Topic: Road racers rake and trail numbers, tree offsets, etc

[gschuld]

I’m putting together some information regarding chassis geometry for road racers.  There are lots of bits and pieces spread through a number of older threads, so I’d like to assemble(and update) the information into one easy to find thread.

Cb750s are my main interest, but rake and trail data and general experience is useful across the model types.

The mainstream settings for road racing cb750s appears to be 14 1/4”.(361mm) to 14 3/4”(374mm) eye to eye rear shocks(assuming a factory k frame, stock rear shock mount location and stock early/mid k stamped swingarm). Some like Rob(Voxonda) have used a longer swingarm along with 45mm offset trees.

Front end typically either 35mm k of f model forks or early GL1000 37mm forks.

The tree offsets generally range from 45mm to 30mm(edit: replacing the 50/60mm offset factory trees).  What I don’t have much info on is the effective front fork length as that differs between the bikes which alters both ride height up front(altering max lean angle) and rake/trail.

For example, I have a mid k model front end with 29 3/4” between center of axle and top of forks at rest.  I’ve seen 77-78f model forks with 30 7/8” at rest. Gl000 forks are longer still.  Depending on the internal mods of the forks(racetech springs/cartridge emulators, spacers, etc, that number often gets shorter)

Overall rake and trail seems to have a target between 24-25 degrees and low 90mm (+/- 3.7”) trail to up over 100mm (+/- 4.0”) trail.

I’d like to find out what others are using for effective front fork length(for my purposes center of axle to top of upper tree/yoke), plus tree/yoke offset and rear shock eye to eye height(please let me know if the swingarm or shock mounting position is altered)

And general thoughts on trail numbers.

Thanks,

George

[scottly]

The tree offsets generally range from 45mm to 30mm.

Stock tree offsets were 60mm, which was reduced to 50mm for F and K7/8 models.

[Captain]

 This is or at least can be a complex subject because many things have an influence over exactly what are the optimum trail numbers..................Rider preference is but one...... But I can tell you this.
 Fast steering is around 95mm and slow is anything above 100mm. At or below 95mm is getting into the serious racing effort and you must use an effective steering damper or you will experience head shake during fast downhill changes of direction or even bumpy high speed flip flop turns.  In our own Superbike we experimented long and hard on this whole handling and turning issue specifically and ended up rebating the steering head by 25 mm steeping the rake by 1.5 deg to put more weight on the front (52-48% F/R) and with custom offset triple clamps settled on 93mm trail. That makes for a very fast turning motorcycle that I would NOT recommend you do unless both the chassis is dead on for steering head to swing arm pivot (perfect 90 deg) and which if you haven't been through the frame to correct, I guarantee will not be accurate.
 This is a very serious and important part of any road racing endeavour and you must be very informed and understand the principals when making any decisions or changes. Otherwise you can go completely the wrong way to what your intentions were.     For example more triple clamp offset reduces trail not the other way around. Many have trouble understanding this fact and believe the opposite but until you do then be very-very careful about anything you are changing.

 Captain

 

 Captain     

[turboguzzi]

Agree with captain. this subject is also very "bike build specific". tires would have a HUGE impact too. Conti radials or Avon/Dunlop Bias? sizes? 17" or 18" or maybe even 19" front if it's "box stock" class? tire width? 4.00", 130, 150?
you get the idea, the numbers you have are a good base, so just build the bike with enough adjust-ability, hit the track and start the long path of setting it up with various triple clamps offsets for trail changing, front end heights, springs rates, sag, etc.

in my 12 years of vintage racing i've seen people on the podium with everything from lazy 30 degrees rake to deadly looking 22 degs.

Easy

[gschuld]

The tree offsets generally range from 45mm to 30mm.

Stock tree offsets were 60mm, which was reduced to 50mm for F and K7/8 models.

Yes, I understand.  I should have been more specific as not to cause confusion(I added an edit above). I was talking specifically about road racers that have swapped out their factory 50-60mm offset trees/yokes for less offset options.  Cornering clearance for road racing requires raising the bike, lifting the rear with longer shocks accomplishes this at the back end.  14.75” is about the limit given the swingarm pivot position and the chain rubbing on the arm becomes intolerable if you go any higher plus the tension on the chain changes more with higher lift causing its own issues.

