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Ok, so i have been trying to figure out the basics of the bike geometry and how it affects handling. So for example lets say i have this bike with two different wheel setup. Whitch setup would you prefer as a cafe racer and why? Would like to hear an opinion from experienced rider.

geometry.jpg
 

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Not the 16R / 18F. 16" rear tires feel weird on sport bikes. They don't like turning in at speed much.

I'd prefer 18's front and rear on an older sportbike. Or 17's F and R if it was feasible.

16" fronts can be ok, but they feel unsure and weird compared to bigger wheels.

Danger, is my business."
 

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Let's assume rake and trail stays the same because your overall tire and wheel height stays the same. If that were the case then the 16" wheel is going to have a massive sidewall and deflection prone. This means, a 16" wheel in either situation is not ideal.

Here is how I would approach it:
Look at the WERA and AHRMA race book and see what wheel sizes in my bike would be limited to in the various classes
then I would look at what tires are available for those wheel sizes and settle on a brand for each.
then I would measure the actual overall height of the different tires
I would then figure out the suspension sag for the bike
then I would figure out the distance from the axle to the bottom of the tire for each combo and set the bike up so the axle was at the correct height for each combo.
I would then take rake and trail measurements for each frame setup. Why would they be different? well even rims the same diameter will have different sidewall tire heights if the widths are different. A 120/60/18 is taller than a 110/60/18 because the aspect ratio (middle number) is a percentage of the width (first number) so 60% of 120 is greater than 60% of 110.
now with the bike setup this way you can also check for crazy combos like your 16" rear 18" front or your 18" rear 16" front.

but there is a quicker way. Find a bike with those wheel sizes already and figure out what it's rake and trail numbers are and see how close you are to them. An early 80's honda vf750F honda interceptor uses a 16" front and an 18" rear, and all the measurements should be online. similarly some HD models use an 18" front 16" rear (although most use a 19" front).

also here is a rake and trail calculator to help you along:
https://www.rbracing-rsr.com/rakeandtrail.html

remember, lowering the front of the bike (even by decreasing the tire height) steepens the rake and raising it increases the rake.
 

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What are your wheel widths. What are your tire options. Unless these are completely custom wheels, it may not be feasible to run them both ways.

Like geeto said, unless the tire diameters are identical, your change the rake.
 

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When you say the rake is the same between the 2 set ups, that would require different rakes for each set up. You couldn't use the same frame and make the wheel changes without changing rake and trail.
 

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Ok, so i have been trying to figure out the basics of pet ownership and how it affects agility. So for example lets say i have a two animals, a brown fuzzy one and a gray smooth one. Which would you prefer to train to run agility courses and why? Would like to hear an opinion from experienced veterinarians.
 

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remember, lowering the front of the bike (even by decreasing the tire height) steepens the rake and raising it increases the rake.
Probably more important than rake, is how adjusting the ride height, front or rear, or a combination of both, affects the front ends static trail measurement.

BTW, it's funny how supposed experts always talk about trail figures. Hardly anyone ever actually measures them. They just quote a figure from a magazine or something. A pro race fabricator once told me the actual figures usually aren't exactly what factories say they are in factory spec sheets.

Danger, is my business."
 

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Discussion Starter #8
This was a theoretical question... Everybody here are always mentioning rake/trail/wheelbase when speaking about geometry and handling but as i see it theres should be lot more factors to be considered. O1mark theoretically you can setup the same bike with different wheels (16/18; 18/16 or 17/17) so that rake and trail will remain the same- you must change the angle of swingarm and fork offset. In my example the numbers are from cbr600f3 specs and it uses 17/17 wheel setup.
Tanshanomi whats your point? Just verbal masturbation in public??
 

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BTW, it's funny how supposed experts always talk about trail figures. Hardly anyone ever actually measures them. They just quote a figure from a magazine or something.....

.,.Everybody talks abot it, nobody knows about it.....
 

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Tanomatic's comments are the motorcycle forum version of this:

The quick brown fox jumps over the lazy dog.

Then his hand slipped off.

Gotta get a grip.

Danger, is my business."
 

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This was a theoretical question... Everybody here are always mentioning rake/trail/wheelbase when speaking about geometry and handling but as i see it theres should be lot more factors to be considered. O1mark theoretically you can setup the same bike with different wheels (16/18; 18/16 or 17/17) so that rake and trail will remain the same- you must change the angle of swingarm and fork offset. In my example the numbers are from cbr600f3 specs and it uses 17/17 wheel setup.
Tanshanomi whats your point? Just verbal masturbation in public??
So what was the point? Nobody is going to do your homework for you and there are lots of variables you can change. If you see it as limited to a few variables then you aren't thinking clearly. but more importantly, why are you asking this questions in a vacuum. I'm not going to do your math homework for you, so if that is what you wanted to start...well..why?

you are talking about angle of swingarm and fork offset, which by the way have the consequence of changing the wheelbase length. you could have changed the aspect ratio of the tire without changing either of those things and you reap the consequence of tire deflection.

have you ever played points based card games? (e.g. pokemon or magic the gathering) Think of your motorcycle chassis a lot like that: each change has a positive and a negative point system. the goal is to make changes where your positive points system is greater than your negative point system. And like any of them you can stack for increased positive or negative.
 

