Question for Frame Geometry guru's

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1/24/2019 11:19 AM

So I was trying to find out what Yamaha did to the 2018 YZ450 frame to make it turn better than the previous generation. I was always under the assumption that a steeper head angle (rake) with a smaller number, and shorter trail, would make the bike turn better. Yamaha 450's have had turning issues since they were invented, but have gotten progressively better with each new model. Here are a few stats that have me scratching my head:

2009 YZ450 rake: 26 degrees
2010-13 YZ450 rake: 26.9 degrees / 4.7 in trail
2014-17 YZ450 rake: 27.08 degrees / 4.4 in trail
2018-19 YZ450 rake: 27.33 degrees / 4.6 in trail

A new RMZ450 has a whopping 27.8 degree rake and 4.7 inches of trail and it turns on a dime.

Am I reading the measurements wrong, or do I have the measurements backwards? I know seat height and bar position come into play also, but what is making this bike turn better?

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1/24/2019 11:28 AM

Didn't they tilt the motor too? Also I think in 2016 they also played around with triple clamp offset and went from the 22mm offset in 2015 to 25mm in 2016. Not sure what offset the 2018+ models run.

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1/24/2019 11:50 AM
Edited Date/Time: 1/24/2019 11:51 AM

Monk wrote:

Didn't they tilt the motor too? Also I think in 2016 they also played around with triple clamp offset and went from the 22mm offset in 2015 to 25mm in 2016. Not sure what offset the 2018+ models run.

In 2010, they reversed the engine, and tilted it back. It turned better than the 2009.
In 2014, they tilted the engine more forward, pulled the steering head in 10mm. It turned better than the previous generation. I know the triple clamp offset has an effect - Yamaha has been back and forth between 25mm and 22mm a few times.

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1/24/2019 3:36 PM

Imo if stew was never on that bike , everyone would have loved them sooner , I have a 15 , and I ve rode an 18 there is a bit of a difference but I really don't think they mad difference in the way they turn . I ve heard the 19 is better but I have nt rode one .

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1/24/2019 3:53 PM
Edited Date/Time: 1/24/2019 3:54 PM

This may help: https://www.cycleworld.com/2015/05/22/cycle-world-tips-and-tricks-understanding-motorcycle-rake-and-trail
Also, remember that head angle affects both rake and trail. I don't have access to those numbers, but I'd guess the YZ and RM-Z are different in that specification. Rake and trail are measured as functions of each other; i.e., trail is the measurement between the tire's contact patch and the point where an imaginary line from the steering head reaches the ground. That point could vary greatly based on the angle at which the steering head is placed, and thus rake and trail would change. (Any change in the head angle would affect both rake and trail - decrease both or increase both.)
Additionally, fork offset changes the trail but not the rake. You could change one but not the other by altering the offset.
Changing fork offset AND head angle to varying degrees can create vastly different handling scenarios.

Finally, don't forget that the motorcycle's wheelbase and center of gravity affect how it turns.

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Braaapin' aint easy.

1/25/2019 7:12 AM

Falcon wrote:

This may help: https://www.cycleworld.com/2015/05/22/cycle-world-tips-and-tricks-understanding-motorcycle-rake-and-trail
Also, remember that head angle affects both rake and trail. I don't have access to those numbers, but I'd guess the YZ and RM-Z are different in that specification. Rake and trail are measured as functions of each other; i.e., trail is the measurement between the tire's contact patch and the point where an imaginary line from the steering head reaches the ground. That point could vary greatly based on the angle at which the steering head is placed, and thus rake and trail would change. (Any change in the head angle would affect both rake and trail - decrease both or increase both.)
Additionally, fork offset changes the trail but not the rake. You could change one but not the other by altering the offset.
Changing fork offset AND head angle to varying degrees can create vastly different handling scenarios.

Finally, don't forget that the motorcycle's wheelbase and center of gravity affect how it turns.

Thanks for the article - and yeah I get that. But 'Rake' and 'Head Angle' are the same thing on most motocross bikes, because the triple clamps are all perpendicular to the steering head.

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1/25/2019 8:05 AM

mxrose3 wrote:

Thanks for the article - and yeah I get that. But 'Rake' and 'Head Angle' are the same thing on most motocross bikes, because the triple clamps are all perpendicular to the steering head.

The triple clamps change the trail, not the rake or head angle. There has been some special bored clamps that change angle, but this is not common.

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Take it to the limit, one more time!

1/25/2019 8:43 AM

More trail seems like it would make a bike turn more slowly, but it is not quite correct. I am grossly oversimplifying, but, it makes the bars heavier, yet gives more front wheel bite. I tried 20s, 18s, and settled on 14mm offset on my KTM and prefer the result. On my Honda I went from 24 to 22 to 20, same.

Does a few other odd things.

It is not easily explained, trying it will be the best way to understand.

BP

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