The raising of the rear decreases the rake a bit and reduces trail significantly, so shorter offset trees/yokes are swapped in to bring trail numbers back up to a functional situation.  The first step is usually 1979+ DOHC trees/yokes which are 45mm offset.  Others (such as Mark Mcgrew and Patrick Bodden) have used modified CBX trees(45mm) and run early goldwing 37mm forks that are longer and raise the front end of the bike some to end up with a good trail number.  Guys like JohnN started out with 60mm, went to dohc 45s, then had 30mm trees custom made.  Nearly without fail, those road racers that have reduced offset on their raised rear road racers have never gone back.  And most settle on 45mm to 30mm depending on their riding style, front fork length(raising the front end increases trail)etc.

Captain,

Thank you for your thoughts.  Well noted.😎


The chart below was posted by Alan(Haybus) a few years ago.  The starting point rear shock length was 13” eye to eye and the front forks were mid 70s k model positioned at the standard height in the trees.

Does anyone have issues with the accuracy of these numbers?  Assuming those numbers are correct, 35mm trees would put the numbers in the mid to high 90s (98mm trail with 14.5” shock length and factory k length forks and position) which seems to be a target area for many.

George

[gschuld]

Agree with captain. this subject is also very "bike build specific". tires would have a HUGE impact too. Conti radials or Avon/Dunlop Bias? sizes? 17" or 18" or maybe even 19" front if it's "box stock" class? tire width? 4.00", 130, 150?
you get the idea, the numbers you have are a good base, so just build the bike with enough adjust-ability, hit the track and start the long path of setting it up with various triple clamps offsets for trail changing, front end heights, springs rates, sag, etc.

Very good points Yossef.  You are right I should narrow this down a bit.  AHRMA f750 class, meaning among other things, 3.0” max rear rim width and 2.5” front rim width.

So let’s assume 130 rear 110 front Avon AM22/23 bias ply tires for now and possibly switching to Continental Road Atrack 3s later in a similar size.

Starting with 77/78 sohc front forks/sliders with Racetech internals, factory swingarm (required) with 14.25”-14.75” range adjustable eye to eye length shocks. The rider is very experienced, (as in also races a well race prepped R1) and will be pushing the lean angle limits, trail braking, all of it😉. The hope it to hit the ground running with a well thought out(and researched) geometry setup that is track proven or at the least very much falls online with known successful combinations.  This is no place to reinvent the wheel😏

So with that in mind....🤣

[turboguzzi]

nice calculation. looks like you need more like a 40mm offset triple to stay in the 25 degs, 95-100 sweet spot area....
dont know how the mark3 contis are, but the mark 2 110 front needed a 3" front rim to work really well. so that might put contis out of the equation for you if you are limited ot 2.5.
In any case, contis are lower both ends by 10mm compared to avon, so you need to raise the whole with them by 10mm

[gschuld]

I did not know the Conti tires were shorter....🤔

(Factory stock setting is 88mm trail FWIW). Edit: That is wrong(sorry😬), the 88mm (and 26.8 degree head angle) was for an otherwise dead stock early/mid K model BUT with an 18” wheel and tire mounted up front. 

http://forums.sohc4.net/index.php?topic=144395.75

Assuming the chart Alan posted was accurate:

With 14.5” shocks, factory length forks:

Offset.     Trail
60mm     70.26mm
55mm.    75.25mm
50mm     80.5mm
45mm.    86.6mm
40mm.    91.6mm
35mm.   97.2mm
30mm.   103mm

35mm offset appears to be about “ideal” given the options.

George

[bwaller]

500/550's head angle is 26 degrees, 750's are one more at 27d.

Stock 750 trail I thought was 95mm??

[gschuld]

Brent,

Thanks for keeping me on the straight and narrow.😉
I just checked again and I was misquoting...
I referenced the 3.48” (88mm) trail number from the thread Haybus created:

http://forums.sohc4.net/index.php?topic=144395.75

My mistake was forgetting the quoted 3.48” was stock EXCEPT for the replacement of the front wheel/tire with an 18” rather than stock 19” front.  So yes, the factory trail number was indeed a bit higher than 3.48”/88mm trail.