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Okay, to treat your question a bit less flippantly, let's plot out the exact figures you've given us to scale:

rake-trail.gif

You didn't say anything about tire sizes, so let's assume they both have the same size tire. A 110/90 tire will have a 99 mm nominal sidewall height, or an additional 198 mm to the overall height of both wheels. If the rake and trail are the same for both, you end up with the steering stem centerline offset back from the axle by a greater amount the larger the wheel gets. In this case, the steering axis is moved farther back by 8.68 mm, or a little over a third of an inch when you go from a 16" to an 18" wheel. What impact does offset have on steering? Well it doesn't, directly, so there's no definite answer. According to Tony Foal's experiments, steering stability is related solely to trail. So these two bikes would be equally prone to return to center after wheel deflection. But the larger 18" wheel will have greater gyroscopic inertia, so it would tend return to center more slowly. This can make the steering feel heavier, but this is only one part of what effects steering feel. Wheelbase, handlebar width, center of gravity height, even the use of leading-axle or trailing-axle forks (which locate the mass of the forks differently) will have as much (and cumulatively more) impact on perceived "steering feel" as the triple clamp offset. And there is nothing that says that the fork centerlines have to be parallel to the steering pivot axis. Lots of stretched-out choppers have "raked" forks, and Harley's touring models have "de-raked" triple clamps that position the steering head in front of the forks.

So, to answer your question purely theoretically, you'd probably notice equal stability and maneuverability with with slightly less effort to initiate a turn with the 16" rim, all else being equal. But "all else being equal" would be phenomenally difficult to achieve in the real world.
 

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OK say you can change the fork offset, front and rear ride heights and keep the trail and rake the same in both instances.

Let's say you are talking about the same dimension tires other than diameter so the 18" wheel/tire combo is taller than the 16" wheel/tire combo.

The effect on the stability of the wheel and the motorcycle is most pronounced when riding over a bump or over and into a hole.


The velocity and acceleration of the tires and wheel (unsprung weights) traveling over bumps and holes will be greater in the smaller diameter combinations leading to a less "comfortable" ride.

The best example of this is trying to ride a scooter with 10 inch wheels over bumpy terrain. Shake your fillings out.
Monster trucks with huge tires driving over a speed bump. Don't even notice it.

Is this what you were getting at?

It's like playing what am I thinking...
 

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OK say you can change the fork offset, front and rear ride heights and keep the trail and rake the same in both instances.

Let's say you are talking about the same dimension tires other than diameter so the 18" wheel/tire combo is taller than the 16" wheel/tire combo.

The effect on the stability of the wheel and the motorcycle is most pronounced when riding over a bump or over and into a hole.


The velocity and acceleration of the tires and wheel (unsprung weights) traveling over bumps and holes will be greater in the smaller diameter combinations leading to a less "comfortable" ride.

The best example of this is trying to ride a scooter with 10 inch wheels over bumpy terrain. Shake your fillings out.
Monster trucks with huge tires driving over a speed bump. Don't even notice it.

Is this what you were getting at?

It's like playing what am I thinking...

:I find a 2.75x21 front and a 4.00x18 rear gets me over pretty much Any bump, if that helps :|
Yes a smaller wheel would be at a disadvantage on a real big bump.



also small wheels would be a bitch for going down steep hills:

 

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You actually make a good point with that picture. It goes beyond just comfort and a bumpy ride.

What happens when you hit that hill with your wheel turned slightly to one side. The steering gets pulled to that side. Why?

It's the rapid change from positive to negative trail when the front of the tire makes contact with the ground.

That change is magnified greatly with smaller diameter wheels because the contact point lands higher on the wheel, farther from the steering axis.


Imagine that on a much smaller scale and you have an idea of the subtle trail changes and the subsequent changes in righting effect while cornering on imperfect surfaces with wheels of different diameters.

:I find a 2.75x21 front and a 4.00x18 rear gets me over pretty much Any bump, if that helps :|
Yes a smaller wheel would be at a disadvantage on a real big bump.



also small wheels would be a bitch for going down steep hills:

 

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Wheelbase has more to do with stability than rake or trail. That's why Vespa scooters work well on dirt trails, bumpy Italian alleys and bumpy roads. Even with small diameter wheels. Vespa's have a relatively long wheelbase to wheel size ratio.

Bike front ends don't cycle back and forth from positive to negative trail. It might vary, but not how you say.

I've ridden powered stand - up small wheeled scooters at near sixty MPH, and their stability is perfectly acceptable. They work fine.

If big wheels were essential for two wheel stability, how can guys do this using tiny wheels?

Razor Dirt Scooter (RDS) Ride Video:

Danger, is my business."
 
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