Along with the longer forks came the possibility of not only locking them but also adjusting the height to make climbing easier (or even possible). What has changed due to slope of the top tube is the seat post length, on modern bikes the seat post is much longer. They now offer extensible seat posts so you can move the saddle down in technically difficult passages when it is only interfering with riding.
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