George

[scottly]

K0-K6 27*rake, 95 trail, offset 60mm
F0/F1 K7/K8 28*rake,115 trail, offset 50mm
F2/F3 27.5*rake, 113.5 trail, offset 50mm
Also, regarding fork length, the fully extended K7 length, measured from the axle centerline to the top of the fork tube is 29.5". This is with no load. The F2/3 forks have a very similar length. As close as I can measure, the loaded length on my bike is about 28.5" with me aboard, with 550 springs and an F2/F3 fork.

[gschuld]

Thanks Scottly . The factory trail numbers are very interesting, especially the F model/late k trail numbers and rake difference.  These head rake and trail numbers came from a factory Honda source?

For fork length I get 29 15/16” on a very early K1 (internals may have been altered) and 30 1/8” on a mid 70s set.

Both fully extended at rest.

[bwaller]

Measuring from center axle to the upper triple clamp is fine to compare until aftermarket clamps are used. Then probably better to measure vertically from the floor to a spot on the front of the frame to compare height.

I'll try to get you my F3 fork info today George.   

[gschuld]

Measuring from center axle to the upper triple clamp is fine to compare until aftermarket clamps are used. Then probably better to measure vertically from the floor to a spot on the front of the frame to compare height.

I'll try to get you my F3 fork info today George.

Good points Brent.

For a while I was measuring front and rear tire radius/diameter, height from ground to bottom of frame neck, and axle to bottom of frame neck.  It’s more accurate and comparable between different setups with different parts like you mentioned.

George

[scottly]

George, yes, I got the rake and trail numbers from the Honda shop manual.
The K7 fork length of 29.5" was without the springs installed, extended until the rebound springs on the damper rods made contact. With the springs installed, the preload compressed the rebound springs, and the length increased to 29 11/16".
I jacked the front wheel off the ground on my bike, and the fully extended length of the F2/F3 fork was also 29 11/16".
I measured a fully extended K1 fork, which by the way does not have rebound springs, at 29 7/8", within a 1/16" of yours.

[scottly]

Measuring from center axle to the upper triple clamp is fine to compare until aftermarket clamps are used. Then probably better to measure vertically from the floor to a spot on the front of the frame to compare height.

The stock uppers have a vertical off-set of about 7/8", so flat aftermarket clamps would have the same effect as sliding the fork tubes 7/8" up above the stock clamps. What really matters is the the distance from the floor to some spot on the frame with the rider aboard, as all dimensions of rake, trail, and fork length will vary as the suspension operates. 

[bwaller]

As Scottly said, assembled F3 forks are 29 11/16" (754mm). Maximum travel for these forks is 133mm, I use 117mm total travel with 15W oil level at 172mm. Stock damper rods with one rebound hole welded shut and replace damper seals every season. Shorter (0.85k) Racetech springs allows a preload spacer to adjust for sag%, plus having preload adjusters gives you the opportunity to fine tune a little for each track. I use emulators with 40in/lb springs set at 3 turns. (but this for a 320lb (dry) motorcycle.)

My best guess is with a stock CB750 frame with front height unchanged and original wheelbase, with shocks 1.75" longer but still at original angle, the rake would decrease by a maximum of 1.5 degrees.

Unfortunately I have experienced the negative results of too little trail. (tankslapper extraordinaire) Don't mess with this.

 

[livefast_dieold]

Thanks George to have created this post.

Here's my data (I'm using Tony Foale motorcycle set up software):



Resulting in exactly 100mm trail. To get to the 25° of rake I had to change the shock mountings, lowering them on the frame by 2.5 cm and using 36 cm eye to eye shocks length. Unfortunately, I don't have the length of the front fork with me now.

[gschuld]

Thanks for joining in.

I suppose it would be most ideal to pick a particular program and have anyone willing to share install their numbers to that program for proper apples to apples comparisons.

I’m curious as to how much trouble you have with chain to swingarm contact.  Sounds like your swingarm angle is pretty much at the limits.  Do you run something large than a 17 tooth front sprocket to gain chain clearance or is a 17 OK?

George

[livefast_dieold]

I'm usually using a 15 teeth sprocket! I've bolted a plastic slider on the swingarm, and it doesn't show any particular sign of wear. I've been using this sprocket for the past 3 seasons.

