4x18 Front and Rear

[Ahks]

Can't find an answer via google, which is strange.

I've got a 76 honda xl250 that I'm converting for street use. Nothing fancy, just want good road rubber for 100% back roads street cruising.

The specs call for a 4.00-18 on the rear. I've laced a rear hoop to my front hub.

What size tire sizes should I run?
I think 120/90 rear and 100/90 front would both fit and be safe.

Being a newb to wheel swapping and vintage anything, I'm seeking the wisdom of the masses. ;D

 

[teazer]

Wisdom of the masses eh? Is that like mob rule?

Welcome.
The starting point for your quest are two places. First what width is that rim? That dictates what tire will fit properly. The next thing you want to look at is width between the fork legs. There is probably enough room up front, but check anyway.

More of an issue is the change in geometry with a smaller diameter wheel. It's been too long since I had my hands on an XL, but I'd expect the smaller wheel to make it more twitchy. Fork angle will steepen and trail will be reduced. Whether that's a good thing or a bad thing I can't say because I don't have that bike to look at and measure, but it's something to consider.

 

[o1marc]

The factory would never have put a rim and tire that big on the front of a bike that small, there is a reason for that. It will negatively effect the way it handles.

 

[Scruffy]

Here's your 4.00-18s for the rear, I'd go with a 3.00/3.25-18 on the front. http://www.americanmototire.com/catalog/index.php?manufacturers_id=714

 

[Ahks]

The starting point for your quest are two places. First what width is that rim? That dictates what tire will fit properly. The next thing you want to look at is width between the fork legs. There is probably enough room up front, but check anyway.

More of an issue is the change in geometry with a smaller diameter wheel. It's been too long since I had my hands on an XL, but I'd expect the smaller wheel to make it more twitchy. Fork angle will steepen and trail will be reduced. Whether that's a good thing or a bad thing I can't say because I don't have that bike to look at and measure, but it's something to consider.

Whats the proper way to measure wheel width? Bead to bead? I'm fairly certain I have room for some tire in there.

The factory would never have put a rim and tire that big on the front of a bike that small, there is a reason for that. It will negatively effect the way it handles.

Is that reason because they didn't have the benefit of another 30 years of R&D? If I only cared about what the OEMs did I'd probably be a sheople driving down the road with blinders on mowing down motorcyclists.

Here's your 4.00-18s for the rear, I'd go with a 3.00/3.25-18 on the front. http://www.americanmototire.com/catalog/index.php?manufacturers_id=714

Too expensive. I'll be converting the wheels to tubeless and running shinko 712's or 230's. I know theres no metric equivelent but 25.4mm to 1 inch, so 25.4x4= 101.6mm. Maybe I should only go as big as 110 on the rear and see if they make a 90/90... which they don't... oh well

 

[Ahks]

Welcome.

And thank you

 

[mydlyfkryzis]

Is that reason because they didn't have the benefit of another 30 years of R&D? If I only cared about what the OEMs did I'd probably be a sheople driving down the road with blinders on mowing down motorcyclists.

30 years of RD changed suspension, tire rubber compounds, braking efficiency...Your bike is still the 30 year old configuration...Unless you re-engineered your bike to modern standards, things like chassis stiffness, swing arm stiffness, suspension compliance, the "old" standard still applies. You cannot just put modern size tires on an old bike and expect handling and performance of a modern bike.

What o1marc said is exactly right....what is right for the bike 30 years ago is still right.

Large tires increase unsprung mass on the suspension which was not designed for it.

If the tires nave increased corning force, the chassis has to withstand it too, and usually isn;t up to the job. You may end up with just a sluggish handling bike, a twitchy handling bike, or maybe one that tends to have uncontrollable wobbles.

Since you haven't done 30 years of RD on your bike, and the nature of your questions to the masses, you really are working in the dark. I hope you end up only with a crappy handling bike, and not something dangerous.

The way to take advantage of 30 yrs of R&D is to but a bike that has those thirty years of R&D applied to it. Your bike is not that.

As far as rim width:

Here's a chart of tire sizes compared to rim width that are acceptable:

http://www.mtsac.edu/~cliff/storage/gs/Motorcycle_Rim_Width_Tire_Size_Chart.pdf


Google is your friend...

