Looking for some experts help here. I have a 2009 Super Glide Custom. The rake of the neck is 29 degrees, I have lowered the rear with 11" progressives and put a 21 x 3.5 tire in the front. I have attempted to use the RB Racing trail calculator however I think my numbers would be changed with the lowering of the rear and the tire change.


Does anybody know what my trail numbers would be if I added a 5 degree triple tree? I would also extend the forks if this would help in maintaining a safe trail number.

 

Lowering your bikes rear end shouldn't change the trail or rake enough to even notice unless you actually measure it, maybe 1/4-1/2" if that. What will change your trail is changing the rake and / or changing the height of the front wheel axle. Changing the front to the 21" does change your trail numbers, but they are still within the "safe numbers" our bikes use. I have the same setup on my street bob and had the same questions, but it all together and figured out the numbers (wish I still had them to give to you). When I added the 21" up front and with the lowered rear, most changes felt are just getting used to the taller wheel up front (about 5-10 minutes of riding). If it sits to high in the front for your liking, you can lower the front end via a suspension kit or just move the legs up in the trees a little (won't change the trail numbers at all). Changing the rake to that of a Wide Glide and then adding Wide Glide legs you would pretty much end up with a Wide Glide setup using the 21" wheel.

 

Go to Howards and use his rake and trail calculator


http://motorcyclemetal.com/gpage23.html


You will find that raising or lowering will cause a change, whether it is enough to be of a concern is up to you...and physics... but his calculator will help guide you.


Different ways of changing the rake actually make a difference in the trail as well, i.e. rake in the trees vs rake in the neck.

 

https://www.rbracing-rsr.com/rakeandtrail.html

 

I tried this one however even when submitting my stock setup info, it was not even close to what the stock trail is of 4.7.

In addition, this site does not take into consideration the lowering and tire change.

I will have to take some measurements to try the motorcycle metal calculator.

To make it easier I wish someone with the same 5 degree setup could weigh in on the handling. I really want the wideglide set up but am worried about affecting handling too much.

 

When using this sites you need to know the actual tire diameter and not the rim size. A 90/90-19" tire will measure right around 25.4". So you would need to put in a diameter of 25.4" or radius of 12.7". If you are going to a 120/70-21" your looking at a diameter of 27.6" or a diameter of 13.8". So the tire change itself is going to lift your axle up 1.1" and change your tree angle (in relation to the ground, not actual) slightly.

To figure out your tire diameter using the measurements on the tire. 90/90-19", the first number (90) is your tires width at 90mm, if you divide it by 25.4 (mm in 1") you'll get the standard measurement of 3.54". The second number (/90) is the percentage of the first number that is used for the side wall height. So 90% of 90mm is 81mm or 90% of 3.54" is 3.19". If you take that number and add it to each side of your rim you'll get the over all tire diameter. So 120mm/70%-21" would look like this 4.72"/3.31"-21".


Using some geometery, your trail will extend out .96" from stock using the 21" tires baised on those measurements above.

 

I put a two over springer on my '12 street bob and I definitely notice a difference in stability at low speeds. nothing horrible, and highway speeds are smooth as butter. Low speeds in town it is different. Just changing to 2+ tubes I don't think it would be a drastic. kicking the front tire forward and adding 2" big change! little changes can effect handling I haven't measured the number on my trail yet but I can feel the difference

 

You must consider steering stem offset in both your trees. You measure that as it is not published. This is very important. It is also harder to do on raked triple trees. It is not the same as neck rake.




You must measure everything including the angle of the bottom frame rails if you want your bike level. You must also consider both ends (F & R) of sag as you must sit on your bike to make it move/ride. You do not need rake & trail to work on the kick stand.

+5º of rake will decrees your trail + you will need to add fork length to put the bike back even. Figure to out before spending one dime as it is free & does not hurt much just thinking it out beforehand.

On Dynas I lengthen the steering stem offset to reduce my trail so the bike handles much better.