General Category >> Technical Documents/Reference >> Fitting MUCH Larger Rear Tire

Message started by Oldfeller2 on 06/10/07 at 10:21:52

Title: Fitting MUCH Larger Rear Tire
Post by Oldfeller2 on 06/10/07 at 10:21:52

The base thread for this instruction is "Drinkin' & thinkn' in Rubber Side Down which contains all comments and discussions as well as some cheap sources for larger tires.

The 165-15 tire in question is very very large, too large to be easily modified to fit on to a Savage.  You may read 165-15 in the text of the thread in several places but DON'T YOU DO IT -- perfect hindsight says a 145-15 would be much much easier to do and more practical to live with.

Remember 145-15.  Save yourself the hassles.

Some comments on mounting an oversized tire.  

You may try to reuse your old tube, but you may find that it has residual damage from being removed/installed that causes you to have a air leak.  If you get a new tube, get a 150/90-15 sized tube (largest one).

Since you are deflecting the sides of a larger tire "inwards" to go down to a small rim width (changing the bead engagement angle of the sidewall to the thread) expect to have to put a lot of extra pressure in the tire to get the bead to seat properly.  

Note: my tire actually reduced its outside diameter slightly during mounting -- it is now exactly 25" in diameter at 35 psi mounted on the rim.  See how small the deflated IRC is now (of course it is completely worn out and lost nearly an inch of diameter off the crown in doing that).

Use lots of extra slick bead slickum to aid in the bead going to the fully seated position, this will reduce the pressure needed to get a full bead seat.  

90-100 psi was what the 165-15 Nanchung VW tire required originally to go to fully seated on the bead.  I deflated pressure while the bead soap was still nice and gooshy to see what would happen and at 10 psi the bead popped down the shoulder again.  Reseating the bead only took 70 pounds, so I might theorize the bead stretches a bit to go into a fully seated position but retains part of that stretch once it has done it the first time.

I used straight dishwashing soap as my bead lube the second time around.  It was heavy enough and slick enough to get the job done and it will eventually dry and be partially absorbed by the bead rubber itself (soaps and waxes are actually components of rubber, BTW)

I intend to use the full 35 psi "max rated" pressure during fit up to  make sure a max bulged tire gets the correct amount of clearance to bolt heads inside the fender well.

Don't forget to balance your now slightly heavier wheel assembly.  

Also, take a stiff brush and scrub & rinse the soap residues off the sides of your newly mounted tire (it will rain sometime when you are riding and that's SOAP caked on the sides of your tire)

We shall now see how it fits in the hole.


Title: Re: Fitting MUCH Larger Rear Tire
Post by Oldfeller2 on 06/10/07 at 10:22:51

When you look under your fender you see that the available real estate has been encroached by some large protruding fasteners.  You can remove some of them and replace them with stainless steel flat head and pan head socket type fasteners from:

The list of possible replacement fasteners goes like this:

 Item # C
 Product #6698
 Metric socket flat head screws, Stainless A-2,   8mm x 1.25 x 16mm
 Quantity: 4 pieces
 Price: $0.54
 Subtotal: $2.16

 Item # B
 Product #6702
 Metric socket flat head screws, Stainless A-2,   8mm x 1.25 x 35mm
 Quantity: 4 pieces
 Price: $0.73
 Subtotal: $2.92

 Item #A
 Product #6657
 Metric socket button head screws, Stainless A-2,  8mm x 1.25 x 25mm
 Quantity: 4 pieces
 Price: $0.96
 Subtotal: $3.84

 Item #D  Welded Nut
 Shorten slightly with 4" grinder
 Overmold (blend a hump) with JB Weld Epoxy
 Paint with silver touch up paint

 Item #E  (not shown) Mounting bolt for Right Buddy Peg
 Shorten slightly with 4" grinder
 Paint with black touch up paint

You will need to buy a HSS countersink to countersink the flat headed screws to be nearly flush to the sheet metal that they will retain.  Close to flush, but not completely flush.  You do want some metal there to clamp up to, right?

Go slowly here == making the countersink too deep & large means you won't hold the sheet metal in place and that is a classic "bad thing" when trying to hold stuff together with flat headed fasteners.

The main fender mounting bolts get replaced with shallow pan head fasteners instead of flat heads.  Why pan heads?  You got these protector bumps in the sheet metal and all you really need to do is get shorter than the bumps.  Pan heads are stronger than flat heads and the fender is a structural element that sees strong vertical vibrational forces and potential tire/fender impact forces.  It needs more fastener strength than the horizontal mounted gingerbread stuff that we flat headed for width clearance.  

Note that the item #A fender mounting holes here aren't simple holes, they are slots.  If you need to shift the fender a bit to avoid a rub this is the time to do it.

Now, do you REALLY need to do all this removal and replacement of fasteners?  It depends on how big your oversize tire is and where it runs when installed on your rim.  