 

[Ahks]

30 years of RD changed suspension, tire rubber compounds, braking efficiency...Your bike is still the 30 year old configuration...Unless you re-engineered your bike to modern standards, things like chassis stiffness, swing arm stiffness, suspension compliance, the "old" standard still applies. You cannot just put modern size tires on an old bike and expect handling and performance of a modern bike.

What o1marc said is exactly right....what is right for the bike 30 years ago is still right.

Large tires increase unsprung mass on the suspension which was not designed for it.

If the tires nave increased corning force, the chassis has to withstand it too, and usually isn;t up to the job. You may end up with just a sluggish handling bike, a twitchy handling bike, or maybe one that tends to have uncontrollable wobbles.

Since you haven't done 30 years of RD on your bike, and the nature of your questions to the masses, you really are working in the dark. I hope you end up only with a crappy handling bike, and not something dangerous.

The way to take advantage of 30 yrs of R&D is to but a bike that has those thirty years of R&D applied to it. Your bike is not that.

Very good points. And points well taken. I'll do some research into my suspension geometry and see if there's anything I can do for it with the resources available. I'm not interested in turning this bike into a Sumo or even a very capable canyon carver. My use of this guy will be back roads day/weekend trips. But that's a bit in the future. Need more info on shocks and forks

As far as rim width:

Here's a chart of tire sizes compared to rim width that are acceptable:

http://www.mtsac.edu/~cliff/storage/gs/Motorcycle_Rim_Width_Tire_Size_Chart.pdf


Google is your friend...

Thanks for those.
Based on the chart a 4x18 came standard on a 2.5" wheel. Once I get home I can confirm that size.

The chart says a 2.5" wheel should be able to run a 3.25, 3.5, 3.6, 4, 90/90, 100/90, 110/80, 110/90, 120/90 and stretching it to the max I would think, a 130/90. While only the 4, 100/90, 110/80 and 110/90 are "standard" for the size. Maybe I should stick to a 100/90 front and 110/90 rear...

And basic suspension, if I'm dropping the front a given value by putting a smaller diameter wheel/tire combo on, then I should be dropping the rear by, what, 75% of the total drop of the front? Just to avoid a super twitchy ride?

 

[teazer]

Rim size is typically stamped into the rim. It will read something like 185-18 or WM2-18 .

 

[o1marc]

Whats the proper way to measure wheel width? Bead to bead? I'm fairly certain I have room for some tire in there.
Is that reason because they didn't have the benefit of another 30 years of R&D? If I only cared about what the OEMs did I'd probably be a sheople driving down the road with blinders on mowing down motorcyclists.
Too expensive. I'll be converting the wheels to tubeless and running shinko 712's or 230's. I know theres no metric equivelent but 25.4mm to 1 inch, so 25.4x4= 101.6mm. Maybe I should only go as big as 110 on the rear and see if they make a 90/90... which they don't... oh well

After 30 years of R&D they are still using much smaller tires on the front. They turn in much faster and are more stable at speed. You'll not find any bikes to my knowledge that run front rim anywhere near the size of the rear. Maybe you can link me to the 30 years of R&D your are referring to that says I'm wrong.

Wheel width is measured bed to bead, tire width is measured from the most outer width of the mounted tire. It's width will be measured from the manufacturer while mounted on the proper size rim. You can always physically put a "big" tire on a smaller rim, it's the other things you need to make sure it will clear ie: fender struts, swing arms, chains, etc.

 

[Ahks]

After 30 years of R&D they are still using much smaller tires on the front. They turn in much faster and are more stable at speed. You'll not find any bikes to my knowledge that run front rim anywhere near the size of the rear. Maybe you can link me to the 30 years of R&D your are referring to that says I'm wrong.

I'm done digging my own hole... The tangent that lead to the initial typing of that was with regards to tire tech. And my hand just kept digging the hole with no regard to chassis tech, or the fact that it's a 30yr old chassis... Now wheres the embarrassed emoticon? :-[

Wheel width is measured bed to bead, tire width is measured from the most outer width of the mounted tire. It's width will be measured from the manufacturer while mounted on the proper size rim. You can always physically put a "big" tire on a smaller rim, it's the other things you need to make sure it will clear ie: fender struts, swing arms, chains, etc.

I'll be taking some measurements in the garage sometime this week to confirm that I've got enough space for newer rubber.

 

[o1marc]

It isn't your 30 year old chassis. Even modern superbikes don't put the same size wheels on nor do they have big tires up front.

