Modern dirtbike front end on old Yamaha Enduro

billy mild

New Member
I have a 1971 Yamaha AT1-c. I'm currently looking to restore/upgrade a bit. The engine will be rebuilt hopefully using a CT1 cylinder and piston kit to make it a 175 cc. I already have a expansion chamber for the bike. I am also looking update the suspension a bit. What is on there now is stock and outdated.

I would like to get some new rear shocks, and possibly do a USD fork conversion. The curent forks on the bike are really small and still use drum brakes. I know the dirt riders back in the day would use a 21" front wheel when racing. I was thinking of getting a set of YZ USD forks to throw on front, but I'm not sure if the steering stem can be converted to work on the old AT1. I thought possibly a set of YZ 80/85 forks along with a 18 or 21" front wheel would work.

Anyone done this?
 

xb33bsa

Over 1,000 Posts
7" or so of wheel travel is all you are gonna be able to do on that tiny frame
the steer head is low and longer travel up front is gonna make it an ill handling choper
whatever you do in front needs to be matched in the rear ,quality,ride height and travel in order to keep the balance of the machine
 

VonYinzer

Over 10,000 Posts
You'd be better suited to find a traditional set of forks that are only slightly longer from something like a later DT. Most modern USD offroad forks will be way too long and most are not meant for street use.
 

billy mild

New Member
I was thinking a smaller modern dirbike front would work fine for this application, but you might be right with the suspension travel aspect.

What type of forks should I look into? I would like to stiffening up the front end a bit. I am not doing a full cafe racer build. I would like to keep the bike to its roots, but just allow it to handle a bit better and still be able to hit some trails once in a while.
 

Shawzie

New Member
I've wanted to do this to my 75 dt175. The "big wheel" 80/85 is a 19" front rim. Normal size is 17" I believe. Let me know how it goes. I very interested
 

makotosun

Avoiding creeping perfection!
You can fit the front end of of the RT/DT (250/360) series on the AT1 frame. Direct bolt up. 34mm fork tubes and really enough travel for that bike. They may be slightly longer, but can be slipped up in the tree a bit to compensate. You probably will need the larger hub also, but I am not sure on that. I have a friend who races an AT1MX converted to the larger front forks and he loves the stability.

There was also a stock steering damper available for certain of the RT/DT models which can be fitted for more stable street use. They fit the AT/CT triples too.

The larger drum brakes fitted with good new shoes and adjuster pad are really quite sufficient for the rest of the running gear.

I have a 1970 AT1 converted to the 175 top end. Make sure you get the proper 175 Top rod bearing as it is different than the 125. Otherwise easy conversion.

With the chamber you can also upsize the carb to a 26mm if you can dind one. That is the MX size carb which makes the motor breath really well. I also put a webco head on mine. It is really just for looks as there were no higher compression heads marketed for the 175 without custom machining.

You should get new clutch disks as the larger displacement will likely make the old disks slip. You can still get Barnett clutch springs for these if you know what to look for.

 

makotosun

Avoiding creeping perfection!
Shawzie said:
I've wanted to do this to my 75 dt175. The "big wheel" 80/85 is a 19" front rim. Normal size is 17" I believe. Let me know how it goes. I very interested
Stock should be 18" on the AT1. The DT1 was stock at 19". The option for both was 21".
 

xb33bsa

Over 1,000 Posts
you cant really do much to that old enduro it is what it is
a trailbike for slow plonking
putting 8-9" travel forks onnit will just unbalance the whole bike
if you want more modern suspension and disc brakes get a newer bike
 

xb33bsa

Over 1,000 Posts
billy mild said:
I'm more looking for a disc brake up front.

I don't want a newer bike. This one has been in my family its entire life.
then show some respect for you're family.don't fucking ruin it :-\ which is what you will end up doing trying to make it something it is not
 

bradj

Over 1,000 Posts
DTT BOTM WINNER
if your forks are 34mm Get rd350 or rd400 lowers and put them on your tubes or swap in the whole fork and front wheel
 

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