If you were rational and picked a 150/80 bike tire you likely won't have to do much at all.  If you were moderate and picked a 145-15 VW tire you wouldn't have to do near as much.  If you plan to put a 175/65R15 Continental on as your next experimental tire after you do yourself a chain conversion -- heck, you may need to flush mount everything you can possibly flush mount.

As always, this information is for theoretical discussion only -- nobody is actually ever going to do any of this silly stuff.  We all got better sense that that, naturally.   ::)


PS   Buy the type of countersink with a single straight cutting edge that is easily resharpened with a standard bench grinder -- you will likely have to resharpen your countersink edge at least once while removing all that metal.

Title: Re: Fitting MUCH Larger Rear Tire
Post by Oldfeller2 on 06/10/07 at 10:24:49

Now, how do you adjust the centering of a wheel assembly inside a fender well?   Especially if you need to shift the tire from right of center (Suzuki standard location) over to the left to get more room for a good bit bigger tire?

Answer, get 3 each of a 17mm I.D. plated steel washer and stack them all up and measure them as a total.  Cut the same total amount off the left side axle spacer (the fat one with the bearing seal riding on it that goes on the outside of the drive pulley).

Now you can move your wheel assembly around by a single washer's distance as you put the varying count of washers on the side that needs to be "thicker" to make the position of the tire be where you want it to be.  The total width of spacers and all 3 washers always remains the same, but the tire assembly moves back and forth at will by "one washer" increments.

That's how you fine tune your rear end "left to right" to make it perfect for whatever fat tire you have mounted on her this time around.

A point to be aware of if you have to get extreme with the tire lateral motion --- as you move left you decrease the amount of the female engagement of the brake drum assembly slot on to the male portion on the swing arm rib.  You can only go so far before you promote a divorce on these critical parts, so if you see the engagement getting a mite slim you might want to back off some on your tire shift.  

Your choices to go further left will need to include an extender plate on the brake drum assembly to reach out the slot to find the rib on the swing arm.  When you go to make such a thing, be aware that a 10-24 flat head socket screw will just span the gap on the inside of the hollow part of the casting such that the threads will sink in on both sides giving you more length of engagement.  The head of a 10-24 flat head just fits flush to the edge of the slot too.

So now you have all the tricks needed to fit a 165-15 OR BIGGER tire.  You can center your tire in the cleaned out fender well and you will still have a bit of clearance on all sides.  And what is neat is you didn't have to put a chain drive conversion on your Savage to do the mod either.


Title: Re: Fitting MUCH Larger Rear Tire
Post by Oldfeller2 on 06/10/07 at 10:25:51

Here is a fully functional huge 165-15 FAT TIRE mounted on a stock Savage bagger using the standard drive belt arrangement.  Extra clearance for the big tire was created in the fender well by removing the protruding bolt heads that Suzuki uses to clutter up the fender well on both sides and by modifying the axle spacers to include adjustment washers.  An adaptor plate was added to the drum brake assembly housing to permit the sideways centering adjustment while retaining the brake tab connection to the swingarm.

There is ample clearance to the top.  There is lots of clearance to the left side.  There is clearance to the belt (but not much).  There is clearance to the belt guard (which is easily told to move over using a flat crowbar,  BTW).

Clearance to the right requires the use of the axle spacers as shown in the previous post.  I put two axle spacers on the right hand side to shift the tire over to the left to become more centered in the fender well.  The rear pulley (and the small belt clearance) moved along with the tire as the tire shifted, but the belt guard required more adjusting with the flat crowbar after the shift was made.

So far handling on the fatter tire is not drastically different.  You can still drag a peg if you want to and braking ability in the wet has not declined to any noticeable degree.  You can skid the back tire if you get on the brakes really really hard (but you could do that with the stock IRC tire, too).

Best running pressure seems to be between 24 and 29 pounds of air.  Below 24 and things begin to feel mushy.  Get up to 29 and hard edges begin to be felt when leaning the bike over in a turn.

(currently I am up to 29 pounds as the edge feel isn't as objectionable as some of the soft running characteristics -- see Drinkin' and thinkin tread)

Does it feel different?   Yes, there is a whole lot more tire back there now and I am still learning how to use it to best advantage.  Taking a steep bank turn and hitting the gas as you exit the turn seems to make the tire want to dig in deeper which is new to me.  

Since I am managing my air pressure in the sweet zone I don't have the hard edges that so many have complained about.  I keep adjusting the pressure but the sweet zone is becoming more defined for me and seems to be heading towards 27-29 pounds.

I would say this mod is certainly doable, but I would also say is not for a beginner rider as you have to use good judgment as you make any modifications to the handling characteristics of your ride.

