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Author Topic: How Much Will Tires Expand When At High Speed???  (Read 597 times)

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Dan_Lockwood

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How Much Will Tires Expand When At High Speed???
« on: February 24, 2021, 03:22:14 PM »

Some of you know that I have a boardtrack bike build finishing up now and I have a slight oversight on my part.

The rear fender is somewhat close to the rear tire.  On one of my first run through the gears on the table lift and scissor jack, I noticed that I lost my tail light, left turn and headlights.  On later inspection I found that my rear tire had a little line in the middle of the tire all the way around.  It was around 1/8" wide.

After some troubleshooting I found that my tail light wires had been melted together.

I ran a flattened 3/8" copper tubing up the inside of my rear fender and bonded it in.  It was just larger enough for my four rear wires to run through to the tail light assembly.  Just about 3" up inside from the bottom of the rear fender, my tire had hit the copper tubing and did the deed on my wires inside the tubing.  It was a bit of a chore getting them broken loose from the inside, but I eventually got the tubing clear again.

I made a couple new standoffs for the rear fender mounting to raise the fender a bit.  I also trimmed the lower front spacer between the fender and the rear loop of the frame where I have it mounted.  I moved it as far as I could, about 1/8" forward.

I've not had an issue since, but I'm leery of true driving speeds and hitting again.

I taped a 3/8" nut on it's side to the tire and it "just" goes around inside the fender.  It's the tightest at the same spot that it hit before. 

My question is: Is the thickness of the nut, around 3/8", going to be enough room for running at speeds?

How much will my 120/70/21 Avon Cobra grow at speed?

I'm a bit limited in how much to move around my fender and rear tire.  The esthetics of the tire/fender come into play.  The fender has a certain radius and the wheel/tire has its own.  If I move too much, the curves will not look good, at least to me...
 
Thanks in advance.
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Dan

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iski

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Re: How Much Will Tires Expand When At High Speed???
« Reply #1 on: February 24, 2021, 03:48:58 PM »

Dan the growth of your tires will depend on how hot they get & what the air pressure is at that max temperature & how much they expand at the centerline at speed.  My rear tire starts out at 40 psi cold & will increase to about 50 psi when hot.  Not sure how much that extra heat made my tire grow.  An easy test would be to fill your tire to what you think the max psi would be & then rotate the tire to make sure you have enough clearance.  After that a short trial run at speed would confirm ok or not.

3/8th sounds a bit small but the key is what is enough clearance to not rub.  Good luck, had that same problem - rear fender wires rubbing - on a dirt bike that was also street legal a long time back.
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Dan_Lockwood

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Re: How Much Will Tires Expand When At High Speed???
« Reply #2 on: February 24, 2021, 05:56:12 PM »

I just tried a bit of pressure changes in the tire to see what might happen.

I started at 30 psi and set the tire on a measured spacer on the table lift and scissor jack. I then inflated to 65 psi and the tire only grew about 1/32".

Does that shed any light on my setup?

Thanks
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Dan

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Re: How Much Will Tires Expand When At High Speed???
« Reply #3 on: February 24, 2021, 11:54:19 PM »

Sounds like you have 11/32nds clearance when your tire is hot - not sure how much more tire distortion occurs at speed in a turn.  I would give it a try at speed - a short run.  A short run the longer runs at speed. From memory the tire will smoke/smell if it rubs the fender. Maybe someone else has a better idea?
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longlast

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Re: How Much Will Tires Expand When At High Speed???
« Reply #4 on: February 25, 2021, 04:52:34 AM »

Just a thought,

Put in your required air pressure (cold) take a measurement from ground to pre marked point.

Do a speed run as soon as you stop take another measurement,..you'll know how much the tire expanded in height use calipers or equivalent for the width.
Plus air pressure to see how much the air has expanded.
In mind air temperature reference 50°c compared to 90°c would make a difference
« Last Edit: February 25, 2021, 04:57:43 AM by longlast »
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Dan_Lockwood

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Re: How Much Will Tires Expand When At High Speed???
« Reply #5 on: February 25, 2021, 01:40:31 PM »

I cannot as of yet ride the bike.  It will need to be inspected by the Troop C Highway Patrol here in MO and then titled etc.

Here's what I have to work with.  This looks like enough clearance, but the copper tubing on the inside takes up about half of my original clearance.  Hindsight now, I would have moved the copper tubing off to the side of the center of the fender.  The fender is flatter than the cross section of the tire, but alas, this was not in my mind at the time.

A half link would help me out, but for a 530 O-ring chain, there are NO offset/half links made.  I even called Diamond Chain and they don't offer one.  I have a Diamond O-ring 530 chain on the bike.  I could switch to a non O-ring 530 pitch chain and "then" use a half link to adjust the tire further back in the fender just a bit.

I "may" be able to lengthen my lower spacer and then extend the fender up and rearward a bit with the offset upper fender mount adapters I made up to get where I am currently.

My thoughts were that if I went from 30 psi up to 65 psi and take the two measurements, that I would replicate a HOT tire with expanded pressure etc.  Again, the tire radius only grew less than a 1/16" between 30 and 65 psi.

Thanks for the input so far, greatly appreciated... :)
 
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Dan

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Dan_Lockwood

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Re: How Much Will Tires Expand When At High Speed???
« Reply #6 on: February 27, 2021, 02:43:30 AM »

I bit the bullet yesterday and moved my wheel/tire back one full chain link.  In doing so I raised the rear of the fender a bit and also moved the front lower rearward about 3/8".  This sound counter-productive if I have a tire clearance issue, but it worked out quite well

I can now stand a 3/8" nut on edge and hold it to the tire with tape.  That's approximately 9/16" tall at that point.  When I rolled it pass the front lower, I just touched the button head I have in the fender through the spacer into the frame.  At this point I have no concerns that my tire will "grow" in diameter 1".  As I said before, with 65 psi versus only 30 psi, there was less than a 1/16" difference being larger with more pressure.

Does everyone agree that with just over 1/2" clearance that I should be okay to run with it?

Thanks for all that took the time to take a look and offer replies etc... :)

Have a great weekend.
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Dan

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iski

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Re: How Much Will Tires Expand When At High Speed???
« Reply #7 on: February 27, 2021, 09:24:08 AM »

Dan that sounds like a good solution & it should work.  Like most all things, the only way to know for sure is to ride it & see what happens. Good luck, BTW that is an interesting looking pic you posted.
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smkymtnboy

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Re: How Much Will Tires Expand When At High Speed???
« Reply #8 on: February 27, 2021, 10:33:35 AM »

will the weight from the remainder of the parts that may be installed to finish the bike and the riders weight influence your clearance?
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motor1

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Re: How Much Will Tires Expand When At High Speed???
« Reply #9 on: February 28, 2021, 09:48:10 AM »

will the weight from the remainder of the parts that may be installed to finish the bike and the riders weight influence your clearance?
It looks like the seat and seat suspension will rest entirely on the frame. I think most of us would love to see more pictures because your work is absolutely beautiful and amazingly detailed.
Great work!
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