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True Temper frame Trek 400 larger tire advice

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True Temper frame Trek 400 larger tire advice

Old 04-18-20, 10:25 AM
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RH Clark
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True Temper frame Trek 400 larger tire advice

Picked up an older frame Trek 400 steel frame road bike for $20 at a thrift store a couple months back. It had 700c x25mm tires that had bad dry rot. I did ride the bike and it is fairly cool with an elliptical 3 ring front, and 9 speeds rear. It's a Deore rear shifter, friction front, with indexed controls on the down tube. I ride quite a lot of dirt and would like tires more suitable for improved forest roads if possible. I ride those roads now on my Surly LHT but would prefer to spare it the sand if I get this 400 to work in the gravel.

Now here's the story of what I did already in an effort to get something accomplished in this pandemic. I live a couple hours drive one way from a bike shop and getting anything done right now is really hard with the virus going on. I went to my local Walmart and bought some 700-c Hybrid tires to try. I will be taking them back because they are too big but I needed some info on size. What I bought were Bell 700X32-45C Reflective Hybrid Bike Tire. I did fit the front and attacked it to the bike and it came close to fitting. I could pull it out of the bottom of the fork drop out and get it to spin freely and the brake to work. It was that close. I even rode it around the driveway like that but didn't like the idea of less than full contact with the locking nut in the drop out and abandoned the whole idea. I thought I might grind the bottom of the fork a little to get it to work but contact with the brake body turned out to be the limiting factor when fully seated into the drop outs. That huge tire would nearly work. It would turn all the way in the drop out but rub hard against the brake body.

I went through all that explanation on those too large tires to perhaps give ya'll a better idea but I could post some pictures if needed. I also hoped someone would be familiar enough with a Trek 400 Chrome Molly frame ,True temper brand ,made in USA Trek to have a good idea of tire size that might fit and do better on gravel.
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Old 04-18-20, 11:29 AM
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You have to have the axle fully in the fork ends or properly in the drop outs for anything you want to consider safe to ride. You also what to have a clearance between the tire are whatever part of your bike it comes the closest to. Wheels and frames can flex. So when you are doing something that makes them flex you don't want the wheel to suddenly jam even if it is only the briefest of moments. What that clearance is can be variable depending on the ride conditions and load on the bike. Most like 4 mm to 6 mm minimum or just about a 1/4 inch.

The title of the tire you bought is not an exact tire size. You need to look on the tire and find the actual width. Look for a number that is embossed or stamped in the sidewall, something like 40-622 or maybe 700 x 40 or some other numbers in that format. The smaller number will be the width of the tire and then you can maybe guesstimate what smaller size you should get to try. The height of the tire will get smaller with the width.
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Old 04-18-20, 11:42 AM
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RH Clark
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Thanks for the input. The size on the actual tire is 700X38c. I knew the wheel needed to be fully seated in the drop outs. I was only playing with it to give me a better idea of what might work. I wouldn't ride with it half way seated. I was only playing around at walking speed.

Any idea how large a tire I might get away with seeing that the 38's just darn nearly fit?
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Old 04-18-20, 11:58 AM
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Before you take them back, look on the box or the actual tire and see what the size really is. Bell's 32-45 is a replacement range. I saw a picture of one and the box had 38c, but the tire will tell you as noted above; xx-622.

Obviously re-mounting you 25c tires and measuring is the best way to know how much clearance you have.

John

Edit added: Didn't see your post. If 38c is too tight, you might get away with a 32c, but real width/height varies by manufacturer. I tried a similar exercise and ended up with Kenda Karves in a 28c. Might be a tad narrow, but they will probably work and priced for a thrift store bike.

Last edited by 70sSanO; 04-18-20 at 12:02 PM.
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Old 04-18-20, 12:13 PM
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One thing to keep in mind is that if you ride on unpaved surfaces your tires will pick up bits of grit. Having just enough space to clear with clean tires is far too little clearance for gravel riding. Reinstall your 700 x 25 tires and measure how much space you have at every place the tire comes close to the frame and fork. My guess is that 700 x 28 may be the biggest size you could mount and have sufficient frame clearance for unpaved roads. Aim for 4-6 mm tire clearance
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Old 04-18-20, 12:13 PM
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A true 32c should work but anything much larger is unlikely. If you want a larger volume tire, you need to think about a 650b conversion but that will cost some money.
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Old 04-18-20, 12:43 PM
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If there are other sizes at one of the big box stores you can get and return, then just keep that ISO sizing number in mind. Since you said you had to drop the tire in the fork end to make it work, then that's probably 4 to 6 mm there.

So conservatively 38 mm - 6 mm you may have dropped in forks = 32 mm. Then assuming you are a light rider and don't routinely jump stumps lets use 4 mm clearance. So 32 mm - 4 mm = 28 mm tire. Which quite happens to be what a lot of 21'st century road frames can handle when not pushed to the absolute limits.

If the tire has an aggressive tread, then that is usually extra mm that is not accounted for by the size. I think size is to the casing, but not certain.

edit....
oops... forgot we're working with one side ..... or essentially radius. So half the numbers 38 -3-2 = 33 mm. Tire height might still get you as I think it's going to be more variable as different tires will have differing thicknesses of tread compound on them..

Last edited by Iride01; 04-18-20 at 12:56 PM.
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Old 04-18-20, 02:13 PM
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Congrats on the score! Those are very nice frames, made in the US from high end US tubing. Trek changed specs a moderate amount so I'm not going to make too precise a guess at the exact tire clearance. You can measure from a known fitting tire to the closest clearance problem. You can use double that distance minus about 6mm or so for safe clearance. Completely guessing, but 32mm is probably safe--I'd measure if trying to go larger. You can also potentially get 650b wheels/tires and use longer reach brakes.
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Old 04-18-20, 02:42 PM
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I think maybe I'll try a set of Kenda commuter 28mm tires at about $21 each unless anyone has a better suggestion. Not as big as I would like but I think very likely to fit. The bike was a hoot for me with rotten 25's. I've enjoy it even if it will never be a real gravel grinder.
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Old 04-18-20, 03:13 PM
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If you are going to do any gravel the Karves are a better tire for that with a shallow knobby pattern. I've used it a lot on pavement and there are no issues except they may not last as long, but go with what you want.

John
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Old 04-18-20, 06:16 PM
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Originally Posted by 70sSanO View Post
If you are going to do any gravel the Karves are a better tire for that with a shallow knobby pattern. I've used it a lot on pavement and there are no issues except they may not last as long, but go with what you want.

John
I appreciate the advice. I was just looking for something cheap that might work. I'll look into the Karves. I don't know anything about tires so all advice is appreciated.
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Old 04-18-20, 10:44 PM
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Originally Posted by RH Clark View Post
I think maybe I'll try a set of Kenda commuter 28mm tires at about $21 each unless anyone has a better suggestion. Not as big as I would like but I think very likely to fit. The bike was a hoot for me with rotten 25's. I've enjoy it even if it will never be a real gravel grinder.
What I like to do is, starting with some tires that do fit, pump them up and use allen wrenches as feeler gauges between the tires and brakes and frame. See where you have the tightest clearance and use that allen wrench plus your current tire size to estimate how much bigger you can go. A lot cheaper than buying tires and finding out they don't fit...
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Old 04-19-20, 11:01 AM
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28mm is a safe bet. Sometimes the clearance is tighter in the back than the front, and you could end up with a 32 fitting the front fork but being too wide for the back. If you post in the C&V section you’ll get an answer from someone who has gone through this with this Trek model.
And these are awesome frames, congrats 🎉 on a sweet ride!
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