Your butt is 100% riding on your good judgment here, you know.  There is serious experimentation going on at the adjusting/riding stages now and I won't take the bike out into heavy street traffic until I get more miles on her and know more about how the new tire reacts to various hard braking and harsh movement scenarios.  


(A good deal of discussion on post modification air pressure and riding techniques are included in The Cafe under the thread " Drinkin' & thinkn' ")

PS    Now that you see all the garbage you have to do to mount a 165-15, realize that a 145-15 doesn't require all this stuff as it doesn't have the issues that drove it on the 165-15 installation.  

Think 145-15.  Save yourself the hassles.


Title: Re: Fitting MUCH Larger Rear Tire
Post by Oldfeller2 on 06/12/07 at 18:51:58

One more minor refinement in the search for "fit" around a really big tall tire.  

I needed another washer thickness worth (1/8") of extra clearance on the left side to resolve some occasional rub issues between the chain guard mount arm and the outside of the big pulley.  

Problem was that the space simply didn't exist.  There was no extra space available to take from Peter to give to Paul by moving a washer.  I was out of room completely between the swingarm posts.  I was packed tight -- no room to move anything.  It was frustrating and perplexing.

So I put a small scissors jack between the two parallel arms of the swingarm and deflected them outwards a tad (bent the "U" open a bit at the mouth in other words).

It worked to create an extra quarter inch of total installed space that didn't exist before.  Now with the extra washer the total of the parts in the location they need to be in works out with enough clearance not to rub into each other.  The tightening of the axle put the free clearance to the new stackup back to the normal amount free slop, so any extra leftovers from a bend job is reclaimed when the axle is tightened and loosened again.

Just some food for thought -- you tend to think about axle space as fixed when really it can be "increased" in small amounts as well as just moved around.


Title: Re: Fitting MUCH Larger Rear Tire
Post by Oldfeller2 on 06/13/07 at 18:23:38

Any tire that is larger or smaller in diameter than the stock IRC rear tire will result in speedometer error.  This is additional error on top of the natural speedo error that we all have on our bikes.

(don't let kph and km fool you, you select mph and miles in the setup menu)

You can get an independently settable, waterproof, durable, easy-to-set-up digital speedo for $15 bucks that does an excellent job of telling you the truth about your real speed and distances traveled.  Best source right now for this little jewel is LL Bean.

Instructions on how to set the speedo up can be found on this excellent web page.  Don't be fooled by the different model numbers, the same company makes both and your 506 is a newer version of the same speedo this guy is talking about.

Lastly, you may need a source for a stronger sensor "sender" magnet that can bridge the larger air gaps between your motorcycle front wheel rim to the side of the front fender.  You can mount your sensor under your fender next to the edge of the rim and put one of these on the top of the rim  (narrow side out)  exactly opposite from the air stem so as to not affect wheel balance adversely.  This magnet is a "lengthwise" oriented magnet so it projects the strongest field distance from the narrow ends.

The Sigma 506 unit is neat for our use because the wire length works out to be exactly what we need on a Savage when we do it with this stronger "sender" magnet to make the tucked under the fender across from the rim mount position for the sensor work correctly.  Buy yourself at least 4 of the little magnets, because they double as BALANCE WEIGHTS for your tires (and they cost a lot less than the chrome plated lead spoke weights they sell for $7.00 EACH)

Do use the mile marker setting trick on the black background web page referred to above.  If you take a trip on an interstate just stop at a mile marker and restart your computer as per his directions and write down all the info you get when you stop at a goodly distant mile marker.  Mile markers are not dead nuts accurate as individuals, but when you are averaging say 40 to 100 of them you get a very very accurate setting for your speedo computer.  

If you can't find it on the black web page, here is the calculator that makes all this math and mm-to-mph translation dirt easy.

Good luck with your larger tire,


Title: Re: Fitting MUCH Larger Rear Tire
Post by Oldfeller on 07/16/08 at 14:23:29











A source for additional information about car tires on bikes in general.


And here is a shout from 2 years out in the future from when I busted the bead on the exact same tire, a Goodrich Radial TA 155/80r15 along with the pictures of the broken bead.   This bead got hung up on silicone that was being used to make the wheel tubeless, so once again an impeded or "hung up" bead was involved.

Despite repeated lubings and inflations, the bead would not go over the rim on one side of the wheel using inflation pressures less than 80 psi.  

Further detailed examination indicates the bead on that side is perhaps defective (undersized for a bike rim) and it is perhaps structurally unsound as well.   The bead failed at 70 psi on about the 6th inflation attempt, which is an unusually low pressure for a bead to break in two.    

This situation makes the tire unsuitable for motorcycle use .....

The previous failure mode on this same particular tire was stated as a sidewall blow out, not a bead breakage.    You'll note the sidewall rips up the side when the bead lets go, so it may have actually been the same stuff, just perceived differently as to "chicken vs the egg" on the root cause.

