SR250 Brat/Street Tracker - First Build (aka Little Trouble Kid)
- Thread starter chiel
- Start date
cosworth
Coast to Coast
Here is the trick to a good front end on the sr250:
Drain and disassemble the forks. Parts washer fired up and hose the shit out. All the old stuff. All the sludge.
Get some 7/8ths tubing. Brass, aluminium etc. Cut two 35mm deburred shanks with a deburred 1/4 hole drilled top and bottom. Drop those in the bottom of the forks. I used brass. If you don't understand what the 35mm tubes does for lowering the forks, go research it.
Assemble the forks but stop once the forks are on the bike and leave the springs out. Make sure the forks are all the way down - they should be with no springs in them. Tight in the triples too.
Fill them with 30w fork oil on both sides, until the top level is 6 inches from the top. Slowly slide the springs in and final assemble is in order.
Once I did this my spring stopped bottoming out on rebound and the bump was great. I'm getting acceptable sag and the front end steers perfectly for the tires siEs and frame angles of the SR250.
And if you didn't do the $20 fork seals...
Then go buy a mini lathe and make some nice brass caps:
Drain and disassemble the forks. Parts washer fired up and hose the shit out. All the old stuff. All the sludge.
Get some 7/8ths tubing. Brass, aluminium etc. Cut two 35mm deburred shanks with a deburred 1/4 hole drilled top and bottom. Drop those in the bottom of the forks. I used brass. If you don't understand what the 35mm tubes does for lowering the forks, go research it.
Assemble the forks but stop once the forks are on the bike and leave the springs out. Make sure the forks are all the way down - they should be with no springs in them. Tight in the triples too.
Fill them with 30w fork oil on both sides, until the top level is 6 inches from the top. Slowly slide the springs in and final assemble is in order.
Once I did this my spring stopped bottoming out on rebound and the bump was great. I'm getting acceptable sag and the front end steers perfectly for the tires siEs and frame angles of the SR250.
And if you didn't do the $20 fork seals...
Then go buy a mini lathe and make some nice brass caps:
Attachments
crazypj
Split personality, I fake being smart
30wt is way too thick, all that does is stop forks working, 15wt would be far better.
Using too thick oil to compensate for soft springs is a bad and stupid idea
You need to cycle fork legs through a couple of strokes after 'filing' then check oil is still 6" from top. Then extend forks all the way before trying to fit springs
Using too thick oil to compensate for soft springs is a bad and stupid idea
You need to cycle fork legs through a couple of strokes after 'filing' then check oil is still 6" from top. Then extend forks all the way before trying to fit springs
cosworth
Coast to Coast
30w is not too thick. This is a weedy little front end with 32mm tubes.
I'm running 30w and 20w wasn't enough. I don't want to sound pissy but I actually own an SR250 that handles like a hot damn because of the front forks.
Have you tried a lowered SR250 with 30w fork oil? Doubt it.
I'm running 30w and 20w wasn't enough. I don't want to sound pissy but I actually own an SR250 that handles like a hot damn because of the front forks.
Have you tried a lowered SR250 with 30w fork oil? Doubt it.
crazypj
Split personality, I fake being smart
Hey Cosworth, I cant help but explain what and why. ;D
Motorcycles have been my 'life' ever since I can remember. my father ot me started when I was about 18months old (so my mother used to tell me)
Apart from 8 yrs working on construction equipment it's the only thing I've ever really done. (and even then I was still 'playing' with bikes)
I trained as a precision machinist so I could make bike parts, fitting BSA bits into Royal Enfield's, parts from different bikes modified to fit something else. The first 'big bore' conversion I did was 1975
I worked in a couple of Yamaha dealers, probably ridden more bikes than you can imagine.
I've ridden SR250 and XT250, (and XS 250, 400, etc) The forks are soft, as were all the bikes designed for California market in the 60's and 70's(and most of the 80's)
I ran dealer workshops but even before that had 'free reign' in all the dealerships I worked at (Honda, Suzuki, Triumph, BMW, Vespa, SILK)
Ive had to pick up plenty of crashed bikes, sometimes serious, mostly 'trivial', I always want to know 'WHY' something happened
Thick oil didn't compensate for soft springs 30+ yrs ago and doesn't compensate for them now.