This reminds me of the stories I heard of the racers BITD racing the CB400f. They kept breaking spokes in the wheel so they decided to run CBR600f1 alloy wheels. As soon as they realized the spoke issue was gone they now realized all the flex in the old spoke wheels was now being generated to the flimsy swing arm. So they braced the swing arm and now found they were transferring all the flex into the flimsy stamped steel frame and this would now require modifying the gas tanks underside to accommodate all the bracing needed in the frame under it.

There's plenty of room for new rubber, why are you so opposed to running what the bike was designed to run with?

Bikes are not necessarily like cars where fatter tires and lowering the CG makes them handle better.

 

[hillsy]

I think 120/90 rear and 100/90 front would both fit and be safe.

I think you'll be OK with those sizes.


But, proof is the pudding as they say - and until you try it and see how it performs we're all just bench racing.

 

[Ahks]

There's plenty of room for new rubber, why are you so opposed to running what the bike was designed to run with?

Bikes are not necessarily like cars where fatter tires and lowering the CG makes them handle better.

The bike is a factory enduro and came with a 21" up front. I want access 100% street rubber. On a budget, had a rear wheel laying around, laced up that hoop to the front hub using the "short" spokes from the 21" and some spokes from the original rear hub lacing. I think I need to talk to Buchanans tho because using the OE spokes like I have has the wheel offset in the forks considerably

I think you'll be OK with those sizes.

But, proof is the pudding as they say - and until you try it and see how it performs we're all just bench racing.

And I did check the clearances. A 120 should only be about 4.7"ish wide. The (what I suspect is original, gotta find a date stamp on it) 4.00x18 on the rear easily has a spare .5" to .75" of clearance on either side and the front looks like it could fit a 160 between the forks

And since everyone like pix...

 

[ako]

You'll not find any bikes to my knowledge that run front rim anywhere near the size of the rear.

Technically you are wrong ;D

The USA made Rokon has a top speed of around 20mph. Which is what the original poster may achieve if he proceeds agains good advice.

Do you want to drive a tractor or a motorcycle?

 

[o1marc]

Technically you are wrong ;D

The USA made Rokon has a top speed of around 20mph. Which is what the original poster may achieve if he proceeds agains good advice.

Do you want to drive a tractor or a motorcycle?

The Rokon is not a street legal bike and with improper tires on your bike it will perform like the Rokon. You can't just lace any old rim to any old hub. he spoke holes are machined at different angles for the intended hub. You can't take a hub from a 21" wheel and expect it to line up properly on an 18" rim designed for a different hub. You have a LOT to learn about wheels and tires but don't want to listen to any one here. Saving money can sometimes get you hurt or worse.

 

[Scruffy]

Same size wheels front and rear? Harley Davidson big twins with 16s, Drifter 1500, Vulcan 800 Classic, Vulcan 1500 Classic and similar. Common theme amongst them? They ride like they are on airbags, but steer like they are on greased glass... On bikes built around ancient technology, I prefer to have the front rim at MINIMUM 1" bigger than the rear rim. My 1963 shipped stock with a 19F and 18R. With the right tires, it handles okay. 19 or 21 front with 15 or 16 rear isn't just for looks, the skinny front tire lets you turn circles around the same bike rolling 16x16, 18x18, etc.

I had a 99 VN800A that rolled with 80/90-21F and 150/80-16R tires, that could take 20mph on ramps at over 50 without issues. I had a 2005 VN800B rolling 130/90-16 F/R that in the same corner wallowed and drug the pegs at 20mph.

Having seen your bike I'd rock a 21F, you can get decent to good 80/90 and 90/90 21 tires pretty easily.

 

[ako]

The Rokon is not a street legal bike and with improper tires on your bike it will perform like the Rokon.

Hence my comment do you want it to ride like a tractor or a motorcycle?

 

[xb33bsa]

i raced motox on a 72 xl250 back in 75 and used to get trophies against maicos huskys buls and czs
i also road it on the street it is a good stiff frame, but will definitely handle like shit the way you are mucking about with it
sorry but you should have just stuck with the 21

 

[Ahks]

sorry but you should have just stuck with the 21

I'm beginning t think that. Dammit

I never really looked at the tire availability for street only applications on a 21" wheel. It would seem there's quite a bit more available than I had thought Original size was a 2.75x21. What about running 90/90-21 street rubber and a 110/90-18 out back? Can a tire thats labeled tubeless be run with a tube?