Let's all jest go back in time about two years and simply say this is the second inflation failure of a BF Goodrich T/A 155/80r15 tire on a Savage rim.   That's two (2) out of a population of 5 and I once again do really believe that this incident took place simply as stated.

That's two and that is a significant trend to me  (my own personal private left wrist is bleeding, and it hurts -- that makes it a significant trend in my book anyway)

I will go back and edit my posts to UNRECOMMEND this tire for anyone to use on a Savage -- it is the least I can do.


Bill, the soapbox is all yours buddy --

I think you got full license to give me a big old "I told you so".

This idea sucks and I'm wiping my own frickin' blood off the keyboard as I tell you this ....

Title: Re: Fitting MUCH Larger Rear Tire
Post by Oldfeller on 09/09/08 at 00:34:11

Personally, I think he really blew it up like he said -- but you believe what you want to.

But his story jives with some interesting points which are available which cause me not to want to recommend that Joe Ordinary List member be messing with high air pressures when mounting tires.

This is from a legal reference source:



Tire bead failure explosions usually occur during the tire mounting process as a result of a latent design defect present in passenger and light truck tires. Most light truck and passenger tires employ a .037 inch weftless bead configuration which is subject to failure at pressures as low as 38 pounds per square inch. Low pressure explosions most often occur if the splice of the bead wire becomes impeded during inflation of the tire. This is commonly referred to as bead hang-up and has been documented in the patent literature, industry documents, and litigation since the mid 1950s."


You will note that Barry REALLY REALLY hung his bead up with too low a pressure at initial mounting, then let it sit overnight to firmly cement that hung up condition in place then actually put the tire on his bike and rode it several miles to find a place with 125-175 pound air pressure available outside to users (kinda dangerous, don't you think to give Joe Ordinary access to 175 psi air when tires really can blow up at those pressures -- what were THEY thinking ??)

Barry's tire certainly meets the bead hang up requirement for a low pressure bead failure as he didn't unseat the bead, relubricate it and try afresh to pressure seat it in one continuous motion with sufficient volume & pressure.


Next, Alcoa Aluminum builds mag wheels and offers us some mounting hints to AVOID EXPLOSIONS.

"Do not use flammable solutions for tire and bead grease. Flammable grease could cause the tire to explode during inflation or driving, causing possible death or serious injury. Use proper tire mounting and inflation equipment.

An inflated tire can separate from the wheel and explode, resulting in death or serious injury. Use a safety cage for inflating tires."


Remember, Barry says he sprayed truck bed liner compound all over the the spoke heads on the inside of his rim trying to air seal them.  He admits some sticky bed liner overspray may have gotten involved in the rim area.

Not only did he cement his mis-seated hung up bead in place with an adhesive, he provided it with a volatile flammable gas source as well.  The solvent used in spray bed liner stuff uses propane and other very flammable items that are supposed to cure out in the open air.  Barry trapped this stuff inside his partially seated tire and gave it some time to migrate out of the solid black goo and intermix with the high pressure air trapped in his spoke sealed rim.  Then he jacked the pressure up and up and up and up and up and up and up.

               Pop goes the weasel ....

Now, why do I no longer recommend anything about tires to anybody?

Folks like Barry is why I don't make recommendations any more.

Even if Barry really is kidding us, there really are folks like "Barry" out there.

How dumb are people?  There are documented cases of people shooting ethyl either (starting fluid) into big truck tires that had lost their bead seal and flipping a match at them.  

                Pop goes the weasel ....

Don't play with them nasty tire irons around car tires folks -- you could hurty yourself


Title: Re: Fitting MUCH Larger Rear Tire
Post by Oldfeller on 11/20/10 at 01:00:49

Hindsight is 20-20 folks --- after 3+ years this topic has generated more vitrol and discord than any other technical item ever posted by anybody.  Even me, the author has extreme reservations about the topic now (and a really achy left wrist with a big 'ol nasty scar on it too).

So, you now notice this thread only contains the bare technicals of getting a bigger tire into the wheel well (which may be of some potential use to someone who wants a larger motorcycle tire size) and some practical warnings about the PERSONALLY KNOWN dangers of separating (breaking in two) the steel bead on a car tire while trying to put it on a motorcycle rim.

Eventually, my wrist might stop hurting .....    


By the way, this thread is locked, so don't bother trying to add to it.

If you want to fight some over car tires -- go to Politics & Religion (Tall Table) and crank up your own thread.

Title: Re: Fitting MUCH Larger Rear Tire
Post by Oldfeller on 07/22/12 at 14:14:38

It is 3 years later on,  new attempts are made using a new style of tire from Nexen (a racing tire for Mini Cooper and TR6).   Difficulties mounting/seating the bead were addressed by trimming off the tubeless bead sealing lip (with only moderate success).   Also contains a mileage comparison of the popular brands of bike tires.

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