What really happens when you fit thick oil is it prevents front end re-bound if you hit a series of bumps or undulations
It's a pretty well known phenomenon known as 'packing'
I also raced motocross and enduro
A far better way to increase spring rate is to cut a section off the closed coils of spring and fit a spacer. Fitting a second spring makes forks softer as you have more working coils.
Your using bikes as a hobby to make cash to support hobby you should start studying what your doing instead of following the herd then recommending some cheap and nasty 'solution'
I'm not trying to piss you off, just explain I do know what I'm doing
Motorcycles have been my 'life' ever since I can remember. my father ot me started when I was about 18months old (so my mother used to tell me)
Apart from 8 yrs working on construction equipment it's the only thing I've ever really done. (and even then I was still 'playing' with bikes)
I trained as a precision machinist so I could make bike parts, fitting BSA bits into Royal Enfield's, parts from different bikes modified to fit something else. The first 'big bore' conversion I did was 1975
I worked in a couple of Yamaha dealers, probably ridden more bikes than you can imagine.
I've ridden SR250 and XT250, (and XS 250, 400, etc) The forks are soft, as were all the bikes designed for California market in the 60's and 70's(and most of the 80's)
I ran dealer workshops but even before that had 'free reign' in all the dealerships I worked at (Honda, Suzuki, Triumph, BMW, Vespa, SILK)
Ive had to pick up plenty of crashed bikes, sometimes serious, mostly 'trivial', I always want to know 'WHY' something happened
Thick oil didn't compensate for soft springs 30+ yrs ago and doesn't compensate for them now.
What really happens when you fit thick oil is it prevents front end re-bound if you hit a series of bumps or undulations
It's a pretty well known phenomenon known as 'packing'
I also raced motocross and enduro
A far better way to increase spring rate is to cut a section off the closed coils of spring and fit a spacer. Fitting a second spring makes forks softer as you have more working coils.
Your using bikes as a hobby to make cash to support hobby you should start studying what your doing instead of following the herd then recommending some cheap and nasty 'solution'
I'm not trying to piss you off, just explain I do know what I'm doing
cosworth
Coast to Coast
So you don't ride an SR250 that's lowered with 30w oil. but you've ridden an SR250 sometime in the past 40 years? Check. You list your past experience in order to invalidate mine. Check.
The OP can take any advice he wants. But your excuse that forks were designed "soft" on the SR250 for the California market alone should tell the OP what advice to take.
The OP can take any advice he wants. But your excuse that forks were designed "soft" on the SR250 for the California market alone should tell the OP what advice to take.
crazypj
Split personality, I fake being smart
If you know anything about history of motorcycles, California market through 1970's was larger than ALL of the rest of USA combined plus most of Europe.
The Japanese are not stupid, they changed , mirrors, brakes, lights for different markets to comply with 'local' regulation but basic bike was the same everywhere ~ designed for California market
Lowered SR with 32mm forks has nothing to do with anything. I'm glad you think it handles great.
Thick oil doesn't compensate for too soft springs.
All that is happening is bottom out tapered stopper isn't 'clanking', you can dress it up any way you like but soft springs are soft springs
The Japanese are not stupid, they changed , mirrors, brakes, lights for different markets to comply with 'local' regulation but basic bike was the same everywhere ~ designed for California market
Lowered SR with 32mm forks has nothing to do with anything. I'm glad you think it handles great.
Thick oil doesn't compensate for too soft springs.
All that is happening is bottom out tapered stopper isn't 'clanking', you can dress it up any way you like but soft springs are soft springs
Cosworth - are you saying to add a spacer to the rebound spring area like in this post (and there are several other posts here and in the WWW explaining the same way to lower a similar fork)? http://www.dotheton.com/forum/index.php?topic=27984.msg300655#msg300655
I can see how this would accomplish lowering the forks by "tricking" the tube action, and why you would need a higher viscosity oil to speed up damping due to smaller fork stroke. I just wonder what advantage this would be for general purpose street riding? I would think a longer stroke would be an advantage with the (generally) poor condition of most public roads to better absorb bumps/holes. Kind of like offroad forks having 9" or so of travel due to the conditions.
I can see how this would accomplish lowering the forks by "tricking" the tube action, and why you would need a higher viscosity oil to speed up damping due to smaller fork stroke. I just wonder what advantage this would be for general purpose street riding? I would think a longer stroke would be an advantage with the (generally) poor condition of most public roads to better absorb bumps/holes. Kind of like offroad forks having 9" or so of travel due to the conditions.
cosworth
Coast to Coast
There is no tricking. You're adding a spacer to drop or shorten the forks. This does two things, it changes your applied head tube angle, quickening the steering.