As for the software, it costs 99 € for a lifelong license (https://motochassis.com/product/motorcycle-setup-software/). It covers probably too much stuff for the sake of comparison, maybe there is a free to use software (or something we can do with an excel spreadsheet) to compare our numbers more easily. I found the attached spreadsheet that easily calculates trail, maybe we can extend it furthermore. If someone has already some formulas that he uses to calculate its frame parameters I can update it myself, so we all use the same approach.

[gschuld]

Wow, 15 tooth on a 520 chain I assume?  That would put your rear sprocket in the 38-40 range?

George

[CR750]

Hey Guys,

I am actually in the midst of this process after upgrading the front end of my VFR.
If you go and have a look here at Tony Foale his site;

https://motochassis.com/freeware/

You will find freeware for the rake and trail etc in basic form + some other handy tools ( I use tube miter quite often ).

You can download those if you go here, and click on the download freeware bundle;

https://motochassis.com/shop/

Please bear in mind it is barebones, you can also do this in an Excelsheet, as the whole thing is based on formula's.
Actually I think a lot of these are to be found on the Web, and he migth have combined them into an .exe, dunno... anyways....handy to have.

Resulting in exactly 100mm trail. To get to the 25° of rake I had to change the shock mountings, lowering them on the frame by 2.5 cm and using 36 cm eye to eye shocks length. Unfortunately, I don't have the length of the front fork with me now.

I am a bit puzzled here...You a referring to the 'ground trail' number here ?
I thought what you design around is your real trail/mechanical trail and that would be 90,6.
That is the number that is important in my opnion ?!

Unless in your more elaborate foale software the rake you have inputted is only your frame rake, and the software takes into account the extra angle changes because your rear tire is bigger then your front by 15 mm, so essentially your rake is further influenced ( steeper rake ) ... but then you end up with some scary numbers for trail on your inputs..??

For example a 'modern' sportsbike 2005 R1 uses 24 degrees of rake, 30 mm offset, and let's assume 315mm for the tire ( actually a little smaller I say about 313mm -theory vs real world ). The real trail would be 97 mm in this case. This is also what Yamaha communicates...Whereas the ground trail would be 107,34 (too much for a sportsbike imo to be real trail )...
So I am puzzled as in were your numbers are in real life.. or is there some misunderstanding going on my behalve ?

I am under the impression you want your Real Trail between the 90-100 ( more like 95-97) area for most of us (as the lower number can get scary twitchy like Captain states ) I believe the 2019 BMW s1000rr sits at 23 degrees rake and 94 trail....

So when we speak of trail and this range....should it be the 'ground trail' or the 'real trail' within that range ?? Makes all the difference ;-)

[livefast_dieold]

Wow, 15 tooth on a 520 chain I assume?  That would put your rear sprocket in the 38-40 range?

George

currently a 44 at the rear!

[slikwilli420]

I'm usually using a 15 teeth sprocket! I've bolted a plastic slider on the swingarm, and it doesn't show any particular sign of wear. I've been using this sprocket for the past 3 seasons.

As for the software, it costs 99 € for a lifelong license (https://motochassis.com/product/motorcycle-setup-software/). It covers probably too much stuff for the sake of comparison, maybe there is a free to use software (or something we can do with an excel spreadsheet) to compare our numbers more easily. I found the attached spreadsheet that easily calculates trail, maybe we can extend it furthermore. If someone has already some formulas that he uses to calculate its frame parameters I can update it myself, so we all use the same approach.

You must be winding the absolute piss out of that engine! I run anywhere from 17/44 to 17/48, or a ratio of 2.59 to 2.82, while yours is in the 2.93 range. Must be very short tracks where you race or you are running to 12-13K RPM.

[livefast_dieold]

Resulting in exactly 100mm trail. To get to the 25° of rake I had to change the shock mountings, lowering them on the frame by 2.5 cm and using 36 cm eye to eye shocks length. Unfortunately, I don't have the length of the front fork with me now.

I am a bit puzzled here...You a referring to the 'ground trail' number here ?
I thought what you design around is your real trail/mechanical trail and that would be 90,6.
That is the number that is important in my opnion ?!

You are correct, 100 mm it's ground trail, so 90,6 real trail.