Rake and trail in Modified class

[gschuld]

I?m on a roll with asking questions so I?m going to keep it up😉

I was studying up on salt flats specific rake trail geometry. 

FWIW this is in reference to a classic class 1970s Cb750 sohc 4 cylinder with a factory wheelbase of 57? and a max modified class wheelbase of 62.7?.  Factory neck rake is 27 degrees.

The accepted range based on my research is minimum 5% and maximum 10% of trail to wheelbase ratio with 7-8% (generally 8?) being ideal.  So my wheelbase limit for Modified class is 63".  A k model cb750 neck rake is 27 degrees(factory trail is 95mm with factory 60mm offset trees).  That's right on 6%

So let's say the bike is lowered some but the frame remains factory level, retaining the 27 degree neck rake.

Factory 27 rake with 60mm is 95mm trail 6% at 63?. WB
Factory 27 rake with 50mm is 106mm trail 6.6%
Factory 27 rake with 45mm is 112mm trail 7% at 63? WB
Factory 27 rake with 35mm is 123mm trail 7.6%
Factory 27 rake with 30mm is 128mm trail 8% at 63? WB
Factory 27 rake with 28mm is 130mm trail 8.2%

I realize lowering the rake is a common approach to achieving a higher trail number especially in A class with a much longer wheelbase(and needing more trail). But all the production class bikes and many of the modified class bikes run factory untouched necks and adjust for trail in other ways.

So according to above, I'd need between 45 and 28mm offset trees to get into the 7-8% trail to wheelbase sweet spot.

Is this sounding right?

Is 8% trail to wheelbase considered ideal at 63? wheelbase if given the choice of tree offsets?

So in my case 28mm or 30mm offset trees are preferable for 8% or just over.

http://www.landracing.com/forum/index.php?topic=3266.0

George

[maj]

What makes you think you need more trail ?

[gschuld]

I don?t necessarily need more trail.  I?m at 6% trail to wheelbase now which is on the lower side of the scale.  But I happen to have a wide selection of ready to go trees with different offsets on hand so I?m looking for what would be ideal.  I do have an interest in shortening my wheelbase a small amount fwd and maximize the stretch out back for a few reasons.  So a set of trees with 30mm offset (from 60mm) will bring the front tire back roughly another inch, and hit the reportedly ideal 8% trail to wheelbase ratio for stability at the same time.  I just want to make sure I?m not missing something.

George

[stay`tee]

Trail comes into play whilst a machine is negoating a curve, it is dictated to by said machines wheelbase and c of g,,

Rake is what your effots should be concentrated on, as rake increase, "slows" steering, therefore mitigating speed wobble, and or over correction 

[gschuld]

Sure, for those building Custom long wheelbase A class bikes using a 35 degree or so rake makes sense.  It also helps put front/rear weight balance where you want it.

But for those who are in Modified Class and not changing their rake from stock on a factory frame...

I my case 27 degrees.  Is the trail number of 7-8% of wheelbase still the go to ideal in this situation on a 63? wheelbase?

George

[gschuld]

I?m perhaps reading too much into this post:

http://www.landracing.com/forum/index.php?topic=3266.0

And granted he was studying the Sonic Arrow crash:

http://www.gregwapling.com/hotrod/land-speed-racing-history/land-speed-racing-sonic-arrow.html

Regardless, increased rake (like 30 degrees and over) is a tried and true high speed stability enhancer with appropriate trail.

But again, I?m talking about what?s ideal for a 27 degree factory neck and a 63? wheelbase.

George

[salt27]

George, If you are participating at an SCTA event the rules in modified do allow the steering head angle to be altered.  7.F

  Don

[gschuld]

I?d be gearing for AMA rules at BMST.

But they to allow an altered rake in modified as well. But I have a few rule based incentives not to do it if I don?t have too.

[Koncretekid]

George,

My A-PG bike that I built from scratch uses CNC billet Honda 350 triple clamps, now with Suzuki forks.  I designed the triple clamps with 45mm offset (60mm is stock) with a 30 degree rake.  However, I have a wheelbase over 75", so the trail is more like 6% of wheelbase.  No handling problems at 130 mph naked (AP), and 153 mph as APS. So yes, by reducing offset you will get more trail with a slight decrease in wheelbase, and should improve the straight line handling, in my opinion.

Tom

[gschuld]

Thanks Tom.

That?s good info.  It?s easy to get sucked into researching this stuff.  Locating the data is more the challenge.  I?ve resorted to scaling and recording rake/wheelbase/etc details from nice side profile shots from Scootershoots.com images.  I can get reasonably accurate info to compare.

George

[bones]

hey gschuld
       I think you are over thinking this
       I've never read of trail being expressed as a percentage of wheelbase.
       I have an A bike which I have run at Bonneville a few times. The wheelbase is 73" rake is 30* and trail is 5"
       This is EXTREMELY stable at 200+ but will still steer. The bike number is 1444  A-G 1350
       I assume you are going to use a CB750 for a classic class with a best class record a little under 145mph.
        Modern sports bikes have aprox 24* and 95mm trail and are happy to go over 170 and stop and turn.
       I think your stock 27* 95mm trail will be fine. If you feel you should have a little more trail maybe add 20mm
         cheers     Bones

[Koncretekid]

Bones,
I'm thinking your bike has modern forks with modern upside forks (much stiffer than'70's Honda).  What is your offset?  Again, I'll bet it has less offset than the old Honda, which along with your 30 degree rake results in more trail (5" as you stated) than the old Honda. I think GS is trying to achieve more trail by reducing his offset (by maybe 20mm that you suggested) thereby also allowing a longer swingarm to stay in M class as well as achieving better high speed handling.
Tom

[bones]

Hey Tom
   Yes I do use USD forks and shortened to 50mm travel on my A bike
I have also used conventional forks. My Yamaha R1 had Yamaha RZ350 forks and had no problems.
My bike uses a 110x17 front tire. The honda is a 19 front. George will have to calculate his trail for the wheel diameter he uses.

   cheers    Bones

[salt27]

My Yamaha R1 had Yamaha RZ350 forks

My RZ350 has R1 forks.   

[mtiberio]

I replaced the 60mm offset triples with a set of custom Kosman 30mm offset triples for this very reason. Shortened the wheelbase while increasing trail and stability.