Secondly, it forces the spring to live in a smaller tube, increasing the spring rate. When you try and put the spring cap back on you'll have to press down on it very hard to get the circlip in. Luckily, the SR250 has a good progressive spring that won't bind with a 35mm spacer.
Now let's talk bump and rebound. You fork now needs more force to compress it over the same static distance. But how fast it compresses and how fast it can decompress is regulated by the oil valving and viscosity. The 30w is recommended by me after trying a 20mm spacer and 20w oil. You are more than welcome to try 20w. When the forks are apart, you'll see that the valving is very simple.
A fork rebuild on theses forks is awesome and simple. You'll be very pleased with the results. Don't drop them too much in the triples though.
But you really have to get the fork oil 6 inches from the top when collapsed with no spring.
Secondly, it forces the spring to live in a smaller tube, increasing the spring rate. When you try and put the spring cap back on you'll have to press down on it very hard to get the circlip in. Luckily, the SR250 has a good progressive spring that won't bind with a 35mm spacer.
Now let's talk bump and rebound. You fork now needs more force to compress it over the same static distance. But how fast it compresses and how fast it can decompress is regulated by the oil valving and viscosity. The 30w is recommended by me after trying a 20mm spacer and 20w oil. You are more than welcome to try 20w. When the forks are apart, you'll see that the valving is very simple.
A fork rebuild on theses forks is awesome and simple. You'll be very pleased with the results. Don't drop them too much in the triples though.
But you really have to get the fork oil 6 inches from the top when collapsed with no spring.
It also reduces fork movement so a fork that used to move (let's say) 4" full-compression-to-extension that is lowered by 35mm (about 1.4") will now only move 2.6" full-compression-to-extension. Similar to putting on shorter rear shocks. This might be fine for a track bike that will see little road surface variation but needs better high-speed suspension control. Not so good for a street bike that might occasionally see stone roads or badly-paved surfaces.
Barkingmadspeedshop
Sell the many to fund the few
So what was done on as my friend says to "old shitty forks" where to do one of two things heavy oil or braise the large holes shut and drill smaller holes in the bottom of the rod. Running 30 weight oil slows the dampening down. He stated that some people would run motor oil. He is an x pro rider and is very accomplished. So what ever floats your boat
crazypj
Split personality, I fake being smart
I started using full synthetic motor oil in forks about 10 yrs ago.
The cheapest full synthetic multi grade oils don't change damping when they get warm and seem to work MUCH better than ATF or single grade fork oils. (until you get to real expensive 'suspension fluids', $26~$30.00 a litre)
They are noticeably smoother operation. kopsicle mentioned a coupe of days ago he had got hold of some and difference in 'feel' was obvious
As for limiting travel, it's generally OK to do exactly how Cosworth says, fit a spacer under top out spring then either leave compression spring stock or chop a piece off and fit a top spacer as well.As your compressing the 'closed coils' I find it simpler to chop a piece off
. Easiest way to 'upgrade springs is get some from a heavier bike with same size forks then cut off a section of closed coils. CB400f or CB360 has 32mm forks and are substantially heavier than SR250, X250/400 would also work
The cheapest full synthetic multi grade oils don't change damping when they get warm and seem to work MUCH better than ATF or single grade fork oils. (until you get to real expensive 'suspension fluids', $26~$30.00 a litre)
They are noticeably smoother operation. kopsicle mentioned a coupe of days ago he had got hold of some and difference in 'feel' was obvious
As for limiting travel, it's generally OK to do exactly how Cosworth says, fit a spacer under top out spring then either leave compression spring stock or chop a piece off and fit a top spacer as well.As your compressing the 'closed coils' I find it simpler to chop a piece off
. Easiest way to 'upgrade springs is get some from a heavier bike with same size forks then cut off a section of closed coils. CB400f or CB360 has 32mm forks and are substantially heavier than SR250, X250/400 would also work
chiel
New Member
Well, thanks for the info and healthy discussion guys ;D Cosworth is right in saying the stock SR front end is flimsy and soft, but since i'm not racing my SR i'm not really interested in a super tight front end. For now the lowered forks make her steer a bit better, which is cool for some sunny sunday driving. But i'll definitely dig into this when she's ready crazypj.
